Scalars 2025: Higgs bosons and cosmology

Sep 22, 2025, 8:00 AM → Sep 25, 2025, 6:00 PM Europe/Warsaw

0.06 (University of Warsaw, Faculty of Physics)

Bohdan Grzadkowski

The conference "Scalars 2025" will be held in Warsaw in September 2025. The meeting is organized jointly by the Faculty of Physics  of the University of Warsaw and the  Division for Physics of Fundamental Interactions of the Polish Physical Society. The meeting will take place at the Faculty of Physics (OCHOTA Campus).   

The meeting will start with the registration on September 22nd  (Monday) in the morning.  The conference "Scalars 2025" is the 7th edition of  "Scalars" organized in Warsaw in 2011, 2013, 2015, 2017, 2019 and 2023. The goal of the meeting is to provide an opportunity to discuss various aspects of physics of scalar particles. In particular we plan to cover the following topics:   
 

• Scalars in cosmology: dark matter, baryogenesis, phase transitions, inflatons, axions, relaxions, radions, etc.,
• SM Higgs boson: theory and experiment,
• Extensions of the minimal scalar sector of the Standard Model, e.g. multi Higgs-doublet models, doublet-singlet models, the Georgi-Machacek model, ...,
• Experimental constraints on possible scalar sectors,
• Future experimental searches for scalars, e.g. at LHC, FCC or ILC.
• Effective field theory in Higgs physics,
• Scale-invariant scalar sectors,
• Composite Higgs bosons,
• Elementary scalars within extra dimensions: gauge-Higgs unification, stabilization of extra  dimensions, radions, bulk scalars, etc.,

 

Previous meetings:

• 2023
• 2019
• 2017
• 2015
• 2013
• 2011

 

         

 

program_plenary_5.pdf

Mon, September 22

8:30 AM → 8:50 AM Registration

8:50 AM → 9:00 AM Opening

8:50 AM Opening

Speaker: Zygmunt Lalak

Prezentacja FUW2025.pdf

9:00 AM → 10:30 AM Plenary 1

Convener: Andrew Long (Rice University)

9:00 AM Gravitational waves from Reheating

We investigate a novel gravitational wave (GW) production mechanism from gravitons generated during the pre-thermal phase of cosmic reheating, where the energy density is dominated by non-thermalized inflaton decay products, dubbed reheatons. We consider multiple production channels, including: i) pure inflaton-inflaton annihilation, ii) graviton Bremsstrahlung from inflaton decay, iii) scatterings between an inflaton and a reheaton, and iv) scatterings among reheatons. To determine the resulting GW spectrum, we solve the Boltzmann equation to obtain the graviton phase-space distribution for each channel. We find that the third channel, iii), dominates due to the large occupation number of reheatons at highly-energetic states during the pre-thermalization phase. Notably, in scenarios with a low inflaton mass, the GW spectrum could fall within the sensitivity range of future experiments such as the Einstein Telescope, the Cosmic Explorer, the Big Bang Observer, and ultimate DECIGO.

Speaker: Nicolás Bernal (New York University Abu Dhabi)

bernal.pdf

9:30 AM Cosmological Collider and Higgs: Primordial Non-gaussianity & Gravitational Waves

Cosmological collider signals of primordial non-Gaussianity arise at tree level when an extra scalar has Hubble mass during inflation. We critically review the formalism finding that a large class of inflationary theories, based on Planck-scale physics, predict a scalar bi-spectrum around the gravitational floor level. This mild signal arises for example in R22 gravity, in the regime where its gravitational scalar has Hubble-scale mass. Signals much above the gravitational floor arise in theories where scalars undergo multiple turns during inflation, thanks to sub-Planckian physics.

Speaker: Anish Ghoshal (University of Wasrsaw, Poland)

talk_SCALARS_2025_final.pdf

10:00 AM On-shell approach to spinning binaries in scalar-tensor theories

The detection of gravitational waves has opened a new era in astronomy. At the same time, certain techniques borrowed from quantum field theory have proven very efficient for calculating classical waveforms for gravitational radiation emitted by systems of compact astrophysical objects. In this talk, I will discuss the application of these techniques to scalar-tensor theories of gravity, where long range interactions are mediated by a massless scalar in addition to the graviton. Such theories are of both theoretical and phenomenological interest, with examples including the Einstein-scalar-Gauss-Bonnet and Dynamical Chern-Simons theories. I will explain how to calculate waveforms and power emitted in gravitational and scalar radiation starting from amplitudes describing emission of a graviton or scalar in matter scattering. The required amplitudes can be calculated in a straightforward manner by using the on-shell and spinor-helicity techniques, also when spins of the compact objects are taken into account. At the leading order in spin the emitted power matches earlier results obtained by classical techniques, while spin-dependent corrections can be readily calculated in the on-shell approach. I will also discuss the effective field theory approach to scalar-tensor theories, and possible UV completions.

Speaker: Adam Falkowski

250922-Warszawa.pdf

10:30 AM → 11:00 AM Coffee

11:00 AM → 12:30 PM Plenary 2

Convener: Michel Tytgat (Université Libre de Bruxelles)

11:00 AM $\tilde\xi$-attractors in metric-affine gravity

We propose a new class of inflationary attractors in metric-affine gravity. Such class features a non-minimal coupling $\tilde\xi \, \Omega(\phi)$ with the Holst invariant $\tilde {\cal R}$ and an inflaton potential proportional to $\Omega(\phi)^2$. The attractor behaviour of the class takes place with two combined strong coupling limits. The first limit is realized at large $\tilde\xi$, which makes the theory equivalent to a $\tilde {\cal R}^2$ model. Then, the second limit considers a very small Barbero-Immirzi parameter which leads the inflationary predictions of the $\tilde {\cal R}^2$ model towards the ones of Starobinsky inflation. Because of the analogy with the renown $\xi$-attractors, we label this new class as $\tilde\xi$-attractors.

Speaker: Antonio Racioppi (NICPB, Tallinn, Estonia)

Racioppi_Scalars_2025_plenary.pdf

11:30 AM Sequential Freeze-in: A Tale of Two Scalars

The process of thermal dark matter production can be significantly more complex and richer than is usually assumed. In this talk I will discuss some of the recent progress in understanding the impact of non-equilibrium effects on multi-component freeze-in. In particular, I will present a sequential freeze-in realisation for a simple model of scalar dark matter coupled to the visible sector through a Higgs portal. Although its freeze-in origin, dark matter in this scenario can be within reach of indirect detection experiments and simultaneously the scalar mediator can be probed in forward physics searches. Finally, consequences of performing the calculation taking into account non-equilibrium distributions will be highlighted.

Speaker: Andrzej Hryczuk (National Centre for Nuclear Research)

Scalars2025.pdf

12:00 PM Realising Dark Matter and NANOGrav data via Dark Branes

In this work we propose a setup for the origin of dark matter based on spacetime with a warped extra dimension and three branes: the Planck brane, the TeV brane and a dark brane, at a (sub)-GeV scale. The Standard Model is localized in the TeV brane, thus solving the Higgs hierarchy problem, while the dark matter χ, a Dirac fermion, is localized in the dark brane. The dark matter has only gravitational interactions with the Standard Mode and we show that it can easily reproduce the thermal relic density by annihilations into radions and avoid direct detection experiments. The dark brane comes with a conformal sector confining at a 1st order phase transition generating a stochastic gravitational waves background which can accommodate the recent NANOGrav signal. A region in the parameter space of dark matter and radion masses is selected.

Speaker: Fotis Koutroulis (IHEP, CAS)

KOUTROULIS_Scalars2025.pdf

12:30 PM → 2:00 PM Lunch

2:00 PM → 4:00 PM Plenary 3

Convener: Mikhail Shaposhnikov

2:00 PM Rarita-Schwinger Fields and Cosmology

The early universe after inflation can have enhanced production, in fact "runaway production," of Rarita-Schwinger fields (spin-3/2 fermions). I will discuss the implication of gravitational production of Rarita-Schwinger fields for dark matter and for the identification of Rarita-Schwinger fields as the gravitino.

Speaker: Edward (Rocky) Kolb (University of Chicago)

Scalars_2025.pptx

Scalars_25_Kolb.pdf

2:30 PM Making dark particles from gravity

Where did the dark matter come from? The phenomenon of cosmological gravitational particle production (CGPP) occurs during and after inflation as quantum fields that feel the cosmological expansion are excited out of their ground state. CGPP is a simple explanation for the origin of dark matter, which might only interact gravitationally, as well as other cosmological relics. In this talk, I'll give a short introduction to CGPP and then discuss a few recent developments.

Speaker: Andrew Long

Andrew_Long--Scalars_2025.pdf

3:00 PM Gravitational Waves from Gravitational Particle Production

The excitation of scalar dark matter during inflation may result in large isocurvature perturbations, which can be avoided by inducing a sizable effective dark matter mass during the inflationary phase. This can be achieved by a direct coupling to the inflaton, through a nonminimal coupling to the curvature, or by a large bare mass. Notably, when the isocurvature is suppressed at CMB scales, a peak arises at small scales, corresponding to modes that leave the horizon near the end of inflation. I will discuss how these large perturbations result in a stochastic gravitational wave background in the sensitivity range of existing and future gravitational wave observatories.

Speaker: Marcos A. Garcia Garcia (Instituto de Física, UNAM)

Scalars_talk.pdf

3:30 PM Dark matter spikes with strongly self-interacting particles

Speaker: Alejandro Ibarra

Ibarra.pdf

4:00 PM → 4:30 PM Coffee

4:30 PM → 6:00 PM Plenary 4

Convener: Zackaria Chacko (University of Maryland, College Park)

4:30 PM On the present status of inflationary cosmology

I will describe basic principles of inflationary cosmology, the problem of initial conditions in the early universe, and a profound role of quantum effects in formation of the global structure of the universe. This will include a discussion of the impossibility to observe the Big Bang, and of the important difference between the universe and the Schrodinger cat. Then I will describe some of the better motivated inflationary models, their predictions, and their observational status.

