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

CERN

Maksym Ovchynnikov

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

United States

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.

Japan

Kensuke Akita

Germany

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.

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

Models of inelastic (or pseudo-Dirac) dark matter commonly assume an accidental symmetry between the left-handed and right-handed mass terms in order to suppress diagonal couplings. We point out that this symmetry is unnecessary because for Majorana fermions the diagonal couplings are not strongly constrained. Removing the requirement of such an ad hoc symmetry instead relaxes the relic density constraint due to additional annihilation modes. We consider a simple UV-complete model realizing this setup and study constraints from (in)direct detection, beam dump experiments and colliders. We identify two viable mass regions for the dark matter mass, around a few hundred MeV and around a few GeV, respectively. The former region will be fully tested by near-future analyses of the NA64 and Belle II data, while the latter turns out to be challenging to explore even with future experiments.

Giovani Garcia

Felix Kahlhoefer

Not-so-inelastic Dark Matter

A systematic and unified analysis of the capability of lifetime frontier experiments to probe the parameter space of hypothetical long-lived particles is crucial for defining the targets of future searches. However, such a comprehensive study has not yet been conducted for dark photons—hypothetical massive particles that kinetically mix with Standard Model photons. Existing studies often rely on outdated assumptions about dark photon phenomenology, leading to inaccurate calculations of the parameter space, including regions already excluded or relevant for future experiments. In this paper, we present a comprehensive calculation of the parameter space for GeV-scale dark photons and light dark matter accessible to lifetime frontier experiments. Our analysis highlights how theoretical uncertainties in dark photon phenomenology can significantly impact experimental sensitivity across coupling and mass parameters. To facilitate further investigation, we provide these results in a publicly accessible form, incorporating updated dark photon phenomenology into the event generator senscalc.

Searches for long-lived dark photons at proton accelerator experiments

For years, it has been believed that the main LHC detectors can play only a limited role of a lifetime frontier experiment exploring the parameter space of long-lived particles (LLPs)—hypothetical particles with tiny couplings to the Standard Model. This paper demonstrates that the LHCb experiment may become a powerful lifetime frontier experiment if it uses the new Downstream algorithm reconstructing tracks that do not allow hits in the LHCb vertex tracker. In particular, for many LLP scenarios, LHCb may be as sensitive as the proposed experiments beyond the main LHC detectors for various LLP models, including heavy neutral leptons, dark scalars, dark photons, and axion-like particles.

Volodymyr Gorkavenko

Spain

India

Brij Jashal

Valerii Kholoimov

Diego Mendoza

Arantza Oyanguren

Volodymyr Svintozelskyi

Jiahui Zhuo

LHCb potential to discover long-lived new physics particles with lifetimes above 100 ps

Axionlike particles (ALPs) emerge in many extensions of the Standard Model as pseudo-Goldstone bosons of a spontaneously broken global symmetry. Understanding their phenomenology in high-energy collisions is crucial for optimizing experimental searches and understanding the exploration potential of future experiments. In this paper, we revise the phenomenology of ALPs with universal couplings to fermions. In particular, we analyze the hierarchy and uncertainty of the various ALP production channels depending on the proton collision energy and the placement of the experiment, and provide improved calculations of the hadronic decay modes.