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.

United Arab Emirates

Nicolás Bernal

China

Quan-feng Wu

Xun-Jie Xu

Germany

Yong Xu

Pre-thermalized gravitational waves

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

Spain

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.

Kuldeep Deka

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 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 2 MeV < < 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.

Brazil

Chee Fong

Colombia

Óscar Zapata

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

Polynomial inflation is a simple cosmological scenario, which fits the cosmic microwave background data well. It provides testable predictions for the tensor-to-scalar ratio and the running of the spectral index. In this work, we investigate the production of Dirac dark matter (DM) within the framework of large-field polynomial inflation. We study all relevant production channels including (<math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mi>i</mi></math>) nonthermal production through inflaton decays and scatterings, and (<math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mi>i</mi><mi>i</mi></math>) thermal production from scattering of standard model particles mediated by inflatons and gravitons. In contrast to small-field polynomial inflation, where inflaton decay dominates DM production, we find that graviton-mediated processes can be dominant in the large-field scenario. For DM lighter than the inflaton, we demonstrate that the interplay between graviton- and inflaton-mediated production channels give rise to nontrivial relations between the DM mass and the reheating temperature required to account for the DM relic abundance.

Julia Harz

Martin A. Mojahed

Graviton- and inflaton-mediated dark matter production after large field polynomial inflation

In this work, we revisit the generation of stochastic gravitational waves (GWs) from interactions in the thermal plasma. We extend the existing literature by incorporating the reheating phase into the thermal history. Our results show that the amplitude of the GW spectrum can be significantly enhanced because the temperature during reheating can be much higher than the reheating temperature. Furthermore, since the temperature evolution during reheating differs from that of free radiation, the peak frequency of the spectrum can also shift. Additionally, the morphology of the spectrum can present characteristic features. We also compute the contribution of the integrated GW spectrum to the effective number of neutrino species, ∆ , which can be substantially larger.

Thermal gravitational waves during reheating

We study the dominant signatures that arise in Higgs physics at colliders when extending the Standard Model (SM) with a Yukawa interaction to heavy neutral leptons, while suppressing their mixing to active neutrinos. We focus on the production of heavy neutral leptons from Higgs bosons that subsequently decay via the Higgs to SM fermions to determine the experimental reach at the LHC detectors and far detectors such as FASER and MATHUSLA. We also determine the impact of precision Higgs constraints on beyond-SM parameters in this scenario.

Discovering heavy neutral leptons with the Higgs boson

We revisit the formation of a thermal population of hadronic axions in nonstandard cosmologies, in light of the recent developments in obtaining continuous and smooth interaction rates for both the gluon and photon couplings. For certain cosmological histories, such as low-temperature reheating (LTR) and kinationlike scenarios, the thermalization of the axion can be severely delayed to higher masses. In the case that thermal equilibrium is achieved, we improve the constraints on LTR for axion masses around the eV scale with respect to previous works and we constrain for the first time early matter-dominated cosmologies. We also point out the possibility of having the coexistence of cold and warm dark matter populations of axions in kinationlike scenarios in the eV mass range.