Speaker: Andrei Linde (Stanford University)

Linde_Scalars2025.pdf

5:00 PM Inflationary potentials with the exponential vs polynomial approach to the plateau

One of the conclusions based ob the Planck data was that inflationary potentials have a plateau, which is approached exponentially fast at the large values of the scalar field. However, recent data from ACT, DESI, and SPT show that the inflationary spectral index n_s is slightly higher than its value based on the Planck data. We will explain why this small deviation, if confirmed, is very significant: it disfavors some of the most popular inflationary models of the last decade, and suggests that the inflaton potentials approach the plateau not exponentially but polynomially.

Speaker: Renata Kallosh

Kallosh_Scalars2025.pdf

5:30 PM Inflationary particle production

I discuss particle production via inflation, which can be the main dark matter production mechanism.

Speaker: oleg lebedev (Uni. Helsinki)

warsaw2025-update.pdf

6:00 PM → 9:00 PM Welcome reception

Tue, September 23

9:00 AM → 10:30 AM Plenary 5

Convener: Manfred Lindner (Max Planck Institute für Kernphysik)

9:00 AM RG-stability of parameter relations in the absence of a conventional symmetry

We examine a quantum field theory of two real scalars in which a tree-level relation among the squared-mass parameters of the scalar potential is stable under renormalization group (RG) running despite the absence of a conventional symmetry to explain the stability. By complexifying the original scalar field theory, one can identify the symmetry that guarantees the RG-stability of a corresponding relation satisfied by the squared-mass parameters of the complexified theory. We can then show that the RG-stability of the parameter relation of the original theory of two real scalars is a consequence of the structure of the beta functions of the parameters of the complexified theory.

Speaker: Howard Haber (University of California, Santa Cruz)

Scalars25_Haber.pdf

9:30 AM Goofy Symmetries

I will discuss the recently discovered global goofy transformations. Originally, goofy transformations were identified in explicit discussions of the 2HDM, but their understanding is mandatory to describe the renormalization group (RG) fixed point structure of QFTs in general. Even though goofy transformations are explicitly broken by the canonical gauge-kinetic terms, the parameter relations they impose on the potential can be stable under RG running to all orders. To supplement this I will show some previously unknown all-order RG stable parameter relations of the 2HDM. I will also briefly discuss goofy transformations in other theories, including their application to the hierarchy problem in the Standard Model.

Speaker: Andreas Trautner (CFTP, IST Tecnico, Lisbon)

Trautner_Goofy_Symmetries_Scalars_25.pdf

10:00 AM Imaginary scaling

Recently, a hitherto unknown class of RG-stable relations between parameters in bosonic field theories have been identified and dubbed as the r0 or ”goofy” symmetries. The 1-loop properties of an invariant 2-Higgs Doublet Model and a minimal symmetric model will be discussed. It is concluded that the symmetry is present at the 1-loop provided the UV cut-off squared transforms non-trivially under r0. The minimal model requires the presence of two real fields.

Speaker: Odd Magne Ogreid (Western Norway University of Applied Sciences)

Ogreid - Warsaw2025.pdf

10:30 AM → 11:00 AM Coffee

11:00 AM → 1:00 PM Plenary 6

Convener: Adam Falkowski

11:00 AM Surprises in Vacuum Decay

Alternative Titles:
1:Vaccum Decay: Bounces, Antibounces and Pseudo-bounces
2:Exploring Vacuum Decay Valleys

In the standard lore the decay of the false vacuum of a single-field potential is
described by a semi-classical Euclidean bounce configuration that can be found using
overshoot/undershoot algorithms, and whose action suppresses exponentially the decay
rate. While this is generically correct, the vacuum decay structure can be far richer.
In some cases there is no bounce and decay proceeds via the so-called pseudo-bounces.
In the general case with bounce, there are 2n+1 bounces, with n ranging from 0 (the
standard case) to ∞. Some of these decays occur via "antibounces" which have the
wrong behavior for overshoot/undershoot algorithms, that can miss them. Bounce and
antibounce configurations form n pairs connected by pseudo-bounces.

Speaker: José Ramón Espinosa (IFT-UAM/CSIC, Marid)

Espinosa_Scalars_2025.pdf

11:30 AM Standard Model Baryon Number Violation at Zero Temperature from Higgs Bubble Collisions

We compute for the first time baryon number violation at zero temperature from Higgs bubble collisions and find that it can be of the same order as that from thermal sphalerons in the symmetric phase at electroweak temperatures. We study the dependence of the rate of Chern--Simons number transitions on the shape of the scalar potential and on the Lorentz factor of the bubble walls at collision via large-scale (3+1)D lattice simulations of the Higgs doublet and SU(2) gauge fields. We estimate the resulting baryon asymmetry assuming some CP-violating source activated by the Higgs-field variation during the phase transition.

Speaker: Marco Gorghetto (DESY)

scalars2025_Gorghetto_1.pdf

12:00 PM A Catalog of First-Order Electroweak Phase Transitions in the SMEFT

A first-order Electroweak Phase Transition (FOEWPT) could explain the observed baryon-antibaryon asymmetry of the Universe, and its dynamics could yield a detectable gravitational wave signature, while the underlying physics would be within the reach of colliders. The Standard Model, however, predicts a crossover transition, so any hope of having a FOEWPT hinges on physics beyond the Standard Model (BSM). Most studies of the possibilities for a FOEWPT consider specific BSM models with new particles around the electroweak scale that help generate a barrier in the effective potential between the true and the false vacuum, facilitating a first-order transition.

On the other hand, the Standard Model Effective Field Theory (SMEFT) is a model-independent effective field theory extension of the SM that encodes new physics at the cutoff scale. It contains all SU(3)xSU(2)xU(1) invariant operators to a given order in the EFT expansion. Previous phenomenological studies of possibilities for a FOEWPT in the SMEFT have considered the case with a tree-level barrier and a negative Higgs quartic coupling. This requires a small new physics scale, which from an EFT perspective is undesirable.

In a recent paper [1] (see also [2]) we have used modern dimensionally-reduced EFT methods with careful power-counting of scale hierarchies to analyze and catalog the different types of FOEWPT that are possible in the SMEFT. These calculations lay the groundwork to performing gauge invariant, properly resummed perturbative expansions addressing the theoretical problems with phase transition calculations.

We find three types of configurations of the scalar potential that allow a FOEWPT: (1) tree-level barriers, (2) radiative barriers, and (3) radiative symmetry breaking through the Coleman-Weinberg mechanism. We also find versions of these with supercooling. We perform a global likelihood scan over the SMEFT Wilson coefficients to identify parameter regions that exhibit these first-order phase transitions and are consistent with experimental and theoretical constraints. We comment on the possibilities for electroweak baryogenesis within the SMEFT, and roughly estimate if the gravitational wave spectra generated by the phase transitions are detectable. In this talk I will also briefly discuss a comparison of the method with the more common 4D QFT calculations of the phase transition parameters, as well as the prospects for probing the allowed parameter space using di-Higgs production in colliders.

[1] E. Camargo-Molina, R. Enberg, J. Löfgren, "A Catalog of First-Order Electroweak Phase Transitions in the Standard Model Effective Field Theory", https://arxiv.org/abs/2410.23210

[2] J.E. Camargo-Molina, R. Enberg, J. Löfgren, “A new perspective on the electroweak phase transition in the Standard Model Effective Field Theory”, JHEP 2021, 127 (2021), https://arxiv.org/abs/2103.14022

Speaker: Rikard Enberg (Uppsala University)

RE scalars 2025.pdf

12:30 PM Supercooled phase transitions from radiative symmetry breaking. Phenomenology, renormalisation and naturalness.

Supercooled cosmological phase transitions typically yield a strong gravitational-wave background, potentially detectable, e.g. in LISA. The scenario of supercooling is naturally realised in models with classical scale invariance, where symmetry is broken radiatively. I will review the phenomenology of the supercooled phase transition and explain how to properly compute the bubble nucleation rate. One can wonder whether introducing mass scales through the on-shell renormalisation could break scale invariance and significantly alter phenomenological predictions of this class of models. In this talk, working with a simple model, I will explain how to renormalise classically scale-invariant models in the on-shell scheme. Understanding the correspondence between various mass scales will shed some light on the naturalness of classically scale-invariant scenarios.

Speaker: Bogumiła Świeżewska (University of Warsaw)

B_Swiezewska.pdf

1:00 PM → 2:00 PM Lunch

2:00 PM → 3:30 PM Plenary 7

Convener: Margarida Nesbitt Rebelo (CFTP/IST, U. Lisboa)

2:00 PM Electroweak baryogenesis in 2HDM without EDM cancellation

We discuss two Higgs doublet models with successful electroweak baryogenesis but without cancellations of electric dipole moments (EDMs). For the baryogenesis, additional scalar bosons are favored to couple mainly with the top quark with CP violations. However, if they also couple to light fermions of the Standard Model, the model is limited severely by EDMs, and additional CP phases irrelevant to the baryogenesis are often introduced to cancel the contributions to the EDMs. We here discuss a scenario where the light-fermion couplings are suppressed to avoid the constraints. We show that our scenario is compatible with the current experimental bounds and is within the scope of future EDM experiments.

Speaker: Shinya Kanemura (The University of Osaka)

Sccalars2025_kanemura.pdf

2:30 PM Hierarchies and conformal UV Completions

The hierarchy problem related to Standard Model embeddings has worsened by the little hierarchy problem, the fact no new physics has so far been observed in LHC data. The talk will discuss how conformal UV completions of the Standard Model and of extensions link these two aspects of the hierarchy problems.

Speaker: Manfred Lindner (Max Planck Institute für Kernphysik)

SCALARS25-Lindner.pdf

3:00 PM Higgs mass and gravity

According to usual calculations (both in flat and curved spacetime) the mass of a scalar particle is quadratically sensitive to the ultimate scale of the theory, the UV physical cutoff. Considering a self-interacting scalar theory on a non-trivial gravitational background, I will show that, once due attention is payed to the path integral measure and to the way the UV scale is introduced, the mass of the scalar particle presents only a (mild) logarithmic sensitivity to this scale. This is obtained without resorting to any supersymmetric embedding of the theory, nor to regularization schemes (as dimensional or zeta-function regularization) where power-like divergences are absent by construction. Based on these results, I will present speculations on the way the Minkowski limit should be approached.

