We consider a simple Higgs portal dark matter (DM) model, where the Standard Model is extended with a complex singlet scalar. The imaginary part of the scalar becomes a massive and stable pseudo-Nambu-Goldstone boson, serving as the DM candidate, while the real part gives rise to a second (dark) Higgs boson. We focus on the freeze-in production of the DM, paying particular attention to low-reheating temperature scenarios, where the dark Higgs can be a long-lived particle (LLP). We also explore the phenomenology of this dark Higgs at the LHC and the Future Circular Collider in hadron-hadron mode, discussing its discovery prospects in regions of parameter space consistent with current DM constraints. Our results emphasize the impact of the cosmic reheating dynamics on the DM freeze-in production, and their critical role in interpreting collider signatures. Furthermore, our findings suggest that LLP searches may provide insights into the fundamental dynamics of reheating.

United Arab Emirates

Nicolás Bernal

Chile

Giovanna Cottin

Bastián Díaz Sáez

Manuel López

Testing frozen-in pNGB dark matter with a long-lived dark Higgs

Thermal dark matter is a compelling setup that has been probed by a multitude of experiments, mostly in the GeV–TeV mass range. The thermal paradigm in the sub-GeV range is about to experience the same experimental test with the next generation of low-energy accelerators and light dark matter detectors. Motivated by this, we investigate thermal dark matter in the <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><msub><mi>L</mi><mi>μ</mi></msub><mo>−</mo><msub><mi>L</mi><mi>τ</mi></msub></math> and assess how the introduction of a matter-dominated era impacts the parameter that yields the correct relic density. Interestingly, we show that the projected experiments, such as the Muon (Synchrotron) Ion Collider, Future Circular Collider-ee, and Light Dark Matter eXperiment, will probe a large region of the viable parameter space that yields the correct relic density. In the GeV–TeV mass regime, the usual large-scale detectors push the sensitivity. Our work highlights the rich interplay between early-Universe dynamics, dark matter phenomenology, and the discovery potential of next-generation experiments.

Italy

Brazil

Jacinto P. Neto

Peru

Javier Silva-Malpartida

Farinaldo S. Queiroz

Enabling thermal dark matter within the vanilla <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mrow><msub><mrow><mi>L</mi></mrow><mrow><mi>μ</mi></mrow></msub><mo>−</mo><msub><mrow><mi>L</mi></mrow><mrow><mi>τ</mi></mrow></msub></mrow></math> model

The freeze-in mechanism provides a compelling framework for dark matter (DM) production, particularly suited to scenarios involving feeble interactions with the Standard Model (SM). In this work, we highlight a possible interplay of a non-instantaneous reheating phase and dark sector self-interactions — specifically 2 → 3 and 3 → 2 cannibalization processes. As an example we study the freeze-in production of a complex scalar DM candidate stabilized by a $\textit{ℤ}$ symmetry permitting cubic self-couplings, enabling number-changing interactions that drive internal thermalization and significantly modify the dark sector number density and temperature evolution. We numerically solve the coupled Boltzmann equations for the DM number density and temperature alongside the evolving SM bath, accurately capturing the dynamics of a prolonged reheating epoch. Our analysis reveals a rich and distinctive phenomenology arising from the interplay between the Universe’s thermal history, Higgs portal-mediated production, and cannibalistic self-interactions. Compared to scenarios with instantaneous reheating or negligible self-interactions, our framework opens new viable regions in parameter space — particularly for light DM — potentially within reach of future probes.

Poland

Esau Cervantes

Kuldeep Deka

Andrzej Hryczuk

Freezing-in cannibals with low-reheating temperature

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 $\textit{reheatons}$. We consider multiple production channels, including: $\textit{i}$) pure inflaton-inflaton annihilation, $\textit{ii}$) graviton Bremsstrahlung from inflaton decay, $\textit{iii}$) scatterings between an inflaton and a reheaton, and $\textit{iv}$) 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, $\textit{iii}$), 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.

