Cosmological dependence of non-resonantly produced sterile neutrinos

Abstract

The detection of a sterile neutrino could constitute the first observation of a particle that could have been produced before Big-Bang Nucleosynthesis (BBN), and could provide information about the yet untested pre-BBN era. The cosmological evolution in this era could be drastically different than typically assumed, in what constitutes the standard cosmology, as happens in a variety of motivated particle models. In this work we assess the sensitivity to different pre-BBN cosmologies in which entropy is conserved of 0.01 eV to 1 MeV mass sterile neutrinos produced in the early Universe via resonant active-sterile oscillations, which requires a large lepton asymmetry. We identify mass ranges where it is possible to have two populations of the same sterile neutrino, one with a colder and one with a hotter momentum spectra, which is in principle an observable effect. Furthermore, we show the regions in mass and mixing where fully resonant production (i.e. simultaneously coherent and adiabatic) can occur. We find that in all the cosmologies we consider, including the standard one, fully resonantly produced sterile neutrinos in the eV-mass range can evade all cosmological constraints.In the absence of a large lepton asymmetry the oscillations are non-resonant and sterile neutrinos are produced via the Dodelson-Widrow (DW) mechanism [45]. In the standard cosmology this mechanism results in a Fermi-Dirac relic momentum distribution of sterile neutrinos, with a reduced magnitude with respect to active neutrinos. Several studies have already been carried out on non-resonantly produced sterile neutrinos in different pre-BBN cosmologies ). This work is complementary to these previous studies in that we concentrate on the production of sterile neutrinos via resonant active-sterile oscillations, an often considered production mechanism that requires a significant lepton asymmetry in active neutrinos (this is the Shi-Fuller mechanism [52], see also Ref. [53,54]). In this case, sterile neutrinos are produced with a colder momentum distribution (i.e. with a lower average momentum) that is different from a Fermi-Dirac spectrum, even in the standard cosmology. In particular, we study the effect of different cosmologies on resonantly produced sterile neutrinos with mass 10 −2 eV < m s < 1 MeV. Since the production rate is usually not fast enough for sterile neutrinos to equilibrate, the final relic abundance and spectrum are fixed by freeze-in.We will analyze resonant sterile neutrino production within several example cosmological models in which entropy is conserved, characterized by the magnitude and temperature dependence of the Hubble expansion rate H in the non-standard cosmological phase. If H is larger than it would be in the standard cosmology, the resonant production of sterile neutrinos during this phase is suppressed with respect to standard production, and if H is lower, the production is enhanced. If the production is fully resonant, which requires adiabaticity and coherence at the resonance, the relic sterile neutrino ...

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].…”

Cosmological Dependence of Sterile Neutrino Dark Matter With Self-Interacting Neutrinos

“…described in terms of two real parameters, η > 0 and β, and the transition temperature T tr , defined as the temperature below which the cosmology becomes standard. Following our previous study of non-resonant sterile neutrino production [40,41], we consider several vastly different specific examples of cosmologies of this type discussed in the literature:…”

“…It is well known that cosmological history can significantly impact expectations for sterile neutrino production, their abundance and momentum distribution (e.g. [38][39][40][41][42][43]). Light nuclei produced during the Big Bang Nucleosynthesis (BBN) are the earliest cosmological remnants that have been detected thus far and the cosmological history of the Universe prior to BBN (i.e.…”

“…In many well motivated models, such as those based on moduli decay [50][51][52][53][54][55], quintessence [56][57][58] or extra dimensions [59,60], the cosmological history could be different than typically assumed. It has been demonstrated [40][41][42] that sterile neutrinos constitute an excellent probe of the yet untested cosmological epoch before BBN. Conversely, this also implies that there is significant uncertainty in the viable sterile neutrino parameter space relevant for searches associated with the unknown cosmological history.…”

Sterile neutrino dark matter production in presence of non-standard neutrino self-interactions: an EFT approach

“…The case in which the entire content of dark matter is constituted by sterile neutrinos is represented by a straight line in this parameter space. More complicated mechanisms of production, such as the Shi-Fuller mechanism [7] which describes resonant production, or non-standard cosmological or particle physics scenarios in which the production occurs [8][9][10][11][12][13][14][15][16], depend on more parameters and modify the region of the parameter space where the condition Ω s h 2 = Ω DM h 2 = 0.12 [17] is satisfied. In the standard DW scenario, the values of m s and sin 2 (2θ) that correspond to Ω s h 2 = Ω DM h 2 = 0.12, are ruled out by constraints coming from X-ray observations [18,19].…”