We provide a fresh look at the cosmological constraints on axion-like particles (ALPs) that couple predominantly to photons, focusing on lifetimes $$\tau _{a} \lesssim 10^{4}\, \textrm{s}$$ and masses $$m_a\lesssim 10\,\textrm{GeV}$$ . We consider Big Bang nucleosynthesis (BBN) and Cosmic Microwave Background (CMB) bounds and explore how these limits depend upon the unknown reheating temperature of the Universe, $$T_\textrm{reh}$$ . Compared with some previous studies, we account for the rare decays of these ALPs into light hadrons and show that this leads to extended constraints for several reheating temperatures. Our limits are cast in a model-independent way, and we identify regions of parameter space where these ALPs could alleviate small tensions in the determinations of $$N_\textrm{eff}$$ and the deuterium abundance. Our Mathematica BBN code is publicly available at .

Switzerland

Miguel Escudero Abenza

Spain

Clara Garcia-Perez

Maksym Ovchynnikov

Nucleosynthesis and CMB bounds on photophilic ALPs: a fresh look

In models of strongly interacting dark sectors, the production of dark quarks at accelerators can give rise to dark showers with multiple dark mesons in the final state. If some of these dark mesons are sufficiently light and long-lived, they can be detected with searches for displaced vertices at beam-dump experiments and electron-positron colliders. In this work we focus on the case that dark quark production proceeds via effective operators, while the dark sector analog of the <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><msup><mi>ρ</mi><mn>0</mn></msup></math> meson can decay via kinetic mixing. We evaluate current constraints from NA62 and BABAR as well as sensitivity projections for SHiP and Belle II. We find that there exists a sizeable parameter region where SHiP may detect several displaced vertices in a single event and thus obtain valuable information about the structure of the dark sector.

United States

Elias Bernreuther

Germany

Nicoline Hemme

Felix Kahlhoefer

Austria

Suchita Kulkarni

CERN

Sub-GeV dark matter and multidecay signatures from dark showers at beam-dump experiments

We delineate the maximal parameter space of sterile neutrino dark matter in the presence of lepton-flavor asymmetries. We focus on large flavor asymmetries with vanishing total lepton asymmetry, which are washed out by neutrino oscillations at MeV temperatures and hence are consistent with big bang nucleosynthesis and cosmic microwave background constraints. We derive a semiclassical Boltzmann equation for sterile neutrinos applicable in this regime and validate it against quantum kinetic equations. For sterile neutrino masses up to 60 keV, the viable range of mixing angles extends by up to 2 orders of magnitude, with broad prospects for tests in forthcoming x-ray, cosmic microwave background, and structure formation observations. We also release a public framework to compute the production of sterile neutrinos, and in particular their momentum distribution, enabling dedicated structure formation analyses.

Japan

Kensuke Akita

Koichi Hamaguchi

Maximal Parameter Space of Sterile Neutrino Dark Matter with Lepton Asymmetries

We propose a scenario to produce large primordial lepton flavor asymmetries with vanishing total lepton asymmetry, based on the Affleck-Dine mechanism with Q-ball formation. This scenario can produce large lepton flavor asymmetries while automatically maintaining the vanishing total lepton number without fine-tuning, evading the current BBN and the CMB constraints by neutrino oscillations at MeV temperature. The asymmetries can be produced at cosmic temperatures of $\textit{T}$ ≳ 1 GeV, early enough to have broad impacts from the early Universe to the present cosmology. This scenario could affect various aspects of early Universe cosmology simultaneously or separately: (i) explaining the observed baryon asymmetry by the same origin as the lepton flavor asymmetries, (ii) affecting the nature of the QCD transition, (iii) opening up a new parameter space of sterile neutrino dark matter by enhancing their production, and (iv) altering the abundance of the light elements, in particular, resolving the recently reported helium-4 anomaly.

Affleck-Dine leptoflavorgenesis

Neutrino direct simulation Monte Carlo (<math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mi>ν</mi><mi>DSMC</mi></math>) is a Monte Carlo method for solving the neutrino Boltzmann equation in the early Universe, designed to track the evolution of cosmic neutrinos across a wide range of cosmological scenarios. We develop a complete <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mi>ν</mi><mi>DSMC</mi></math> solver that consistently incorporates the effects of the electron mass, three-flavor neutrino oscillations, and finite-temperature QED corrections to the thermodynamics of the electromagnetic plasma. As a first application, we perform a high-precision calculation of neutrino decoupling in the standard cosmological model and obtain <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><msub><mi>N</mi><mi>eff</mi></msub><mo>=</mo><mn>3.0439</mn><mo>±</mo><mn>0.0006</mn></math>, in excellent agreement with state-of-the-art results.

