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.

Japan

Kensuke Akita

Germany

Gideon Baur

CERN

Maksym Ovchynnikov

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.

Primordial Neutrinos and New Physics: Novel Approach to Solving the Neutrino Boltzmann Equation

Dark photons, hypothetical feebly interacting massive vector bosons, appear in many extensions of the Standard Model. This study investigates their production and subsequent decay during supernova explosions. We demonstrate that the decay of dark photons, with masses ranging from 200 to 400 MeV, can lead to the emission of neutrinos with energies surpassing those emitted by supernovae. These neutrinos therefore serve as a distinct signal of new physics, allowing for the exploration of previously uncharted regions of the dark photon parameter space and complementing both accelerator-based searches and other astrophysical constraints. The signal is largely unaffected by the specifics of the supernova’s temperature and density radial profiles outside the SN core. Our results indicate that searching for high-energy neutrinos accompanying supernova explosions provides a novel approach to probe physics beyond the Standard Model, including dark photons, heavy neutral leptons, and other feebly interacting particles with masses in the hundreds of MeV range.

Denmark

Oleg Ruchayskiy

High-energy neutrino signals from supernova explosions: A new window into dark photon parameter space

The hot big bang model predicts the existence of a cosmic neutrino background. The number of particles and antiparticles in this primordial bath of neutrinos can be different—a memory of processes that took place at very early epochs. If neutrinos were massless, this asymmetry would not change once neutrinos froze out. However, in the case of massive particles, the asymmetry is not protected by conservation laws and can get erased via helicity-flipping scatterings off matter inhomogeneities. We evaluate this helicity-flipping rate and demonstrate that if relic lepton asymmetry ever existed, it would remained largely intact in the Earth’s neighborhood for realistic values of neutrino masses.

Switzerland

Robin Würsch

Lepton number survival in the cosmic neutrino background

We investigate how hypothetical particles—sterile neutrinos—can be produced in the interior of exploding supernovae via the resonant conversion of <math><msub><mover><mi>ν</mi><mo>¯</mo></mover><mi>μ</mi></msub></math> and <math><msub><mover><mi>ν</mi><mo>¯</mo></mover><mi>τ</mi></msub></math>. The novelty of our treatment lies in the proper account of the resulting lepton number diffusion. We compute the yield of sterile neutrinos and find that even after taking into account backreaction, sterile neutrinos can carry out a sizeable fraction of the total energy of the explosion comparable to that of active neutrinos. The production is, however, sensitive to the temperature in the inner supernovae regions, making robust predictions of challenging. In order to understand whether this production affects supernova evolution and can therefore be constrained, detailed simulations including the effects of sterile neutrinos are needed.

Ukraine

Alexey Boyarsky

Resonance production of keV sterile neutrinos in core-collapse supernovae and lepton number diffusion

The existence of feebly interacting massive particles (FIMPs) could have significant implications on the effective number of relativistic species <math><msub><mi>N</mi><mi>eff</mi></msub></math> in the early Universe. In this work, we investigate in detail how short-lived FIMPs that can decay into neutrinos affect <math><msub><mi>N</mi><mi>eff</mi></msub></math> and highlight the relevant effects that govern its evolution. We show that even if unstable FIMPs inject most of their energy into neutrinos, they may still decrease <math><msub><mi>N</mi><mi>eff</mi></msub></math>, and identify neutrino spectral distortions as the driving power behind this effect. As a case study, we consider heavy neutral leptons (HNLs) and indicate which regions of their parameter space increase or decrease <math><msub><mi>N</mi><mi>eff</mi></msub></math>. Moreover, we derive bounds on the HNL lifetime from the cosmic microwave background and comment on the possible role that HNLs could play in alleviating the Hubble tension.

United Kingdom

Nashwan Sabti

When feebly interacting massive particles decay into neutrinos: The <math><msub><mi>N</mi><mi>eff</mi></msub></math> story

The extension of the Standard Model with two gauge-singlet Majorana fermions can simultaneously explain two beyond-the-Standard-model phenomena: neutrino masses and oscillations, as well as the origin of the matter-antimatter asymmetry in the Universe. The parameters of such a model are constrained by the neutrino oscillation data, direct accelerator searches, big bang nucleosynthesis, and requirement of successful baryogenesis. We show that their combination still leaves an allowed region in the parameter space below the kaon mass. This region can be probed by the further searches of NA62, DUNE, or SHiP experiments.