Speaker: Vincenzo Branchina

WARSAW-25-Branchina.pdf

3:30 PM → 4:00 PM Coffee

4:00 PM → 5:00 PM Plenary 8

Convener: Igor Ivanov (Sun Yat-sen University)

4:00 PM (In)consistency of the Real 2HDM

The Real 2HDM with a softly-broken Z_2 symmetry is expected to receive divergent CP-violating radiative corrections from the CP phase in the CKM matrix, as emphasized by Fontes et al. four years ago. I will describe a new approach combining the properties of renormalization group equations, the momentum structure of divergences in Feynman diagrams, and approximate symmetries between individual diagrams that we used to demonstrate the cancellation of these CP-violating divergences through six loops. By identifying the couplings that break the approximate symmetries at seven loops, we can predict the leading parameter dependence of the divergent CP violation.

Speaker: Heather Logan (Carleton University)

CPV-2HDM-SCALARS25.pdf

4:30 PM Critical Unstable Qubits

By employing the Bloch-sphere formalism, I will present a novel class of unstable qubits, which are called Critical Unstable Qubits (CUQs). The characteristic property of CUQs is that the energy-level and decay-width Pauli vectors, E and Γ, are orthogonal to one another, and the key parameter r = |Γ|/(2|E|) is less than 1. A remarkable feature of CUQs is that they exhibit atypical behaviours like coherence-decoherence oscillations when analysed in an appropriately defined co-decaying frame of the system. In the same frame, I will show how a unit Bloch vector b describing a pure CUQ sweeps out unequal areas during equal intervals of time, while rotating about the vector E. These phenomena emerge beyond the usual oscillatory pattern due to the energy-level difference of a standard two-level quantum system. I will illustrate how these new features are relatively robust and persist even for quasi-CUQs, in which the vectors E and Γ are not perfectly orthogonal to each other. I discuss potential applications of our results to quantum information and to unstable meson-antimeson and other systems.

Speaker: Apostolos Pilaftsis (University of Manchester)

PILAFTSIS-CUQs-Warsaw._v2.pdf

6:00 PM → 7:30 PM A visit to the Chopin Museum and a piano concert

Wed, September 24

9:00 AM → 10:30 AM Plenary 9

Convener: Manuel Pérez-Victoria (University of Granada)

9:00 AM Unifying the Higgs Mass Hierarchy, the Cosmological Constant, and the Axion Quality Puzzles

The smallness of the Higgs boson mass, the cosmological constant, and the explicit breaking of the Peccei–Quinn symmetry are usually regarded as independent theoretical challenges. We show that these puzzles become interrelated when Einstein–Cartan gravity is incorporated into the Standard Model in a Weyl-invariant way. The resulting framework introduces only a single additional scalar degree of freedom — beyond the graviton and the Standard Model fields — which behaves as an axion-like particle. The smallness of this particle’s mass induced by gravity, together with that of the Higgs boson mass and the cosmological constant, is linked to tiny values of the local Lorentz gauge couplings.

Speaker: Mikhail Shaposhnikov (EPFL)

Scalars_Warsaw.pdf

9:30 AM SMEFT at the LHC?

It is argued that the SMEFT cannot be used to derive constraints on weakly-coupled heavy new particles from LHC data. Depending on the UV-complete model, the bounds derived using the SMEFT implementation can be considerably stronger or much weaker than the true constraints on the model. This is true even for an "SMEFT-friendly" scenario without 2 --> 1 resonance production. Combinatorial issues arising from the very large number of free parameters of the SMEFT are also briefly discussed.

Speaker: Manuel Drees (Bonn U.)

Drees.pdf

10:00 AM General Form of Effective Operators from Hidden Sectors

I discuss a model-independent analysis of the dimension-six terms that are generated in the low energy effective theory when a hidden sector that communicates with the Standard Model through a specific portal operator is integrated out. I work within the Standard Model Effective Field Theory (SMEFT) framework and consider the Higgs, neutrino, and hypercharge portals. I show that, for each portal, the forms of the leading dimension-six terms in the low-energy effective theory are fixed and independent of the dynamics in the hidden sector. For the Higgs portal, two independent dimension-six terms are generated, one of which has a sign that, under certain conditions, is fixed by the requirement that the dynamics in the hidden sector be causal and unitary. In the case of the neutrino portal, for a single generation of SM fermions and assuming that the hidden sector does not violate lepton number, a unique dimension-six term is generated, which corresponds to a specific linear combination of operators in the Warsaw basis. For the hypercharge portal, a unique dimension-six term is generated, which again corresponds to a specific linear combination of operators in the Warsaw basis. For both the neutrino and hypercharge portals, under certain conditions, the signs of these terms are fixed by the requirement that the hidden sector be causal and unitary. I perform a global fit of these dimension-six terms to electroweak precision observables, Higgs measurements, and diboson production data and determine the current bounds on their coefficients.

Speaker: Zackaria Chacko

effective_operators_Warsaw.pdf

10:30 AM → 11:00 AM Coffee

11:00 AM → 1:00 PM Plenary 10

Convener: Howard Haber (University of California, Santa Cruz)

11:00 AM CP-violation in the Weinberg 3HDM potential

We explore the phenomenology of Weinberg's Z2 times Z2 symmetric three-Higgs-doublet potential, allowing for spontaneous violation of CP due to complex vacuum expectation values. An overview of all possible ways of satisfying the stationary-point conditions is given, with one, two or three non-vanishing vacuum expectation values, together with conditions for CP conservation in terms of basis invariants. All possible ways of satisfying the conditions for CP conservation are given. Scans of allowed parameter regions are given, together with measures of CP violation, in terms of the invariants. The light states identified in an earlier paper are further explored in terms of their CP-violating couplings. Loop-induced CP violation in $WWZ$ couplings, as well as charge-asymmetric scattering are also commented on.

Speaker: Margarida Nesbitt Rebelo (CFTP/IST, U. Lisboa)

mnrScalars25Warsaw24Sept25.pdf

11:30 AM 3HDM based on CP symmetry of order 4: the story so far

CP4 3HDM is a peculiar three-Higgs-doublet model in which a single symmetry, the CP symmetry of order 4, leads to tight constraints on the scalar and Yukawa sectors. The model does not possess the decoupling limit; moreover, tree-level flavor-changing neutral couplings are unavoidable although they can be suppressed. Thus, it remains an open question if the model can pass experimental constraints or runs into immediate conflict with experiment. In this talk, I report on how well the experimental constraints on the scalar, Yukawa, and flavor sectors can be accommodated in this model. Interestingly, out of 8 possibilities, only one CP4-invariant Yukawa sector can pass these constraints in a rather narrow part of the parameter space and leads to very characteristic top-Higgs couplings to be fully probed in future.

Speaker: Prof. Igor Ivanov (Sun Yat-sen University)

Igor-Ivanov-Scalars-2025.pdf

12:00 PM Maximal CP violation from long-range forces

CP violation plays a crucial role in explaining the observed matter–antimatter asymmetry of the Universe. While standard baryo-/leptogenesi scenarios typically predict small CP asymmetries arising from the interference between tree-level and loop-level diagrams, we demonstrate that scatterings of non-relativistic particles mediated by a light force carrier can generate order-1 CP asymmetries and cross-sections that saturate partial-wave unitarity bounds. Notably, the (nearly) maximal values of the cross-sections and CP asymmetries can be reached even for weak coupling strength, at sufficiently low velocities.

Speaker: Anna Socha

Scalars_ASocha.pdf

12:30 PM Di-Higgs and New Physics

I review the sensitivity of the di-Higgs process at the Large Hadron Collider to fundamental theories of the electroweak scale.

Speaker: Stefano Moretti

Moretti-Final.pdf

1:00 PM → 1:05 PM Conference photo

1:05 PM → 2:00 PM Lunch

2:00 PM → 4:00 PM Parallel 1

Convener: Nicolás Bernal (New York University Abu Dhabi)

2:00 PM Indications for New Higgs Bosons in Associated Di-Photon Production

Statistically significant excesses exist at around 152 GeV in associated di-photon production contained in the sidebands of SM Higgs analyses. They are pronounced in several signal regions and can be explained by the Drell-Yan production of a new Higgs boson in association with a charged Higgs boson. In this context, we first examine the excesses in a simplified model approach, considering the decays of a charged Higgs to tau-nu, WZ and top-bottom. We then specialize our analysis for the real Higgs triplet and two-Higgs doublet models, resulting in a combined significance of 4 and 4.3 sigma, respectively.

Speaker: Sumit Banik (Universität Zürich)

Scalars_Conference_2025.pdf

2:15 PM General CP-violating 2HDM in light of the excesses in di-photon searches at the LHC

Recently, statistically significant excesses in inclusive and associated di-photon searches have been observed at the LHC, accumulating at around 95 GeV and 152 GeV, respectively. In this context, I will argue how the most general CP-violating 2HDM in the Yukawa alignment limit can account for these observations. In the Higgs basis, where the two scalar doublets are identified as $H_1$ and $H_2$, the lagrangian term $Z_7 H_1^\dagger H_2 H_2^\dagger H_2 + {\rm h.c.}$ enters the branching ratios to di-photon of the new physics (mostly) CP-even scalar ($H$) and the (mostly) CP-odd scalar ($A$) of the model. While the $\Re[Z_7]$ contributes to $H\to\gamma\gamma$, the $\Im[Z_7]$ affects $A\to\gamma\gamma$ and can be correlated with the observation of nonzero electric dipole moments.

Based on: https://inspirehep.net/literature/2854614.

