Improved big bang nucleosynthesis constraints on heavy neutral leptons

An allowed window for heavy neutral leptons below the kaon mass

“…Their subsequent decays can affect light element abundances and establish an upper bound on the lifetime of HNLs, i.e. a lower bound on their mixing angles [80][81][82][83][84][85][86][87][88][89][90][91][92].…”

“…1. There has been a significant improvement of the Big Bang Nucleosynthesis constraints, based on the meson driven conversion effect [92] (as compared to τ < 0.1 s bound in refs. [25,106,107]).…”

“…On the other hand, a requirement that the presence of HNLs in the primordial plasma would not lead to the over-production of light elements (Deuterium, Helium-4) provides a lower bound on the HNL mixing angles. For HNLs heavier than π ± -mesons the strongest BBN bound of the HNL lifetime is due to n ↔ p meson driven conversion [92]. Pions and kaons produced in HNL decays at the time when free neutrons are present in the plasma modify the resulting freeze-out ratio of neutron to proton abundances, leading to a larger values of 4 He abundance as compared to the Standard Model BBN.…”

“…Pions and kaons produced in HNL decays at the time when free neutrons are present in the plasma modify the resulting freeze-out ratio of neutron to proton abundances, leading to a larger values of 4 He abundance as compared to the Standard Model BBN. If meson production is kinematically allowed, the following constraint can be derived [92]:…”

“…Implementation of different mixing patterns changes the value of Br N →h only, since Y N ζ depends on processes at high temperature, where all lepton species are in equilibrium, and P conv is solely related to mesons. The value of Y N ζ varies in 10 −3 − 10 −2 , therefore we use the conservative lower bound Y N ζ = 10 −3 [92]. In terms of (U 2 e , U 2 µ , U 2 τ ) = U 2 (x e , x µ , x τ ), the branching ratio can be parametrized in the following way:…”