China

Quan-feng Wu

Xun-Jie Xu

Germany

Yong Xu

Pre-thermalized gravitational waves

We investigate the properties of the radially excited charged pion, with a specific focus on its electromagnetic form factor (EFF) and its box contribution to the hadronic light-by-light (HLbL) component of the muon's anomalous magnetic moment, $ a_{\mu} $ . Utilizing a coupled non-perturbative framework combining Schwinger-Dyson and Bethe-Salpeter equations, we first compute the mass and weak decay constant of the pion's first radial excitation. Initial results are provided for the Rainbow-Ladder (RL) approximation, followed by an extended beyond RL (BRL) analysis that incorporates meson cloud effects. Building on our previous work, this analysis demonstrates that an accurate description of the first radial excitation can be achieved without the need for a reparametrization of the interaction kernels. Having demonstrated the effectiveness of the truncation scheme, we proceed to calculate the corresponding EFF, from which we derive the contribution of the pion's first radial excitation to the HLbL component of the muon's anomalous magnetic moment, producing $ a_{\mu}^{\pi_1-\text{box}}(\text{RL}) = -(2.03 \pm 0.12) \times 10 ^{-13} $ , $ a_{\mu}^{\pi_1-\text{box}}(\text{BRL}) = $ $ -(2.02 \pm 0.10) \times 10 ^{-13} $ . Our computation also sets the groundwork for calculating related pole contributions of excited pseudoscalar mesons to $ a_{\mu} $ .

Spain

Mexico

Angel S. Miramontes

K. Raya

A. Bashir

P. Roig

G. Paredes-Torres

Radially excited pion: electromagnetic form factor and the box contribution to the muon's $\textit{g}$-2 * A. S. Miramontes acknowledges $\textit{Consejo Nacional de Ciencia, Humanidades y Tecnología}$ (CONAHCyT), Mexico, for the financial support received through the program '$\textit{Postdoctorados Nacionales por México}$', additionally, has been partially funded by the 'PROMETEO' programme of the 'Generalitat Valenciana' grant CIPROM/2022/66, and by the Spanish 'Agencia Estatal de Investigación', MCIN/AEI/10.13039/501100011033, through the grants PID2020-113334GB-I00 and PID2023-151418NB-I00. A. Bashir thanks $\textit{Coordinación de la Investigación Científica}$ of the $\textit{Universidad Michoacana de San Nicolás de Hidalgo}$ Grant No. 4.10., CONAHCyT grant CBF2023-2024-3544 and $\textit{Ayudas Beatriz Galindo}$, Spain. P. Roig acknowledges Conahcyt (México) funding through project CBF2023-2024-3226 as well as Spanish support during his sabbatical year through projects MCIN/AEI/10.13039/501100011033, grant PID2020-114473GB-I00, and Generalitat Valenciana grant PROMETEO/2021/071. G. Paredes-Torres acknowledges CONAHCyT, Mexico, for the financial support provided to him through the program '$\textit{Beca de Posgrado en México}$'. The work of A. Bashir and K. Raya is supported by Spanish Ministry of Science and Innovation (MICINN grant no. PID2022-140440NB-C22) and Junta de Andalucía (grant P18-FR-5057)

The transition from the end of inflation to a hot, thermal Universe, commonly referred to as (re)heating, is a critical yet often misunderstood phase in early Universe cosmology. This short review aims to provide a comprehensive, conceptually clear, and accessible introduction to the physics of (re)heating, tailored to the particle physics community. We critically examine the standard Boltzmann approach, emphasizing its limitations in capturing the intrinsically non-perturbative and non-linear dynamics that dominate the early stages of energy transfer. These include explosive particle production, inflaton fragmentation, turbulence, and thermalization; phenomena often overlooked in perturbative treatments. We survey a wide range of theoretical tools, from Boltzmann equations to lattice simulations, clarifying when each is applicable and highlighting scenarios where analytic control is still feasible. Special attention is given to model-dependent features such as (pre)heating, the role of fermions, gravitational couplings, and the impact of multifield dynamics. We also discuss exceptional cases, including Starobinsky-like models and instant (pre)heating, where (re)heating proceeds through analytically tractable channels without requiring full non-linear simulations. Ultimately, this review serves both as a practical guide and a cautionary tale, advocating for a more nuanced and physically accurate understanding of this pivotal epoch within the particle physics community.