Speaker: Guglielmo Coloretti (University of Zurich (UZH))

Scalars2025_GC_notsplit.pdf

Scalars2025_GC.pdf

2:30 PM Deciphering the CP nature of the 95 GeV Higgs boson

The excesses observed by CMS in the $\tau\tau$ and $\gamma\gamma$ channels around 95 GeV, together with the $b\bar{b}$ excess reported by the LEP collaboration in the same mass region, have sparked significant interest in the possibility of new physics beyond the Standard Model (BSM). Several BSM frameworks featuring a non-standard Higgs-like state lighter than 125~GeV have been proposed to explain the anomalies observed around 95~GeV. Notable among them are the two Higgs doublet model (2HDM) with specific Yukawa textures, as well as minimal extensions of the 2HDM. As all these frameworks include in their particle spectra a CP-even and a CP-odd scalar, or as well as possible mixtures thereof, determining the CP nature of the 95~GeV Higgs state becomes crucial.

In this talk, I will address the problem of determining the CP nature of the 95~GeV resonance within a simplified model. Where the additional scalar state of mass 95 GeV has Yukawa couplings, scaled by the SM Yukawa coupling, and the interaction Lagrangian is constructed to include both scalar and pseudoscalar components, which can be tuned to a particular CP state through an appropriate mixing angle. Assuming that the 95~GeV anomalies persist in the high-luminosity phase of the LHC, I will demonstrate that the $\tau\tau$ decay mode of the 95~GeV Higgs-like state can serve as a unique probe of its CP nature, distinguishing whether it is a CP-even, CP-odd, or a CP-mixed state.

Speaker: Prasenjit Sanyal (CQUeST, Sogang University)

Prasenjit Talk.pdf

2:45 PM Phenomenological Study of the Nambu–Jona-Lasinio Composite Model at the LHC and HL-LHC

Within a new physics scenario, four-fermion operators of the Nambu–Jona-Lasinio (NJL) type exhibit a strong-coupling ultraviolet (UV) fixed point at the TeV scale [1]. At this scale, a quark and a lepton—two Standard Model (SM) elementary fermions—form a bound state, resulting in a composite bosons, which couples to its constituents via effective contact interactions at the compositeness scale $\Lambda \approx \mathcal{O}(\text{TeV})$. As a first step, we implement this model in the Universal FeynRules Output (UFO) format and focus on the collider phenomenology of scalars at the LHC, including its High-Luminosity upgrade.

Initial studies have explored the phenomenology of leptoquarks (LQs), which can be produced at hadron colliders through various mechanisms, including quark or gluon fusion, t-channel exchange, and single production processes. In our detailed phenomenological analysis [2], we compute LQ production cross sections for both proton-proton ($pp$) and photon-proton ($\gamma p$) collisions, as relevant to LHC experiments. For $\gamma p$ scenarios, we consider both elastic and inelastic photon emission. Additionally, we investigate lepton-induced processes by exploiting the possibility of lepton emission from protons. A targeted study of LQs coupling to a $\mu$–$c$ quark pair demonstrates improved sensitivity relative to existing search strategies. Based on this enhanced discovery potential, we recommend incorporating dedicated LQ search channels in future LHC analyses.

Ongoing research is focused on quark-antiquark interactions at the LHC. Specifically, we are exploring the production mechanisms of composite scalar particles that decay into diphoton or diquark final states. A central goal is to extract constraints on these final states using existing experimental data.

Speaker: Sehar Ajmal (INFN, Perugia (IT))

Compositemodel_Sehar_Scalars2025.pdf

3:00 PM Probing Co-Scattering of scalar dark matter in an EFT framework with monophoton searches

We investigate the co-scattering mechanism for dark matter production in an EFT framework which contains new $Z_2$-odd singlets, namely two fermions $N_{1,2}$ and a real scalar $\chi$. The singlet scalar $\chi$ is the dark matter candidate. The dimension-5 operators play a vital role to set the observed DM relic density.We focus on a nearly degenerate mass spectrum for the $Z_2$ odd particles to allow for a significant contribution from the co-scattering or co-annihilation mechanisms. We present two benchmark points where either of the two mechanisms primarily set the DM relic abundance. The main constraint on the model at the LHC arise from the ATLAS mono-$\gamma$ search. We obtain the parameter space allowed by the observed relic density and the mono-$\gamma$ search after performing a scan over the key parameters. We find the region of parameter space where the relic abundance is set primarily by the co-scattering mechanism while being allowed by the LHC search. We also determine how the model can be further probed at the HL-LHC via the mono-$\gamma$ signature.

Speaker: Dr Abhishek Roy (Center for Quantum Spacetime (CQUeST), Sogang University)

Scalars2025_Abhishek_Roy.pdf

3:15 PM Searches for light exotic scalars at the e+e- Higgs factory.

The physics program of the future electron-positron Higgs factory will focus on the measurement of the 125 GeV Higgs boson, with the Higgs-strahlung process being the dominant production channel at 240-250 GeV. However, similar production of light exotic scalars, in a scalar-strahlug process, is still not excluded by the existing experimental data, provided their coupling to the SM gauge bosons is sufficiently small. This was selected as one of the focus topics of the ECFA Higgs/Top/EW factory study. Resulting from my analysis are the expected scalar production cross section limits from the search in the $b\bar{b}$ decay channel, based on a full simulation of the International Large Detector (ILD), assuming 250 GeV ILC running scenario. I will also include results on the expected sensitivity in di-tau and invisible decay channels, as well as limits expected in the decay independent approach.

Speaker: Bartłomiej Brudnowski (University of Warsaw)

scalars_2025_bb_final.pdf

3:30 PM Enhancing prospects of sub-GeV majoron at intensity frontier experiments through flavor-changing processes

Majoron ($J$) is a feebly-interacting axion-like particle that can realize type-I seesaw mechanism, which also leads to flavor violating processes (FVP). We revisit prospects of sub-GeV majoron at intensity frontier experiments. We show that FVP allow to probe the majoron mass region $\sim 100$ MeV to $1$ GeV, complementary to astrophysical bounds and dedicated experiments probing FVP. We also study muon-beam experiments, where for the majoron lighter than muon, the $\mu \to e + J$ decays can put limits competitive with those set by supernovae.

Speaker: Krzysztof Jodlowski (IBS CTPU-PTC)

Jodlowski_Scalars25.pdf

3:45 PM Clockwork Axions at Hadron Colliders

Multi-axion scenarios have interesting applications in cosmology, especially in the context of inflation. An interesting manifestation of such scenarios arises within the framework of the clockwork mechanism wherein the light axion is localized towards one end of a theory space lattice composed of $N$ scalar fields through a specific form of near-neighbour interactions. This effectively generates a large decay constant for the axion without the need for invoking a large Peccei-Quinn (PQ) symmetry breaking scale. Therefore, even if the light axion is nearly invisible at a high energy collider setting, the associated heavy axion-like particles (ALPs) in such scenarios may be observable by virtue of their decay constants being characterized by a low-scale PQ symmetry breaking. In this talk, I will discuss the possibility of certain interesting as well as unique signatures pertaining to the resonant production of the clockwork ALP spectrum within a QCD axion setup at hadron colliders, particularly in the diphoton channel at the LHC and its forthcoming upgrades. Notably, for ALP masses below the electroweak scale, there exist scenarios where the ALP mass-splittings are small, in which case the collection of resonances could appear as a band of closely-spaced peaks in the invariant mass distribution, or in some cases even smaller such that the resonances effectively appear as a single broad resonance. In addition to these features, a realization of the clockwork axion on a two-dimensional lattice further engenders the possibility of having long-lived ALPs in the spectrum which can be within the sensitivity reach of some of the upcoming displaced vertex detectors such as MATHUSLA.

Speaker: Suvam Maharana (Tata Institute of Fundamental Research, Mumbai)

Warsaw_Suvam_Talk.pdf

2:00 PM → 4:00 PM Parallel 2

Convener: Andrzej Hryczuk (National Centre for Nuclear Research)

2:00 PM On the CP Properties of spin-0 dark matter

Aiming to uncover the CP properties of spin-0 particle Dark Matter (DM), we explore a two-component DM scenario within the framework of 3-Higgs Doublet Models (3HDMs), a well-motivated set-up previously studied due to the complementarity of its collider and astrophysical probes. We devise benchmark points in which the two components of DM have same CP in one case and opposite CP in another. We then show several cross section distributions of observables at collider experiments where the two cases are clearly distinguishable.

Speaker: Atri Dey (Uppsala University)

Scalar2025_Atri.pdf

2:15 PM Electroweak baryogenesis from charged current anomalies in $B$ meson decays

We demonstrate for the first time that new physics explaining the long standing charged $B$ meson anomalies, $R(D^{(*)})$, can be the source of CP violation that explains the observed baryon asymmetry of the universe (BAU). We consider the general two Higgs doublet model with complex Yukawa couplings and compute the BAU in the semiclassical formalism, using a novel analytic approximation for the latter. After imposing constraints from both flavor observables and the electron electric dipole moment (eEDM), we find that a significant BAU can still be generated for a variety of benchmark points in the parameter space, assuming the occurrence of a sufficiently strong first order electroweak phase transition. These scenarios, which explain both the $R(D^{(*)})$ flavor anomalies and the BAU, can be probed with future eEDM experiments and Higgs factories measurements.

Speaker: Dr Cristian Felipe Sierra Fonseca (Shanghai Jiao-Tong University)

Scalars2025_Cristian_v2.pdf

2:30 PM Bubble wall dynamics and the electroweak phase transition

The early Universe provides a unique environment to explore fundamental physics, offering extreme conditions that allow theoretical models to be tested at energy scales far beyond the reach of current and future accelerators. Among the various cosmological phenomena, first-order phase transitions play a prominent role as they may have left a variety of experimentally accessible signatures. A first order phase transition proceeds through a process of nucleation, growth and merger of bubbles of true vacuum in a false vacuum background. The dynamics of this process is governed by the density perturbations generated by the propagation of the bubble wall. A precise determination of this dynamics, and in particular of the wall expansion velocity and width, is crucial to assess the experimental signatures of the transition. In this talk, I will report on recent advances in the quantitative theoretical description of bubble dynamics. Adopting typical benchmark models, I will present numerical results for the bubble wall velocity and the plasma and fields profiles that describe the phase transition dynamics, as well as results for some of the cosmological relics of the transitions, namely the gravitational wave spectrum and baryogenesis.

