Abstract
We examine the cosmological and astrophysical signatures of a ``dark baryon,'' a neutral fermion that mixes with the neutron. As the mixing is through a higher-dimensional operator at the quark level, production of the dark baryon at high energies is enhanced so that its abundance in the early universe may be significant. Treating its initial abundance as a free parameter, we derive new, powerful limits on the properties of the dark baryon. Primordial nucleosynthesis and the cosmic microwave background provide strong constraints due to the interconversion of neutrons to dark baryons through their induced transition dipole, and due to late decays of the dark baryon. Additionally, neutrons in a neutron star could decay slowly to dark baryons, providing a novel source of heat that is constrained by measurements of pulsar temperatures. Taking all the constraints into account, we identify parameter space where the dark baryon can be a viable dark matter candidate and discuss promising avenues for probing it.
Highlights
We identify a novel late-time heating mechanism of neutron stars (NS): if the decay of stellar neutrons into dark baryons is very slow, equilibrium between the two species is not achieved over the NS lifetime, and heat is generated by the removal of neutrons from their Fermi sea followed by their replacement
While terrestrial data rules out large parts of this parameter space for mass splittings above roughly 700 keV, cosmological data from the cosmic microwave background (CMB) and big bang nucleosynthesis (BBN) probe larger mχ, even for a small initial χ abundance, where the small Δm leaves little energy for visible particles produced in association with χ in neutron decay
We show constraints from the COBE satellite as well as the region that could be probed by the proposed PIXIE experiment, which mostly come from the so-called y distortions that occur when χ decays after Compton scattering freezes out at τC [36,53]
Summary
New states with masses around a GeV that mix with the standard model (SM) baryons have been considered recently for a number of compelling reasons, such as mirror matter scenarios [1], the baryon asymmetry of the universe [2,3,4,5,6], models of dark matter [2,7,8,9,10], the neutron lifetime anomaly [9,11,12,13,14], the recent XENON1T excess [15], 21 cm cosmology [16], and general baryon-number violating phenomenology [17,18]. The simplicity of the “minimal” dark baryon model at low energy is somewhat deceptive: processes in neutron stars (NS) tend to equilibrate n and χ [20,21], which together with the observations of the most massive NSs seems to require additional repulsive interaction between χ particles [20] This may in turn require, e.g., a composite nature of χ itself and/or the existence of a new “dark vector” force. We identify a novel late-time heating mechanism of NSs: if the decay of stellar neutrons into dark baryons is very slow, equilibrium between the two species is not achieved over the NS lifetime, and heat is generated by the removal of neutrons from their Fermi sea followed by their replacement We exploit this phenomenon to place powerful constraints using NS temperature measurements. V we provide discussion on future prospects from cosmological, astrophysical, and terrestrial probes
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