Abstract
According to the Standard Model (SM), we expect to find a proton for each decaying neutron. However, the experiments counting the number of decayed neutrons and produced protons disagree. This discrepancy suggests that neutrons might have an exotic decay channel to Dark Sector (DS) particles. In this paper, we explore a scenario where neutrons decay to a dark Dirac fermion $\ensuremath{\chi}$ and a non-Abelian dark gauge boson ${W}^{\ensuremath{'}}$. In our proposed scenario, the DS has three portals with the SM sector: (1) the fermion portal coming from the mixing of the neutron with $\ensuremath{\chi}$, (2) a scalar portal, and (3) a nonrenormalizable kinetic mixing between photon and dark gauge bosons which induces a vector portal between the two sectors. We discuss the cosmological implications of this scenario assuming DS particles are produced via freeze-in. The fermion and the scalar portal leads to the overproduction of DS particles by the time of the Cosmic Microwave Background (CMB), and thus we disable these two portals in the early universe. For that, we require the maximum temperature of the universe to be lower than ${m}_{\ensuremath{\chi}}$. We rely on the vector portal to connect the two sectors in the early universe, and we discuss the phenomenological bounds on the model. The main constraints come from the Big Bang Nucleosynthesis, ensuring the right relic abundance of dark matter, and the observation of large neutron stars.
Highlights
Even though the Standard Model (SM) of particle physics can explain almost all observed phenomena, we are certain there exists physics beyond the SM
We show that the χ − n mixing with strength 10−13 MeV is small enough that χ counts as a dark matter (DM) in the data from the Cosmic Microwave Background (CMB), and it is big enough that leads to the overproduction of χ in the early universe
We presented a model that can explain the discrepancy between the total decay width of the neutron and its decay width to protons
Summary
Even though the Standard Model (SM) of particle physics can explain almost all observed phenomena, we are certain there exists physics beyond the SM. Since there are severe constraints on the baryon number violating models [13,14], we assume χ has a baryon charge of þ1 In this setup, the effective Lagrangian including the new degrees of freedom becomes. Since Φi, with i 1⁄4 1, 2 are charged under SUð3Þc, we expect their number density in the early universe to match that of photons (e.g., we expect them to be in thermal equilibrium with thermal bath) Through their couplings with the dark sector, the production of χ and subsequently φ and W0 should occur in abundance. Having closed the Higgs portal, we need to discuss the evolution of dark sector particles in the early universe, and how much they contribute to the relic abundance of the total DM.
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