Speaker: Carlo Branchina (Università della Calabria)

BranchinaC_scalars.pdf

2:45 PM Finite-temperature bubble-nucleation with shifting scale hierarchies

Focusing on supercooled PTs in models with classical scale symmetry, we investigate the limitations of derivative expansions in constructing a thermal EFT description for bubble nucleation. We show that derivative expansion for gauge field fluctuations diverges because the gauge field mass varies strongly between the high- and low-temperature phases. By computing the gauge fluctuation determinant, we show that these effects can be captured while accounting for large explicit logarithms at two loops. We show how this construction can improve nucleation rate calculations, providing a more robust framework for describing GW from supercooled PT in models like the SU(2)cSM.

Speaker: Maciej Kierkla (University of Warsaw)

mkierkla_scalars25_wwa.pdf

3:00 PM Electroweak phase transition in a non-abelian vector dark matter scenario

We study a scenario where the Standard Model is extended by a SU(2) gauge group in the dark sector. The three associated dark gauge bosons are stabilised via a custodial symmetry triggered by an addition dark SU(2) scalar doublet, thus making them viable dark-matter candidates. After considering the most recent constraints for this model, we analyse the phase transition dynamics and compute the power spectrum of resulting stochastic gravitational-wave background. Finally, we find regions of the parameter space yielding the observed dark-matter relic density while also leading to strong enough phase transition with an associated gravitational-wave signal reaching the sensitivity of future space-based gravitational-wave detector, such as LISA, DECIGO, BBO, TianQin or Taiji.

Speaker: Maimoona Razzaq (University of Calabria and INFN Cosenza, Italy)

Scalars_2025_maimoona.pdf

3:15 PM Equation of State of the Universe after a First Order Phase Transition

Cosmological first-order phase transitions are caused by a scalar field that tunnels to a new vacuum state, triggering the nucleation and expansion of bubbles. In scenarios where the scalar field interacts only weakly with the surrounding plasma, the assumption of instantaneous reheating after the transition breaks down. As a result, the evolution of the universe may become dominated by oscillations of the scalar field. Such scenarios are often presumed to result in a phase of matter domination. In this talk, I examine this assumption using results from lattice simulations that track the scalar field’s energy distribution over time. By analyzing the system’s equation of state after the transition, I will show that it depends on the mean bubble separation, with large separations leading to sizable deviations from matter domination. These insights carry significant implications for the universe's later evolution and potentially the production of dark matter.

Speaker: Henda Mansour (Karlsruhe Institute of Technology)

Scalars2025_HendaMansour.pdf

Slides

3:30 PM Baryogenesis in the 2HDM+a

There are a multitude of Standard Model (SM) extensions that accommodate an electroweak first-order transition (EWFOPT) in the Early Universe, with the aim of providing an explanation for the observed baryon asymmetry of the Universe (BAU). Using a well-known SM extension featuring two Higgs doublets and a SM-singlet pseudoscalar (2HDM+a) that evades electric dipole moment (EDM) constraints, we investigate regions of parameter space that may potentially produce the observed BAU and their resulting interplay with theoretical and experimental collider constraints. More specifically, the BAU is realised via transitional CP-violation during a strong first-order phase transition, and, we provide upper and lower bounds on the BAU across parameter space by bounding the wall velocity of the bubble walls that occur during the transition. The 2HDM+a can readily accommodate the observed BAU, however, our results highlight why common assumptions and estimates used to calculate the BAU in the literature can become inaccurate and often overestimate its magnitude.

Speaker: Tom Gent (University of Sussex)

Scalars Presentation.pdf

3:45 PM Gravothermalizing into Primordial Black Holes

Very little is known about the universe’s history from after the end of inflation until the Big Bang nucleosynthesis, which spans more than $10^{39}$ orders of magnitude in time scales. In this work, we show that if during this unknown period there was a long period of matter domination by a massive scalar field, and if the particle causing the matter domination has moderate self-interactions, the matter particles can undergo gravothermal collapse to form exotic states as primordial black holes (PBHs), boson stars, and cannibal stars. We found that for some choice of parameters, our model can predict an amount of PBHs surviving until today comparable to dark matter. For an optimistic estimate of PBH abundance, we also find that PBHs with masses less than $10^9$ g can reheat the universe before BBN. From the bounds on the PBH abundance, we also constrain the models of massive scalar fields in a large range of parameters.

Speaker: Daniele Perri (Warsaw University)

SCALARS_2025.pdf

2:00 PM → 4:00 PM Parallel 3

Convener: Alejandro Ibarra (Technical University of Munich)

2:00 PM Is our vacuum global in economical 331 models?

The so called 331-models are SM extensions based on $SU(3)_c\times SU(3)_L\times U(1)_X$-gauge group. The scalar potential of 331-models is typically quite complicated, due to presence of three scalar triplets.
Although the 331 models have been extensively studied, the existence of multiple minima and metastability of the 331 scalar potential has not been studied in great detail in the literature, previous studies concentrating on overly simplified scalar potentials that are not used in phenomenology. We study the typical scalar potential of 331-model with three scalar triplets, which is often used in the phenomenological studies. We classify the potential minima and determine the conditions under which the electroweak vacuum is global with the help of orbit space methods. For the case the electroweak vacuum is not global, we calculate bounds on the scalar couplings from metastability. This talk is based on work which will be published soon.

Speaker: Niko Koivunen (NICPB)

Scalars_2025_Koivunen.pdf

2:15 PM Light states in real multi-Higgs models with spontaneous CP violation

In models with multiple Higgs doublets that trigger spontaneous electroweak symmetry breaking, one expects that the abundance of dimensionful quadratic couplings in the scalar potential allows for a regime where, apart from the would-be Goldstone bosons and a neutral Higgs-like state, all new scalars can have masses much larger than the electroweak scale. For models where CP invariance holds at the lagrangian level but is broken by the vacuum, it can be shown that such a reasonable expectation does not hold. Through perturbativity requirements on the dimensionless quartic couplings, the spectrum of the new scalars includes one charged and two additional neutral states whose masses cannot be much larger than the electroweak scale.

Speaker: Miguel Nebot Gómez (U. Valencia - IFIC)

Nebot_LightStates-RealnHDM_Scalars2025.pdf

2:30 PM Pseudo NG bosons from finite modular symmetry

Pseudo Nambu-Goldstone (pNG) bosons can play important roles in particle physics, such as being a light dark matter (DM), the QCD axion to solve the strong CP problem, and so on. I point out that such a pNG boson is naturally realized by the finite modular symmetry, which may originate from the geometry of extra dimensions in the superstring models. An accidental global U(1) symmetry arises due to the residual $Z_N$ symmetry, when the modulus is stabilized near a fixed point of the finite modular symmetry. To illustrate, I will show the realization of the KSVZ axion model to solve the strong CP problem, where the modulus is stabilized by the radiative potential generated by the vector-like quarks, based on arXiv:2402.02071 (JHEP) and 2405.03996 (JHEP). Since the finite modular symmetries were originally used to explain the flavor structure, this observation suggests that there are non-trivial connections between the pNG mode, which may be the DM, and flavor physics. If time permits, I will discuss the existence of such pNG mode in other stabilization mechanisms and possible applications to particle physics based on 2409.19261 (JHEP) and 2412.18435 (JHEP).

Speaker: Junichiro Kawamura (Waseda University)

slide_modular_talk.pdf

2:45 PM Lie symmetry analysis of the field equations in multi-Higgs models

We show how to apply Lie symmetry analysis of partial differential equations (PDEs) to the Euler-Lagrange equations of multi-Higgs models, to determine their scalar Lie point symmetries. A Lie point symmetry is a structure-preserving transformation of the spacetime variables and the fields of the model, which is also continuous and connected to the identity. The Lie point symmetries can be divided into strict variational symmetries, divergence symmetries and non-variational Euler-Lagrange symmetries, where the first two are collectively referred to as variational symmetries. Variational symmetries lead to conserved currents and are usually lifted to the quantized theory. We demonstrate that there are no scalar divergence or non-variational symmetries in the two-Higgs-doublet model (2HDM), and re-derive its well-known strict variational symmetries, thus confirming the consistency of our implementation. Moreover, we consider the scalar Lie point symmetries of the standard model (SM) augmented with one or two real, scalar gauge singlets. Lie symmetry analysis of PDEs is a broadly applicable method for finding Lie symmetries, while missing discrete symmetries can be identified through the automorphism groups of the Lie symmetry algebras thus obtained.

Speaker: Marius Solberg (Norwegian University of Science and Technology)

Lie symmetry analysis of the field equations in multi-Higgs models.pdf

3:00 PM Constrained instantons in scalar field theories

In QFT, a metastable vacuum state can decay through quantum tunnelling. The calculation of the decay rate relies on instantons — a kind of non-trivial classical solution in the Euclideanized theory. However, sometimes theories with metastable vacua do not have any instanton solutions, thus rendering the usual method of calculating decay rates unusable. An important example of such a theory is the Electroweak theory, where vacuum decay is directly related to baryon number violation, but where there are no instanton solutions to mediate the decay.

In this talk, I will discuss a method for computing the decay rate in such theories using constrained instantons. It is based on a perturbative approach by Affleck from the 1980s, which we have generalised and made fully non-perturbative. I will begin by outlining the method in general, and I will then apply it to a simple toy model - a single, real, massive scalar field in 4 dimensions.

