Can Sterile Neutrinos Be the Dark Matter?

Abstract

We use the Ly-α forest power spectrum measured by the Sloan Digital Sky Survey (SDSS) and high-resolution spectroscopy observations in combination with cosmic microwave background and galaxy clustering constraints to place limits on a sterile neutrino as a dark matter candidate in the warm dark matter (WDM) scenario. Such a neutrino would be created in the early universe through mixing with an active neutrino and would suppress structure on scales smaller than its free streaming scale. We ran a series of high-resolution hydrodynamic simulations with varying neutrino mass to describe the effect of a sterile neutrino on the Ly-α forest power spectrum. We find that the mass limit is ms > 14keV at 95% c.l. (10keV at 99.9%), which is nearly an order of magnitude tighter constraint than previously published limits and is above the upper limit allowed by X-ray constraints, excluding this candidate as dark matter in this model. The corresponding limit for a neutrino that decoupled early while in thermal equilibrium is 2.5keV (95 % c.l.). One of the major unsolved mysteries in cosmology is the nature of the dark matter in the universe. Observational evidence points towards cold dark matter (CDM), for which random velocities are negligible. Two of the leading particle physics candidates, the lightest supersymmetric partner and axions, both require extensions beyond the standard model. At the same time, neutrino experiments over the past decade have shown that neutrinos oscillate from one flavor to another, which is only possible if they have mass. Current data from atmospheric and solar neutrino experiments [1,2] are compatible with mixing between the three active neutrino families. Perhaps the simplest way to incorporate these neutrino phenomena into the standard model is to add right-handed neutrinos, just as for other fermions.Given this extension of the standard model it is natural to ask if these (almost) sterile right-handed neutrinos can also explain the dark matter [3]. At least two sterile neutrinos are required to explain the origin of neutrino mass and existence of different mass mixing scales in solar and atmospheric neutrinos, so in a model with three families of sterile neutrinos a third one can act as dark matter [4]. Such neutrinos free stream and erase all fluctuations on scales smaller than the free streaming length. This length is roughly proportional to the temperature and inversely proportional to the mass of neutrinos. Thus if the neutrino mass is sufficiently high, or the temperature sufficiently low, then it acts just like CDM and can satisfy all of the observational constraints from structure formation. Current constraints require the neutrino mass to be above 1.8keV [5,6]. This is below the 5-8keV upper limits from the absence of detection of X-ray photons from radiative decays [7,8,9,10]. A massive neutrino in the keV range has also been suggested as a possible explanation for high pulsar velocities [11] and such a model can possibly explain baryon asymmetry in the universe [12].A sterile neutri...

Limits on the radiative decay of sterile neutrino dark matter from the unresolved cosmic and soft x-ray backgrounds

“…Since sterile neutrinos behave as increasingly warm dark matter for lighter particle masses, observations of the lack of deviations from absolute cold dark matter clustering constrain lighter particle masses. Two recent analyses of a single measurement of the flux power spectrum of the Lyα forest from the Sloan Digital Sky Survey (SDSS) [43] find roughly similar constraints m s > 14 keV [44] and m s 9 keV [45] 1 in the DW model. These improved considerably relative to previous constraints at m s 2 keV, which used the inferred linear matter power spectrum from the SDSS and higher-resolution Lyα flux power spectra [46,47].…”

Constraints on the presence of a 3.5 keV dark matter emission line fromChandraobservations of the Galactic centre

“…Observations of small scale structure from the Lyman α forest can provide limits on the mass if the velocity distribution is known Viel et al 2005Viel et al , 2006Seljak et al 2006). The velocity distribution of the sterile neutrinos depends on their production mechanism in the early Universe, and while the originally proposed non-resonant production (Dodelson & Widrow 1994) is ruled out, in general masses above 2 keV are allowed (Boyarsky et al 2009a;Horiuchi et al 2014).…”

Probing the nature of dark matter with cosmic x rays: Constraints from “dark blobs” and grating spectra of galaxy clusters

“…In the simplest Dodelson-Widrow production scenario of the sterile neutrinos [5], lower mass bounds on the mass can be obtained from studies of the Ly-forest [39][40][41], which combined with the x-ray constraints rule out the full parameter space. However, there are other production scenarios, for example, via inflation [42], for which the Lyresults cannot be applied.…”