Abstract
For a long time there were 3 main experimental indications in favor of the existence of sterile neutrinos: appearance in the beam in the LSND experiment, flux deficit in comparison with theoretical expectations in reactor experiments, and ve deficit in calibration runs with radioactive sources in the Ga solar neutrino experiments SAGE and GALEX. All three problems can be explained by the existence of sterile neutrinos with the mass square difference in the ballpark of 1 eV2. Recently the MiniBooNE collaboration observed electron (anti)neutrino appearance in the muon (anti)neutrino beams. The significance of the effect reaches 6.0σ level when combined with the LSND result. Even more recently the NEUTRINO-4 collaboration claimed the observation of oscillations to sterile neutrinos with a significance slightly higher than 3σ. If these results are confirmed, New Physics beyond the Standard Model would be required. More than 10 experiments are devoted to searches of sterile neutrinos. Six very short baseline reactor experiments are taking data just now. We review the present results and perspectives of these experiments.
Highlights
Oscillations of the three neutrino flavors are well established
Several effects observed with about 3σ significance level can be explained by active-sterile neutrino oscillations
Mueller et al [4] made new estimates of the reactor νe flux which is about 6% higher than experimental measurements at small distances. This deficit is the so called “Reactor antineutrino anomaly” (RAA). Both anomalies can be explained by activesterile neutrino oscillations at Very Short Baselines (VSBL) requiring a mass-squared difference of the order of 1 eV2 [5]
Summary
Oscillations of the three neutrino flavors are well established. Two mass differences and three angles describing such oscillations have been measured [1]. The best point in the sterile neutrino parameter space corresponds to a very large mixing (sin2 2θ = 0.92) and a small mass square difference of ∆m214 = 0.041eV2 (see Figure 1). This region in the sterile neutrino parameter space is disfavored by other experiments and only a small area with larger mass square differences up to 2 eV2 and smaller mixing is still allowed by the global fits [9, 10].
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