Speaker: Kinga Gawrych (Imperial College London)

K Gawrych - Scalars 2025 24.09.2025.pdf

3:15 PM The Geometry of Interactions: Curved Field Space of Scalars at One Loop

We present a systematic study of one-loop quantum corrections in scalar effective field theories from a geometric viewpoint, emphasizing the role of field-space curvature and its renormalisation. By treating the scalar fields as coordinates on a Riemannian manifold, we exploit field redefinition invariance to maintain manifest coordinate independence of physical observables. Focusing on the non-linear sigma model (NLSM) and (\phi^4) theory, we demonstrate how loop corrections induce momentum- and scale-dependent shifts in the curvature of the field-space manifold. These corrections can be elegantly captured through the recently proposed geometry-kinematics duality, which generalizes the colour-kinematics duality in gauge theories to curved field-space backgrounds. Our results highlight a universal structure emerging in the contractions of Riemann tensors that contribute to renormalisation of the field-space curvature. In particular, we find explicit expressions and a universal structure for the running curvature and Ricci scalar in simple models, illustrating how quantum effects reshape the underlying geometry. This geometric formulation unifies a broad class of scalar EFTs, providing insight into the interplay of curvature, scattering amplitudes, and renormalisation.

Speaker: Mr Dominik Haslehner (Max Planck Institute for Physics & Technical University of Munich)

Haslehner - Scalars2025.pdf

3:30 PM Neutrino masses and mixed dark matter from doublet and singlet scalars

We consider the extension of the Standard Model with an inert scalar doublet, three right-handed neutrinos, and singlet scalar fields, $\varphi$ and $S$. In this model, neutrino masses are zero in the limit of the unbroken $Z_4$ discrete symmetry. We show that when the singlet scalar field $\varphi$ gets a VEV, the $Z_4$ symmetry is broken to $Z_2$, and neutrino masses are generated at one-loops due to the mixings between the neutral components of the inert scalar doublet and the singlet scalar field $S$. There is a dark matter candidate from the lightest neutral scalar field, which is a mixture of the inert scalar doublet and the singlet scalar field $S$, in general. The $Z_4$ breaking mass terms are constrained by electroweak precision data and direct detection (DD) bounds for dark matter, favoring small mixings or almost degenerate masses for the DM scalars. As a result, we discuss the implications of the results for small neutrino masses and DD-safe dark matter.

Speaker: Rojalin Padhan (Chung-Ang University, Seoul)

scalars25_Rojalin.pdf

3:45 PM A Horizontal Three-Higgs-Doublet-Model

I'll present a variant of the Three-Higgs-Doublet Model (3HDM), referred to as the BGL-3HDM, which incorporates a $U(1)_1\times U(1)_2$ symmetry that leads to a 'horizontal' Yukawa structure, where a particular doublet is the primary source of mass for a particular fermion generation. In the version considered here, the Yukawa matrices in the down-quark and charged-lepton sectors are diagonal, thereby confining tree-level FCNCs interactions mediated by nonstandard scalars to the up-quark sector only. Additionally, apart from the free BSM parameters that appear in Natural-Flavor-Conservation (NFC) versions of 3HDMs with a $U(1)_1\times U(1)_2$ symmetry, no new BSM parameters are introduced by the Yukawa sector of the model, making it as economical as the NFC versions of 3HDM. However, even in the down-quark and in the charged-lepton sectors, flavor-diagonal but nonuniversal Higgs couplings set this model apart from the NFC versions of the 3HDMs.

Speaker: Anugrah Prasad (Indian Institute of Technology Indore)

Anugrah_presentation.pdf

4:00 PM → 4:30 PM Coffee

4:30 PM → 5:45 PM Parallel 4

Convener: Shinya Kanemura (The University of Osaka)

4:30 PM LHC Signatures of a Vector-Like Top Partner and Charged Higgs in the 2HDM-II

I will present the collider phenomenology of a vector-like top partner (VLT) in the Type-II Two-Higgs-Doublet Model (2HDM-II) extended by a vector-like quark doublet, with emphasis on final states involving a charged Higgs boson. The analysis is performed in the alignment limit, where the light CP-even Higgs boson exhibits Standard Model-like properties. In this framework, the VLT predominantly decays via the exotic channel $T \to H^+ b$, yielding signatures with high $b$-jet multiplicity and leptonic components. Results are based on Monte Carlo simulations of VLT pair production at the 14 TeV LHC, covering both fully hadronic and semi-leptonic final states. The projected discovery sensitivity reaches up to $m_T \sim 1.6$ TeV, depending on the charged Higgs mass, integrated luminosity, and systematic uncertainties.

Speaker: Dr Mohammed Boukidi (IFJ PAN)

Boukidi_scalars2025.pdf

4:45 PM Vector-like T Bounds at the LHC: Impact of 2HDM-II

In our study, we explored the impact of incorporating vector-like T (VLT) quarks into Two-Higgs-Doublet Models (THDMs), which introduces new decay modes beyond the Standard Model (SM) channels ($T \to Zt$, $T \to ht$, $T \to Wb$). These new beyond Standard Model (BSM) decay channels include the CP-even $T \to Ht$, CP-odd $T \to At$, and charged $T \to H^+b$, which have not been experimentally investigated at the LHC, as existing vector-like quark (VLQ) searches focus solely on SM channels. Our analysis focused on how these BSM decay modes affect the stringent mass limits of VLT quarks. For THDM type II (THDM-II) with a singlet $T$, the mass bound is relaxed from approximately 1.43 TeV to 1.27 TeV. For THDM-II with a doublet $(T, B)$, the mass limit is significantly reduced from about 1.54 TeV to 0.99 TeV, driven primarily by the dominance of the $T \to H^+b$ decay, which can achieve a branching ratio of nearly 98\%.

Speaker: Mohamed Ech-chaouy (Polydisciplinary Faculty, Laboratory of Fundamental and Applied Physics, Cadi Ayyad University, Sidi Bouzid, B.P. 4162, Safi, Morocco)

main.pdf

5:00 PM Vector-like Quark $B$ Bounds at the LHC: Impact of THDM-II

We examined the integration of vector-like bottom (VLB) quarks into the Two-Higgs-Doublet Model (THDM), enabling new decay channels distinct from the Standard Model (SM) processes ($B \to Zb$, $B \to hb$, $B \to Wt$). These Beyond Standard Model (BSM) decays, including CP-even $B \to Hb$, CP-odd $B \to Ab$, and charged $B \to H^-t$ modes, remain unprobed at the LHC, where VLB searches currently target only SM channels. Our analysis assessed the impact of these BSM decays on the stringent mass constraints for VLB quarks in THDM type II (THDM-II) under the alignment limit. For a singlet $B$, the mass limit drops from approximately 1.43 TeV to 1.00 TeV, while for a doublet $(T, B)$, it decreases from about 1.55 TeV to 0.98 TeV, driven primarily by the $T \to H^-b$ decay, which can achieve a branching ratio of nearly 100%.

Speaker: Ms khawla salime (Polydisciplinary Faculty, Laboratory of Physics, Energy, Environment, and Applications, Cadi Ayyad University, Sidi Bouzid, B.P. 4162, Safi, Morocco)

Vector-like Quark B Bounds at the LHC_ Impact of 2HDM.pdf

Vector-like Quark B Bounds at the LHC_ Impact of 2HDM.pdf

5:15 PM Probing the general 2HDM with flavor violation through $A \to ZH$

We investigate the LHC discovery prospects for a second Higgs doublet through $A \to ZH$ weak decay. The latter is identified as the smoking gun signature of two Higgs doublet models (2HDMs) with first-order electroweak (EW) phase transition, a necessary condition for EW baryogenesis. In the general 2HDM (G2HDM) that has flavor-changing neutral Higgs couplings, $H$ may decay dominantly via $t\bar c + \bar tc$ final states, giving rise to trilepton signals. By a phenomenological analysis, we show that $A \to ZH$ in $\ell^+ \ell^-t\bar c$ or $\ell^+ \ell^-\bar t c$ final states could be a promising probe of G2HDM at the LHC with flavor violation.

Speaker: Mohamed Krab (National Taiwan University)

Krab_scalars25.pdf

5:30 PM Probing neutrinophillic scalars with high-energy muon beams

High-energy muon beams can generate intense and energetic muon neutrino fluxes through muon decays. The energy and intensity of the beam, as well as its well-known energy spectrum, provide a unique opportunity to study neutrino properties and interactions, potentially uncovering new physics beyond the Standard Model.
In this talk, I will discuss the prospects for detecting new scalar mediators that couple predominantly to neutrinos and have masses ranging from a few MeV to tens of GeV. These neutrinophilic scalars, which may mediate interactions between neutrinos and a hidden sector, are compelling candidates in the context of neutrino portal dark matter. A characteristic experimental signature involves neutrino charged current scattering events accompanied by positively charged muons and apparent lepton number violation.

Speaker: Jyotismita Adhikary (NCBJ,Poland)

MuCol-3.pdf

4:30 PM → 6:15 PM Parallel 5

Convener: Pyungwon Ko (KIAS)

4:30 PM Bubble Wall Velocity from Hydrodynamics

Cosmological scalar phase transitions are ubiquitous in particle physics models. If they are first-order, they can be tested with gravitational-wave signal and baryon asymmetry they produce. These however, crucially depend on the velocity that nucleated new-phase bubbles reached. In this talk I will present generalized description which builds a bridge between numerical simulations, often used within the community, and the analytical estimates of this parameter. Our method allows one to determine the wall velocity without the necessity of performing full real-time simulations. Moreover, I will explain why some stationary solutions discussed in the literature are not dynamically realized, and provide a selection rule determining their fate.

Speaker: Ignacy Nałęcz (Faculty of Physics, University of Warsaw)

Scalars2025_Nalecz.pdf

4:45 PM Nano-Hertz gravitational waves and sub-GeV dark matter from a classically conformal phase transition

Strong first-order phase transitions offer a compelling explanation for the stochastic gravitational wave background in the nano-Hertz range measured by pulsar timing arrays (PTA). In this talk, I will consider a dark Higgs mechanism in a classically conformal dark sector where the symmetry breaking of a dark $U(1)^{\prime}$ gives rise to a gravitational wave background that can fit the PTA data. The vacuum expectation value of the dark Higgs field is found to lie in the MeV-GeV range and sources the mass of a stable fermionic sub-GeV dark matter candidate. The dark sector is coupled to the Standard Model via kinetic mixing of the $U(1)^{\prime}$ gauge boson with hypercharge, which is tightly constrained by laboratory searches. I will discuss these accelerator constraints as well as cosmological constraints on the decay of the dark Higgs boson after the phase transition. Finally I will present the results of a global fit and show that the model has viable parameter space where it fits the PTA data, reproduces the observed relic abundance and satisfies all relevant constraints.

