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

 

         

 

Mon, September 22

8:50 AM Opening

Plenary 1

10:30 AM Coffee

Plenary 2

1:00 PM Lunch

Plenary 3

4:00 PM Coffee

Plenary 4

6:00 PM Welcome reception

Tue, September 23

Plenary 5

10:30 AM Coffee

Plenary 6

1:00 PM Lunch

Plenary 7

3:30 PM Coffee

Plenary 8

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

Wed, September 24

Plenary 9

10:30 AM Coffee

Plenary 10

1:00 PM Lunch

Parallel 1

1 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))

2 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)

3 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)

4 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))

5 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)

6 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)

7 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)

8 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)

Parallel 2

9 On the 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)

10 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)

11 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)

12 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)

13 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)

14 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)

15 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)

16 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)

Parallel 3

4:00 PM Coffee

Parallel 4

17 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)

18 Vector-like T Bounds at the LHC: Impact of THDM

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)

19 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)

20 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)

21 Constraining Monopole–Dipole Potential: New Bounds using tests of gravity

Ultralight axion-like particles (ALPs) can mediate macroscopic spin-dependent forces through monopole–dipole interactions. In this work, we consider the Earth–Sun system as a natural laboratory to probe such long-range forces. By treating the Earth as a polarized source — due to its geomagnetic field , we explore how these polarized electrons interact with unpolarized nucleons in the Sun to generate a monopole–dipole potential between the Earth and the Sun. This exotic interaction can influence planetary dynamics and photon propagation, manifesting in phenomena such as perihelion precession, light bending, and Shapiro time delay.

We present two distinct approaches to constrain the monopole–dipole coupling strength. In the first, we derive—for the first time—a direct astrophysical bound using Earth's perihelion precession, yielding $g_S\,g_P \lesssim 1.75 \times 10^{-13}$ for ALPs with mass $ m_a \lesssim 10^{-18}$ eV. This bound is tighter than those obtained from light bending and Shapiro delay constraints. In the second, we adopt a hybrid strategy: using Mars’ perihelion shift to constrain monopole–monopole coupling $\left( g_S \lesssim 3.51 \times 10^{-25}\right)$ and Gaia DR2 data on red giant branch tips in $\omega$ Centauri to bound dipole–dipole couplings $\left(g_P \lesssim 1.6 \times 10^{-13} \right)$. This yields a combined hybrid limit on the monopole–dipole interaction: $g_S\,g_P \lesssim 5.61 \times 10^{-38}$.

Our hybrid bound is 3 orders of magnitude more stringent than the Eöt-Wash experiment and 1 order of magnitude stronger than the current hybrid $(Lab)^N_S\times(Astro)^e_P $ limit.

Speaker: Debashis Pachhar (Physical Research Laboratory)

Parallel 5

Parallel 6

8:00 PM Conference dinner

Thu, September 25

Plenary 11

10:30 AM Coffee

Plenary 13

1:00 PM Lunch

Plenary 13

4:00 PM Coffee

Plenary 14