Sterile neutrinos

Neutrino masses are usually described by adding to the Standard Model some SU(2)singlet fermions that have the Yukawa couplings, as well as some Majorana mass terms. The number of such fields and the scales of their Majorana masses are not known. Several independent observations point to the possibility that some of these singlets may have masses well below the electroweak scale. A sterile neutrino with mass of a few keV can account for cosmological dark matter. The same particle would be emitted anisotropically from a cooling neutron star born in a supernova explosion. This anisotropy can be large enough to explain the observed velocities of pulsars. A lighter sterile neutrino, with mass of the order of eV, is implied by the LSND results; it can have profound implications for cosmology. We review the physics of sterile neutrinos and the roles they may play in astrophysics and cosmology. PACS: 14.60.St,13.15.+g,14.60.Pq,95.35.+d UCLA/07/TEP/5 STERILE NEUTRINOS IN PARTICLE PHYSICS The name sterile neutrino was coined by Bruno Pontecorvo, who hypothesized the existence of the right-handed neutrinos in a seminal paper [1], in which he also considered vacuum neutrino oscillations in the laboratory and in astrophysics, the lepton number violation, the neutrinoless double beta decay, some rare processes, such as μ → eγ , and several other questions that have dominated the neutrino physics for the next four decades. Most models of the neutrino masses introduce sterile (or right-handed) neutrinos to generate the masses of the ordinary neutrinos via the seesaw mechanism [2]. The seesaw lagrangian L = LSM + N̄a ( iγ∂μ ) Na − yαaH L̄αNa − Ma 2 N̄c aNa +h.c. , (1) where LSM is the lagrangian of the Standard Model, includes some number n of singlet neutrinos Na (a = 1, ...,n) with Yukawa couplings yαa. Here H is the Higgs doublet and Lα (α = e,μ,τ) are the lepton doublets. Theoretical considerations do not constrain the number n of sterile neutrinos. In particular, there is no constraint based on the anomaly cancellation because the sterile fermions do not couple to the gauge fields. The experimental limits exist only for the larger mixing angles [3]. To explain the neutrino masses inferred from the atmospheric and solar neutrino experiments, n = 2 singlets are sufficient [4], but a greater number is required if the lagrangian (1) is to explain the LSND [5], the r-process nucleosynthesis [6], the pulsar kicks [7, 8, 9], dark matter [10, 11, 12, 13], and the formation of supermassive black holes [14]. The scale of the right-handed Majorana masses Ma is unknown; it can be much greater than the electroweak scale [2], or it may be as low as a few eV [5, 13, 15]. Even if some of the right-handed Majorana masses are much larger than others, for example, if some of the Ma (a = 1, ...,nl) are smaller than 100 GeV, while some others (a = nl, ...,n) are much greater than 100 GeV, both classes can have a non-negligible contribution to the active neutrino masses. Obviously, this does not contradict the usual decoupling theorems, because the heavy states decouple from all the physical processes at low energies, but they can still contribute to the values of the active neutrino masses if the corresponding Yukawa couplings are large enough.