Speaker: Jonas Matuszak (Karlsruher Institute of Technology (KIT))

Scalars2025_JMatuszak.pdf

5:00 PM RGE effects on new physics searches via gravitational waves

In models beyond the Standard Model, a first-order phase transition could be realized. Gravitational waves produced in such a phase transition may provide a window into new physics through their observable signatures. However, it is known that there are significant theoretical uncertainties in the spectrum of gravitational waves, such as those arising from RGE effects.
In this talk, we consider effective field theories, including the SMEFT, and evaluate the impact of new physics on the gravitational wave spectrum through RGE effects, as well as the theoretical uncertainties.

Speaker: Katsuya Hashino (National Institute of Technology, Fukushima College)

Scalars2025_KH.pdf

5:15 PM Anti-de Sitter Wormholes as seeds for Higgs Inflation

The current experimental data suggest that the Standard Model Higgs potential is metastable, with a second, deeper AdS minimum emerging at high scales. Assuming that the Higgs boson initially resides in this AdS minimum, this talk will explore how the presence of axionic fields or magnetic radiation can catalyze the onset of inflation through asymptotically AdS Euclidean wormholes. The initial conditions for inflation are naturally set in place.

Speaker: Ioannis Gialamas (Laboratory of High Energy and Computational Physics, NICPB, Tallinn)

scalars_2025_Gialamas.pdf

5:30 PM Geometrical destabilization during inflation

In multi-field inflationary models couplings between fields are not limited to a potential of the model, but can also be present in kinetic terms. In such a case, they can be interpreted as a non-trivial structure of the space of fields. Non-vanishing curvature of this space can lead, if negative, to a new phenomenon called geometrical destabilization.

For example, $\alpha$-attractors are inflationary models in which geometrical destabilization takes place after inflation, during preheating. It causes intensive production of spacial fluctuations of fields which efficiently siphon energy from homogeneous inflaton mode, leading to nearly instantaneous reheating.

The geometrical destabilization not necessarily happens after inflation. It can also take place during inflationary epoch. One may suppose that it poses a~threat to a successful inflation, as it may lead to its premature end. However, our studies have shown that this is not the case. The instability is shut down by the so-called kinematic backreaction and the inflation proceeds further as a ‘side-tracked' inflation.

Speaker: Tomasz Krajewski (Institute of Fundamental Technological Research PAS)

Geometrical_destabilization_during_inflation.pdf

5:45 PM Freezing-in Cannibal Dark Matter during early matter domination

If Dark Matter is produced via the freeze-in mechanism during a period of early matter domination, the required portal coupling is enhanced to compensate for the faster expansion of the Universe—especially when the reheating temperature is low. This can lead to experimentally testable scenarios. Moreover, any theory involving scalar Dark Matter naturally includes self-interactions, which induce number-changing processes. These reactions can drive the Dark Matter toward a vanishing chemical potential, even in the absence of thermal equilibrium with the Standard Model. In this talk, I will discuss Dark Matter production under these conditions within a specific model of cannibalistic, self-interacting scalar Dark Matter. I will emphasize how the self-interactions influence both the dynamics of production and the prospects for detection.

Speaker: Esau Cervantes (NCBJ (Warsaw))

FI_CannibalDM.pdf

6:00 PM Neutron star heating by dark matter in an ALP mediated lepton-flavor-violating model

We investigate the capture of fermionic dark matter by neutron stars in scenarios where the dark matter interacts with leptons via a pseudoscalar mediator that violates lepton flavor in the $\mu$–$e$ sector. We demonstrate that such interactions can lead to significant heating of the star, potentially raising its surface temperature to $\sim!2000,\mathrm{K}$. This level of heating may be detectable in the near future with upcoming infrared telescopes, including the James Webb Space Telescope (JWST), the Thirty Meter Telescope (TMT), and the European Extremely Large Telescope (E-ELT). Our analysis accounts for both kinetic energy deposition from dark matter capture and heating from annihilation in the stellar core. We also address the subtleties involved in inelastic capture processes, with particular attention to the implications of lepton-flavor-violating interactions.

Speaker: Jaime Hoefken Zink (Narodowe Centrum Badań Jądrowych (NCBJ))

SCALARS_NS_heating_JHZ_final.pdf

4:30 PM → 6:15 PM Parallel 6

Convener: Antonio Pich (IFIC, CSIC-UV)

4:30 PM Stabilizing dark matter with quantum scale symmetry

In the context of gauge-Yukawa theories with trans-Planckian asymptotic safety, quantum scale symmetry can prevent the appearance in the Lagrangian of couplings that would otherwise be allowed by the gauge symmetry. Such couplings correspond to irrelevant Gaussian fixed points of the renormalization group flow. Their absence in the theory implies that different sectors of the gauge-Yukawa theory are secluded from one another, in similar fashion to the effects of a global or a discrete symmetry. As an example, we impose the trans-Planckian scale symmetry on a model of Grand Unification based on the gauge group SU(6), showing that it leads to the emergence of several fermionic WIMP dark matter candidates whose coupling strengths are entirely predicted by the UV completion, while the scalar potential at low energy is similar to a 2HDM+2S model.

Speaker: Rafael Robson Lino dos Santos (NCBJ, Warsaw)

Talk_Scalars_2025.pdf

4:45 PM Unifying Seesaw and Radiative Neutrino Mass Mechanisms with A₄ Symmetry: A Pathway to Dark Matter Stability

To address the smallness of neutrino masses and the observed large neutrino mixing, we propose a hybrid framework that synergistically combines the canonical seesaw mechanism with radiative mass generation. This model is embedded in the A₄ non-Abelian discrete flavor symmetry, whose spontaneous breaking generates correct neutrino mixing patterns and stabilizes dark matter through a conserved residual symmetry. We investigate the phenomenological consequences of this "discrete dark matter" paradigm, analyzing both Dirac and Majorana neutrino mass scenarios. The interplay between the seesaw and radiative mechanisms is shown to yield testable predictions for neutrinos (e.g., mixing angles, mass-squared differences) and dark matter (relic density, direct detection signatures). This work bridges high-scale symmetry-based models with low-energy observables, offering a unified approach to neutrino mass and dark matter stability.

Speaker: Biswajit Karmakar (University of Silesia in Katowice)

Karmakar.pdf

5:00 PM Electroweak baryogenesis in the three-loop radiative seesaw model with dark matter

The Standard Model successfully describes particle physics but cannot explain neutrino oscillations, the baryon asymmetry of the universe (BAU), and dark matter. The Aoki-Kanemura-Seto (AKS) model is a new physics model that can explain these three phenomena simultaneously at TeV scale testable by future experiments. However, in the original model published in 2009, the baryon number was not evaluated. In the present work [Enomoto, Kanemura, Taniguchi, JHEP06(2025)036 and Aoki, Enomoto, Kanemura, Taniguchi in preparation], we introduced CP violation to the original AKS model for evaluating the baryon number and found a benchmark scenario to avoid the current constraint on the electron electric dipole moment while keeping a large enough CP-violating phase for electroweak baryogenesis. In this talk, we evaluate the baryon number in this model, using the previously found viable parameter regions. Furthermore, we present benchmark points that can simultaneously explain neutrino masses, dark matter, and BAU, under various experimental and theoretical constraints. We also discuss prospects for testing the model at future experiments. The new particles predicted by the AKS model, including the charged singlet scalar $S^\pm$ and the additional Higgs bosons $H_2$ and $H_3$, could be produced at future collider experiments such as the High-Luminosity LHC and electron-positron Higgs factories, and are expected to be tested there. In addition, since electroweak baryogenesis requires a strong first-order electroweak phase transition, which can generate characteristic gravitational wave signals, we also explore the potential for detection of such signals by future space-based observatories such as DECIGO.

Speaker: Sora Taniguchi (The University of Osaka)

Scalars2025_Taniguchi.pdf

5:15 PM Is Standard Model effective field theory enough for Higgs pair production?

We study Higgs pair production in lowest order (LO) Higgs effective Field Theory (HEFT) and Standard Model Effective Field Theory (SMEFT) at dimension 6. Inspired by the ``Loryon'' models, we study which effective field theory one should concretely use and whether one might be able to probe models that realise HEFT rather than SMEFT. We find that in certain region of the parameter space the appropriate EFT to be used is HEFT while SMEFT is non predictive for this class of UV models.

Speaker: Lorenzo Tiberi (University of Perugia & INFN)

Tiberi_scalars.pdf

5:30 PM Towards automatizing Higgs decays in BSM models at one-loop in the decoupling renormalization scheme

High-precision calculations of Higgs boson observables can be used to constrain models of Beyond the Standard Model (BSM) physics. Motivated by the non-observation of light BSM particles at the LHC, I will discuss a renormalization scheme that enables precise predictions of Higgs boson decays in the presence of moderately heavy BSM physics at the one-loop level. I will outline the basics of the decoupling renormalization scheme and present the renormalization conditions for a generic model. Furthermore, I will demonstrate its application to a specific model to explore its effects. This calculation is implemented in the FlexibleSUSY spectrum-generator generator and will be automatically applied to any user-defined BSM model in the future.

Speaker: Jonas Lang (National Center for Nuclear Research Poland)

scalars25_JL.pdf

5:45 PM Application of the Decoupling Scheme to the Decay h -> ll in the 2HDM within $\texttt{FlexibleDecay}$

$\texttt{FlexibleDecay}$ is an extension of the spectrum-generator generator $\texttt{FlexibleSUSY}$ which allows to calculate Higgs boson decay rates and branching ratios for their respective decay channels in arbitrary BSM models. For that purpose, the so-called Decoupling Renormalization Scheme is employed. It is constructed such that non-decoupling effects in the case of heavy BSM-physics are circumvented, while advantageous features of the $\overline{\mathrm{MS}}/\overline{\mathrm{DR}}$ scheme in the context of spectrum generators are kept. The publicly available version of $\texttt{FlexibleDecay}$ currently implements a LO calculation amended by some important NLO corrections. It is to be extended to a complete NLO calculation for the leptonic decay channel $h \longrightarrow \ell^+ \ell^-$. This requires a reconsideration of details of the Decoupling Scheme in order to ensure its desired properties.

