Sterile neutrinos as subdominant warm dark matter

Abstract

In light of recent findings which seem to disfavor a scenario with (warm) dark matter entirely constituted of sterile neutrinos produced via the Dodelson-Widrow (DW) mechanism, we investigate the constraints attainable for this mechanism by relaxing the usual hypothesis that the relic neutrino abundance must necessarily account for all of the dark matter. We first study how to reinterpret the limits attainable from X-ray non-detection and Lyman-alpha forest measurements in the case that sterile neutrinos constitute only a fraction fs of the total amount of dark matter. Then, assuming that sterile neutrinos are generated in the early universe solely through the DW mechanism, we show how the X-ray and Lyman-alpha results jointly constrain the mass-mixing parameters governing their production. Furthermore, we show how the same data allow us to set a robust upper limit fs < 0.7 at the 2 sigma level, rejecting the case of dominant dark matter (fs = 1) at the ~ 3 sigma level.Comment: Minor changes; added references; version accepted for publication in Phys. Rev.

Lepton-number-driven sterile neutrino production in the early universe

“…(As discussed above, we take L e0 = L µ0 = L τ 0 .) Some ranges of the sterile neutrino parameters can be ruled out by the x-ray observations [2,3,4,5,6,7,8,9,10,11] or by observations of smaller-scale large scale structure (e.g., the Lyman-α forest) coupled with structure formation calculations [9,42,43,44]. Bounds on the lepton numbers from big bang nucleosynthesis considerations [21,22,23,24] are much larger than those employed in our calculations.…”

Constraining light dark matter with diffuse X-ray and gamma-ray observations

“…We show our bounds for both these cases in the left and right panel of figure 5, respectively, in the m νs − sin 2 2θ plane. In addition, we show existing bounds from the observation of the Lyman-α forest [49] and the overclosure region, in which the neutrino density produced by the DW mechanism exceeds the observed DM density. We also show the region where the sterile-neutrino lifetime is shorter than the age of the Universe, and hence it cannot act as DM.…”

“…Above (below) the black solid line, the neutrino lifetime is shorter (longer) than the age of the Universe. Within the green boundaries, the sterile neutrino is ruled out by Ly-α forest data [48,49]. Two cases for the sterile-neutrino energy density are assumed.…”

WMAP five-year constraints on lepton asymmetry and radiation energy density: implications for Planck

“…For both, non-resonant zero lepton number production and enhanced resonant production with initial cosmological lepton number, the keV mass sterile neutrino produced via small mixing angle oscillation conversion of thermal active neutrinos [16] provides a valuable Dark Matter (DM) candidate, alleviating the accumulating contradiction between the ΛCDM model predictions on small scales and observations by smearing out the small scale structure [17,18,19,20]. A recent analysis of X-ray and Lyman-α data indicate that keV sterile neutrinos can be considered valuable DM candidates only if they are produced via resonant oscillations with non-zero lepton number or via other (non-oscillatory) production mechanisms [21]. On the other hand, the possible existence of new particles such as axions and gravitons, the time variation of the physical constants and other non-standard scenarios (see e.g.…”