v3: content matches published version in International Journal of Modern Physics D (discussion generally extended, discussions on separate seesaw and on vacuum alignment added, Q_6 scheme added, acknowledgements changed to comply with the referee's requests); 100 pages, 8 figures
We review the model building aspects for keV sterile neutrinos as Dark Matter candidates. After giving a brief discussion of some cosmological and astrophysical aspects, we first discuss the currently known neutrino data and observables. We then explain the purpose and goal of neutrino model building, and review some generic methods used. Afterwards certain aspects specific for keV neutrino model building are discussed, before reviewing the bulk of models in the literature. We try to keep the discussion on a pedagogical level, while nevertheless pointing out some finer details where necessary and useful. Ideally, this review should enable a grad student or an interested colleague from cosmology or astrophysics with some prior experience to start working on the field.
Considering the gauge model SU(2)_L x U(1) with the extension of the lepton sector by right-handed sterile Majorana neutrinos, we distinguish a "minimal parametric mixing scenario" for a complete matrix of 6 x 6 light (active) and sterile neutrinos, which depends, in addition to experimentally measured physical parameters, only on the masses of sterile neutrinos. For such a scenario, improved cosmological bounds are provided, resulting from the lifetime of sterile neutrinos and the fraction of energy carried by sterile neutrino dark matter.
Neutrino-neutrino interactions, as mediated by standard model Z exchange, can drastically enhance the Dodelson-Widrow mechanism for the production of sterile neutrinos in the region of temperature $5-15$MeV. For a sterile mass in the KeV region with the usual type of active-sterile neutrino coupling, the production can be copious enough to provide ample dark matter, even when the active-sterile coupling is small enough to avoid the bounds set by laboratory results.
Sterile neutrinos can be produced in the early universe via interactions with their active counterparts. For small active-sterile mixing angles, thermal equilibrium with the standard model plasma is not reached and sterile neutrinos are only produced via flavor oscillations. We study in detail this regime, taking into account matter potentials and decoherence effects caused by elastic scatterings with the plasma. We find that resonant oscillations occurring at temperatures $T\lesssim 10\,\mathrm{GeV}$ lead to a significant enhancement of the sterile neutrino production rate. Taking this into a…
We consider the possibility of the lightest sterile neutrino dark matter which has dipole interaction with heavier sterile neutrinos. The lifetime can be long enough to be a dark matter candidate without violating other constraints and the correct amount of relic abundance can be produced in the early Universe. We find that a sterile neutrino with the mass of around MeV and the dimension-five non-renormalisable dipole interaction suppressed by $Λ_5 \gtrsim 10^{15}$ GeV can be a good candidate of dark matter, while heavier sterile neutrinos with masses of the order of GeV can explain the active…
Sterile neutrinos can be a possible candidate for dark matter. Sterile neutrinos are radiatively unstable and can inject photon energy into the intergalactic medium (IGM). The injection of photon energy into IGM can modify the temperature and ionization history of IGM gas during cosmic dawn. Theoretical models based on the ΛCDM framework predict an absorption profile in the 21 cm line during the cosmic dawn era. Recently, the Experiment to Detect the Global Epoch of Reionization Signature (EDGES) collaboration confirmed such an adsorption signal. Injection of energy into IGM can modify the abs…
We examine the possibility that direct dark matter detection experiments find decay products from sterile neutrino dark mater in $U(1)_{B-L}$ and $U(1)_R$ models. This is possible if the sterile neutrino interacts with a light gauge boson and decays into a neutrino and the light gauge boson with a certain lifetime. This decay produces energetic neutrinos scattering off nuclei with a large enough recoil energy in direct dark matter detection experiments. We stress that direct dark matter detection experiments can explore not only WIMP but also sterile neutrino dark matter.
We study the phenomenology of a light (GeV scale) sterile neutrino sector and the pseudo-Goldstone boson (not the majoron) associated with a global symmetry in this sector that is broken at a high scale. Such scenarios can be motivated from considerations of singlet fermions from a hidden sector coupling to active neutrinos via heavy right-handed seesaw neutrinos, effectively giving rise to a secondary, low-energy seesaw framework. Such scenarios allow for rich phenomenology with observable implications for cosmology, dark matter, and direct searches, involving novel sterile neutrino dark matt…
Using an analytic treatment, we discuss the parameter regions of large active-sterile neutrino mixing angles where sterile neutrinos produced in non-resonant flavour oscillations can approach thermalization in several cosmologies. We show that thermalization affects only large active-sterile neutrino mixing already rejected by different limits. Hence, the allowed sterile neutrino parameter regions are unaffected.
We investigate how hypothetical particles - sterile neutrinos - can be produced in the interior of exploding supernovae via the resonant conversion of $\barν_μ$ and $\bar ν_τ$. The novelty of our treatment lies in the proper account of the resulting lepton number diffusion. We compute the yield of sterile neutrinos and find that even after taking into account back reaction, sterile neutrinos can carry out a sizeable fraction of the total energy of the explosion comparable to that of active neutrinos. The production is, however, exponentially sensitive to the temperature in the inner supernovae…
We obtain non-linear kinetic equations for sterile neutrino occupancies and lepton minus baryon numbers by matching real time correlation functions of thermal fluctuations computed in an effective description to those computed in thermal quantum field theory. After expanding in the sterile-neutrino Yukawa couplings, the coefficients in the equations are written as real time correlation functions of Standard Model operators. Our kinetic equations are valid for an arbitrary number of sterile neutrinos of any mass spectrum. They can be used to describe, e.g., low-scale leptogenesis via neutrino o…
Motivated by the capability of the KATRIN experiment to explore the existence of KeV neutrinos in the $[1-18.5]$ KeV mass range, we explore the viability of minimal extensions of the Standard Model involving sterile neutrinos (namely the 3 + $N$ frameworks) and study their possible impact in both the beta energy spectrum and the neutrinoless double beta decay effective mass, for the two possible ordering cases for the light neutrino spectrum. We also explore how both observables can discriminate between motivated low-scale seesaw realizations involving KeV sterile neutrinos. Our study concerns…
We review sterile neutrinos as possible Dark Matter candidates. After a short summary on the role of neutrinos in cosmology and particle physics, we give a comprehensive overview of the current status of the research on sterile neutrino Dark Matter. First we discuss the motivation and limits obtained through astrophysical observations. Second, we review different mechanisms of how sterile neutrino Dark Matter could have been produced in the early universe. Finally, we outline a selection of future laboratory searches for keV-scale sterile neutrinos, highlighting their experimental challenges a…
Neutrinos, being the only fermions in the Standard Model of Particle Physics that do not possess electromagnetic or color charges, have the unique opportunity to communicate with fermions outside the Standard Model through mass mixing. Such Standard Model-singlet fermions are generally referred to as "sterile neutrinos''. In this review article, we discuss the theoretical and experimental motivation for sterile neutrinos, as well as their phenomenological consequences. With the benefit of hindsight in 2020, we point out potentially viable and interesting ideas. We focus in particular on steril…
We study a model of a keV-scale sterile neutrino with a relatively large mixing with the Standard Model sector. Usual considerations predict active generation of such particles in the early Universe, which leads to constraints from the total Dark Matter density and absence of X-ray signal from sterile neutrino decay. These bounds together may deem any attempt of creation of the keV scale sterile neutrino in the laboratory unfeasible. We argue that for models with a hidden sector coupled to the sterile neutrino these bounds can be evaded, opening new perspectives for the direct studies at neutr…