Ukraine

Oleksii Ihnatenko

Precision calculation of <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><msub><mi>N</mi><mi>eff</mi></msub></math> with neutrino direct simulation Monte Carlo

A class of extensions to the Standard Model adds hypothetical long-lived particles (LLPs) that have mass- or kinetic-mixing with neutral mesons, such as pions or rho mesons. The mixing can contribute significantly to the production of LLPs at proton accelerator experiments, and no consistent description of these production modes exists in the literature. In this paper, we develop a framework for studying different LLPs—dark photons, vector mediators coupled to the baryon current, and axionlike particles with different coupling patterns. In particular, we implement the production mechanisms in pythia8, study how the overall flux and kinematic distributions depend on the LLP’s mass, and compare various subprocesses where the mixing contributes—proton bremsstrahlung, meson decay, and production in the fragmentation chain. We find that our new description of LLP production predicts an integrated flux that differs from current approaches by one to two orders of magnitude, and highlight the unavoidable theoretical uncertainties coming from poor knowledge of the properties of heavy mesons.

Yehor Kyselov

Stephen Mrenna

New physics particles mixing with mesons: Production in the fragmentation chain

Searches for axionlike particles (ALPs) with masses in the GeV range are a central objective of present and future intensity-frontier experiments. Interpreting these searches demands a reliable description of ALP production and decay. The prescription currently adopted by the community depends on unphysical chiral-rotation parameters, wrongly describes the mass scaling of the ALP flux, and ignores mixing with heavy pseudoscalar resonances. We introduce the framework that treats GeV-scale ALP interactions self-consistently, includes their mixing with heavier excitations <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mi>π</mi><mo stretchy="false">(</mo><mn>1300</mn><mo stretchy="false">)</mo></math>, <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mi>η</mi><mo stretchy="false">(</mo><mn>1295</mn><mo stretchy="false">)</mo></math>, and <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mi>η</mi><mo stretchy="false">(</mo><mn>1440</mn><mo stretchy="false">)</mo></math>, and properly describes production channels. When applying our description to proton beam experiments, we find that existing bounds and projected sensitivities shift by up to an order of magnitude relative to earlier estimates. We further delineate the dominant theoretical uncertainties, which originate from the still-incomplete experimental knowledge of the light-meson spectrum.

Andrii Zaporozhchenko

Advancing the phenomenology of GeV-scale axionlike particles

We investigate the cosmological impact of hypothetical unstable new physics particles that decay in the MeV-scale plasma of the early Universe. Focusing on scenarios where the decays produce metastable species such as muons, pions, and kaons, we systematically analyze the dynamics of these particles using coupled Boltzmann equations governing their abundances. Our results demonstrate that the metastable species can efficiently annihilate or interact with nucleons, often leading to their disappearance before decay. The suppression of decay significantly alters the properties of cosmic neutrinos, impacting cosmological observables like big bang nucleosynthesis and the cosmic microwave background. To support further studies, we provide two public codes: the Mathematica code that traces the evolution of these metastable particles, as well as the python-based unintegrated neutrino Boltzmann solver that uses this evolution as an input and may be applied to a broad range of scenarios. We then utilize them for studying a few particular new physics models.

Gideon Baur

Thomas Schwetz

Netherlands

Vsevolod Syvolap

Dynamics of metastable standard model particles from long-lived particle decays in the MeV primordial plasma

We investigate decays of hypothetical unstable new physics particles into metastable species such as muons, pions, or kaons in the early Universe, when temperatures are in the MeV range, and study how they affect cosmic neutrinos. We demonstrate that the nontrivial dynamics of metastables in the plasma alters the impact of the new physics particles on the neutrino population, including the effective number of neutrino degrees of freedom, <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mrow><msub><mrow><mi>N</mi></mrow><mrow><mi>eff</mi></mrow></msub></mrow></math>, modifies neutrino spectral distortions, and may induce asymmetries in neutrino and antineutrino energy distributions. These modifications have important implications for observables such as big bang nucleosynthesis and the cosmic microwave background, especially in light of upcoming cosmic microwave background observations aiming to reach percent-level precision on <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><msub><mi>N</mi><mi>eff</mi></msub></math>. We illustrate our findings with a few examples of new physics particles and provide a computational tool available for further exploration.

New Physics Decaying into Metastable Particles: Impact on Cosmic Neutrinos

Understanding how new physics influences the dynamics of cosmic neutrinos during their decoupling is crucial in light of upcoming precise cosmological observations and the need to reconcile cosmological and laboratory probes. Existing approaches to solving the neutrino Boltzmann equation are often model dependent, computationally inefficient, and yield contradictory results. To solve this problem, we introduce a novel method to comprehensively study neutrino dynamics. We apply this method to several case studies, resolving the discrepancy in the literature about the impact of nonthermal neutrinos on <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mrow><msub><mrow><mi>N</mi></mrow><mrow><mi>eff</mi></mrow></msub></mrow></math> and providing important insights about the role of decaying new physics particles on MeV plasma.