Revisiting cosmological bounds on sterile neutrinos

Abstract

We employ state-of-the art cosmological observables including supernova surveys and BAO information to provide constraints on the mass and mixing angle of a non-resonantly produced sterile neutrino species, showing that cosmology can effectively rule out sterile neutrinos which decay between BBN and the present day. The decoupling of an additional heavy neutrino species can modify the time dependence of the Universe's expansion between BBN and recombination and, in extreme cases, lead to an additional matter-dominated period; while this could naively lead to a younger Universe with a larger Hubble parameter, it could later be compensated by the extra radiation expected in the form of neutrinos from sterile decay. However, recombination-era observables including the Cosmic Microwave Background (CMB), the shift parameter RCMB and the sound horizon rs from Baryon Acoustic Oscillations (BAO) severely constrain this scenario. We self-consistently include the full time-evolution of the coupled sterile neutrino and standard model sectors in an MCMC, showing that if decay occurs after BBN, the sterile neutrino is essentially bounded by the constraint sin 2 θ 0.026(ms/eV) −2 .

Production of heavy sterile neutrinos from vector boson decay at electroweak temperatures

“…These values are in significant tension both with the results that we obtained above and the bounds of ref. [83]. In particular with M s few MeV and such large mixing angle our result (VII.25) indicates that N s /N ν ≃ 1 suggesting full thermalization, furthermore, as is discussed above, our results provide a lower bound for the abundance of sterile-like heavy neutrinos.…”

“…However they can be suitable as the MeV sterile neutrinos that are conjectured to provide a solution to the 7 Li problem. This is with the caveat that there seems to be tension among the various bounds available in the literature [79,83]. Just as these heavier neutrinos may decay by injecting energy into the plasma as the solution to this problem, we also conjecture that they may also decay into lighter ≃ KeV sterile neutrinos, with a much smaller branching ratio, that could be suitable candidates for warm dark matter.…”

Global constraints on a heavy neutrino

“…This scenario is very strongly constrained (see, for example, Refs. [2][3][4][5][94][95][96][97][98][99][100][101]). However, constraints using the ν 4 decay products are model-specific.…”

“…4(a) and exclude |U e4 | 2 < O(10 −3 ) when 1 keV m 4 450 keV. 4 • An experiment performed at TRIUMF used the kinematics of Γ(π → eν) to place limits on |U e4 | 2 < O(10 −8 ) at 90% CL for 10 MeV m 4 55 MeV [60,61].…”

“…Throughout, we will assume that there is no relation between the different mixing-matrix elements U αi , nor any relation between U αi and the different neutrino masses m i . To date, much attention has been paid to a single Majorana sterile neutrino augmenting the SM, with mass 10 eV m 4 1 TeV, where the model responsible for its decay is identical to the model that dictates its production, i.e., neutrino production and decay are governed uniquely by the weak interactions. As discussed, for example, in Refs.…”