Non-thermal cosmic neutrino background
Mu-Chun Chen, Michael Ratz, Andreas Trautner

TL;DR
This paper explores the possibility of a non-thermal cosmic neutrino background for Dirac neutrinos, which could be detectable and provide insights into the early universe, distinct from the standard thermal background.
Contribution
It introduces the concept of a non-thermal relic neutrino background, discusses its potential abundance, and proposes a scenario for its generation during inflationary preheating.
Findings
Non-thermal neutrinos can have a density up to half of photon density.
Such neutrinos can increase the effective number of neutrino species beyond 3.046.
Future experiments like PTOLEMY could detect this non-thermal background.
Abstract
We point out that, for Dirac neutrinos, in addition to the standard thermal cosmic neutrino background (CB) there could also exist a non-thermal neutrino background with comparable number density. As the right-handed components are essentially decoupled from the thermal bath of standard model particles, relic neutrinos with a non-thermal distribution may exist until today. The relic density of the non-thermal (nt) background can be constrained by the usual observational bounds on the effective number of massless degrees of freedom , and can be as large as . In particular, can be larger than 3.046 in the absence of any exotic states. Non-thermal relic neutrinos constitute an irreducible contribution to the detection of the CB, and, hence, may be discovered by future experiments such as PTOLEMY. We…
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