India

Basabendu Barman

Javier Rubio

Two or three things particle physicists (mis)understand about (pre)heating

Heavy neutral leptons (HNLs) provide a compelling extension to the Standard Model, addressing the neutrino masses, baryogenesis, and dark matter problems. We perform a model-independent collider study, decoupling the active-sterile mixing angle (<math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mrow><mi>V</mi></mrow></math>) from the Yukawa coupling (<math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mi>y</mi></math>), and explore sensitivities at the HL-LHC for prompt and displaced decays. We also consider the possibility of HNLs being long-lived particles decaying in far detectors as FASER. In addition, we study the expected reach at FCC-ee for the prompt and displaced cases. For zero mixing, FCC-ee and HL-LHC sensitivities to <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mi>y</mi></math> are comparable, with Higgs width measurements imposing the strongest constraints. With nonzero mixing, sensitivities are dominated by <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mi>V</mi></math>, significantly constraining parameter space. This work highlights the importance of precision Higgs studies and displaced searches in probing HNLs at current and future colliders.

Marta Losada

Heavy neutral leptons without prejudice

The dynamics of cosmic reheating, that is, on how the energy stored in the inflaton is transferred to the standard model (SM) thermal bath, is largely unknown. In this work, we show that the phenomenology of the nonbaryonic dark matter (DM) ultraviolet freeze-in production strongly depends on the dynamics of the cosmic-reheating era. Using a general parametrization for the Hubble expansion rate, and SM temperature, we thoroughly investigate DM production during reheating, not only recovering earlier findings that focused on specific cases but also exploring alternative scenarios. Additionally, we derive a generalized framework for DM production via inflaton decays and identify the viable parameter space, while simultaneously addressing constraints from cosmic microwave background observations. As illustrative examples, we explore gravitational DM production through scatterings of SM particles or inflatons, deriving well-defined parameter regions for these scenarios.

Dark matter ultraviolet freeze-in in general reheating scenarios

We consider a simple extension of the standard model with fermionic dark matter (DM) and a $\textit{Z}$′ gauge boson acting as a mediator. We also assume a scenario where cosmic reheating occurs at low temperatures due to the decay of a massive inflaton into standard model states. To follow the evolution of the background and the dark sector states, we implement the required Boltzmann equations in the code micrOMEGAs to explore both the freeze-out and freeze-in mechanisms. We determine the parameter space of the model that satisfies the relic density constraint under different assumptions for the reheating dynamics, and examine current constraints from DM direct detection, taking special care of the scenarios where DM was produced during the reheating era. Large regions of the parameter space favored by low-temperature reheating cases are already probed or will be within the reach of future experiments, both for the WIMP and the FIMP paradigms.

France

Geneviève Bélanger

Russia

Alexander Pukhov

$\textit{Z}$′-mediated dark matter with low-temperature reheating

The parameter space of freeze-in dark matter (DM) with mass $\textit{m}$ through light dark photon (“minimal freeze-in DM”) is currently being probed by direct detection experiments through electron and nuclear recoil. Exploring the DM production in the mass range 10 MeV < $\textit{m}$ < 10 TeV, we quantify the impact of quantum statistics and the reheating dynamics (beyond the instantaneous reheating approximation) on the DM production in the early universe, in particular, the dependence on the cosmic equation of state and the scaling of the temperature of the Standard Model bath during reheating. Special cases corresponding to matter-domination and kination are carefully studied. To fit the entire observed DM relic abundance, low-temperature reheating scenarios require an increase in the coupling between dark and visible sectors which, in turn, enhances the regions of the parameter space that are already tested and will be probed by next-generation direct detection experiments for diverse reheating scenarios.

Chee Fong

Colombia

Óscar Zapata

Probing low-reheating scenarios with minimal freeze-in dark matter