While the first of the two complementary talks on $\texttt{FlexibleDecay}$ focusses on the philosophy behind the construction of the decoupling scheme in the context of a complete NLO calculation as well as the application to simple extensions beyond the SM, this talk is dedicated to present some subtleties in more complex models. As a concrete example, the type-II Two-Higgs-Doublet Model will be discussed. After recapitulating the decoupling limit in this model, the definition of the renormalization scheme will be elucidated. Especially complications occuring in extended Higgs sectors like the treatment of mixing angles describing the mixing among Higgs states will be clarified. Finally, also numerical results will be presented.

Speaker: Johannes Wünsche (IKTP, TU Dresden)

Scalars2025_Wuensche_2HDM_Decoupling.pdf

6:00 PM Automatizing the path from Lagrangian to Higgs physics constraints

FlexibleSUSY is a framework for an automated computation of physical quantities in non-supersymmetric and supersymmetric models starting from the most basic model building blocks, namely the particle content and the Lagrangian. Among a plethora of observables that it supports, it is also capable of computing decay widths of Higgs sector particles with precision comparable to the current experimental accuracy.
In this talk I will discuss the recently created interface between FlexibleSUSY and HiggsTools/ Lilith that allows to asses the global agreement of a BSM model Higgs sector with experimental data. This extension provides a fully automatized chain leading directly from a user defined BSM model to the determination of the viability of that model in terms of a p-value.

Speaker: Wojciech Kotlarski (National Centre for Nuclear Research (PL))

Scalars 2025.pdf

6:30 PM → 11:00 PM Conference dinner

Thu, September 25

9:00 AM → 10:30 AM Plenary 11

Convener: ANTONIO DELGADO (University of Notre Dame)

9:00 AM Physics Beyond the SM : The case of vector-like qiarks.

We argue that VLQ are a plausible extension of the SM, and we point out a similarity
between VLQ and right-handed neutrinos entering in the seesaw mechanism type one.

Speaker: Gustavo Branco

gcbScalars25Warsaw25Sept25gcbScalars25Warsaw25Sept25.pdf

9:30 AM RockStar Baryogenesis: baryogenesis from primordial CP violation

Speaker: Venus Keus (Dublin Institute for Advanced Studies)

VKeus_Scalars2025.pdf

10:00 AM Rare decays with missing energy and light invisible scalars

We discuss possible light invisible scalars in the context of rare decays with missing energy. We first explore the windows remaining in B to K + inv and K to pi + inv decay modes. We construct simple UV-complete extensions and discuss some resulting phenomenology in parameter regions where the scalars are consistent with dark matter constraints.

Speaker: Prof. German Valencia (Monash University)

scalars2025_Valencia.pdf

10:30 AM → 11:00 AM Coffee

11:00 AM → 1:00 PM Plenary 12

Convener: Andreas Trautner (CFTP, IST Tecnico, Lisbon)

11:00 AM NNLO QCD corrections to $\bar{B} \to X_s \gamma$ without interpolation in $m_c$

Speaker: Mikolaj Misiak

Mikolaj_Misiak.pdf

11:30 AM Phenomenological status of the Aligned Two-Higgs-Doublet model

An updated global fit of the parameter space of the (flavour) Aligned Two-Higgs-Doublet model will be presented, taking into account improved theoretical constraints and recent LHC data. Particular attention will be devoted to the possibility that neutral and charged scalars lighter than the 125 GeV Higgs boson might exist. Sizeable regions of the parameter space remain compatible with all current data.

Speaker: Antonio Pich (IFIC, CSIC-UV)

A2HDM_Pich_Scalars2025.pdf

12:00 PM Non-invertible Peccei-Quinn symmetry, natural 2HDM alignment, and the visible axion

We identify m12 as a spurion of non-invertible Peccei-Quinn symmetry in the type II 2HDM with gauged quark flavor. Thus a UV theory which introduces quark color-flavor monopoles can naturally realize alignment without decoupling and can furthermore revive the Weinberg-Wilczek axion. As an example we consider the SU(9) theory of color-flavor unification, which needs no new fermions. This is the first model-building use of non-invertible symmetry to find a Dirac natural explanation for a small relevant parameter.

Speaker: ANTONIO DELGADO (University of Notre Dame)

Delgado-Scalars.pdf

12:30 PM Deconstructing flavour

Speaker: Stefan Pokorski

SCALARS2025.pokorski.pdf

1:00 PM → 2:00 PM Lunch

2:00 PM → 4:00 PM Plenary 13

Convener: Mikołaj Misiak (University of Warsaw)

2:00 PM Triple Higgs Boson Production at the LHC and Beyond

Triple Higgs boson(HHH) production is a new and active area of Higgs physics at the LHC and beyond. While the standard model cross section of the HHH production is very small, it could be significantly enhanced in theories beyond the standard model, both via resonant and non-resonant production. In particular, HHH production offers sensitivity to trilinear Higgs boson coupling, lambda3, complementary to HH production. In this talk, I'll review recent results and new ideas on how to pursue this channel at the LHC and beyond.

Speaker: Greg Landsberg (Brown University)

Scalars2025-HHH.pdf

2:30 PM Higgs boson measurements and searches for new scalars with ATLAS'

This talk provides an updated review of the progress made at the ATLAS experiment at the LHC, concentrating particularly on the scalar sector and searches for new particles.

Speaker: Trevor Vickey (University of Sheffield)

TrevorVickey_Scalars_September_2025.pdf

3:00 PM Linear Electron Positron Colliders: Vision and Prospect

Speaker: Prof. Aleksander Filip Żarnecki (Faculty of Physics, University of Warsaw)

lcvision_scalars2025_v2.pdf

3:30 PM Muon-Induced Di-Tau Production: A Probe of New (Pseudo)scalars

High-energy muon beams have been extensively used to investigate light new physics in the NA64-mu experiment at CERN. Recently, a new impetus for these efforts emerged with the possible use of forward TeV-scale muons at the LHC. The future could bring significant progress in this direction with dedicated, intense muon beams planned for proposed muon experimental facilities.

In this talk, we will illustrate the physics potential of employing such muon beams in dedicated searches using an active target material. Of particular interest will be muon-induced di-tau production, which can be used to probe BSM (pseudo)scalars and perform measurements of new physics contributions to the anomalous magnetic moment of the tau lepton.

Speaker: Sebastian Trojanowski (National Centre for Nuclear Research, Poland)

STrojanowski_SCALARS2025.pdf

4:00 PM → 4:30 PM Coffee

4:30 PM → 6:00 PM Plenary 14

Convener: Manuel Drees

4:30 PM On new physics contributions to $H \rightarrow l^+ l^- \gamma$

Motivated by a small (probably fading away) but intriguing excess observed in the decay mode $H \rightarrow l^+l^- \gamma$ reported by both the ATLAS and CMS collaborations, I explore the possibility that new physics contributes directly to the effective $H \rightarrow l^+l^- \gamma$ coupling rather than modifying the $Z$ peak. Concretely, I consider a dimension-8 operator that could arise from new particles via box diagrams. Such non-resonant contribution may provide an alternative origin for current or future excesses. I examine how experimental cuts may distinguish between possible modifications of the $Z$ peak and non-resonant contributions. Current measurements requires that the new physics scale, if any, is relatively low ($\Lambda \sim v$). I illustrate this scenario using a simplified model in the form a new scalar field coupled to vector-like fermions, a model motivated by the dark matter problem, and discuss some of its other experimental constraints or predictions.

Speaker: Michel Tytgat (Université Libre de Bruxelles)

Scalars2025_tytgat.pdf

5:00 PM Model building from the bottom up

I will discuss the systematic exploration of perturbative UV completions of effective theories with new heavy particles of spin 1.

Speaker: Manuel Pérez-Victoria (University of Granada)

BottomUp_MPV.pdf

5:30 PM Linking the KM3-230213A Neutrino Event to Dark Matter Decay and Gravitational Waves Signals

The KM3NeT collaboration recently reported the detection of an ultra-high-energy neutrino event, dubbed KM3-230213A. This is the first observed neutrino event with energy of the order of O(100)PeV, the origin of which remains unclear. We interprete this high energy neutrino event as the Dirac fermion dark matter (DM) $\chi$ decay through the right-handed (RH) neutrino portal assuming the Type-I seesaw mechanism for neutrino masses and mixings. Furthermore, dark matter $\chi$ is assumed to charged under $U(1)$ dark gauge symmetry, which is sponetaneously broken by the vacuum expectation value of the dark Higgs $\Phi$. In this scenario, the DM can decay into a pair of Standard Model (SM) particles for $v_\Phi \gg m_\chi$, which we assume is the case. If the DM mass is around 440 PeV with a lifetime $\sim 5 \times 10^{29}$ sec, it can account for the KM3-230213A event. However, such heavy DM cannot be produced through the thermal freeze-out mechanism due to overproduction and violation of unitarity bounds. We focus on the UV freeze-in production of DM through a dimension-5 operator, which helps in producing the DM dominantly in the early Universe. We have also found a set of allowed parameter values that can correctly account for the DM relic density and decay lifetime required to explain the KM3NeT signal. Moreover, we have generated the neutrino spectra from the two-body decay using the HDMSpectra package, which requires the dark Higgs vacuum expectations value (VEV) to be much larger than the DM mass. Finally, the large value of the dark Higgs field VEV opens up the possibility of generating GW spectra from cosmic strings. We have found a reasonable set of parameter values that can address the KM3NeT signal, yield the correct value of the DM relic density through freeze-in mechanism, and allow for possible detection of GW at future detectors.

Speaker: Pyungwon Ko (KIAS)

Scalars 2025_Ko 2.pdf