Sterile neutrino states

Abstract

Abstract.Neutrino masses are likely to be a manifestation of the right-handed, or sterile neutrinos. The number of sterile neutrinos and the scales of their Majorana masses are unknown. We explore theoretical arguments in favor of the high and low scale seesaw mechanisms, review the existing experimental results, and discuss the astrophysical hints regarding sterile neutrinos. Sterile neutrinos in particle physicsThe term 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 lagrangianwhere L SM is the lagrangian of the Standard Model, includes some number n of singlet neutrinos N a with Yukawa couplings y αa . Here H is the Higgs doublet and L α (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], dark matter [9,10,11,12], and the formation of supermassive black holes [13]. The scale of the right-handed Majorana masses M a is unknown; it can be much greater than the electroweak scale [2], or it may be as low as a few eV [5,14]. The seesaw mechanism [2] can explain the smallness of the neutrino masses in the presence of the Yukawa couplings of order one if the Majorana masses M a are much larger than the electroweak scale. Indeed, in this case the masses of the lightest neutrinos are suppressed by the ratios H /M a .However, the origin of the Yukawa couplings remains unknown, and there is no experimental evidence to suggest that these couplings must be of order 1. In fact, the Yukawa couplings of the charged leptons are much smaller than 1. For example, the Yukawa coupling of the electron is as small as 10 −6 .