The discovery channel at the Neutrino Factory: $\nu_\mu\to\nu_\tau$ pointing to sterile neutrinos

Abstract

We study the potential of a Neutrino Factory in constraining the parameter space of a scheme with one sterile neutrino separated from three active ones by an O(1) eV2 mass-squared difference. We present approximated analytic expressions for the oscillation probabilities, showing that the greatest sensitivity to sterile neutrinos at a Neutrino Factory can be achieved using the νμ → νμ and the νμ → ντ oscillations. We have studied two setups: a Neutrino Factory with 50 GeV (20 GeV) stored muons, with two detectors of the Hybrid-MIND type (a magnetized ECC next to a magnetized iron calorimeter), located at L = 3000, 7500 km (L = 4000, 7500 km) from the source. Four channels have been used: νe → νμ,ντ; νμ → νμ,ντ. The relevant backgrounds, efficiencies and systematic errors have been taken into account, and we have discussed dependence of the sensitivities on the systematic errors. We have found that the 50 GeV (20 GeV) setup can constrain sin2 2θ(4fam)13 ≤ 7 × 10−5 (2 × 10−4); θ34 ≤ 12° (14°); and θ24 ≤ 7.5° (8°). Our results hold for any value of Δm2SBLgtrsim0.1 eV2. Eventually we have shown that, if a positive signal is found, the proposed setup is able to measure simultaneously θ34 and δ3 with a precision of few degrees and few tens of degrees, respectively, solving the so-called "intrinsic" and "sign degeneracies". Combination of νμ disappearance and of the νμ → ντ channel, that will be called "the discovery channel'', at the two baselines is able to measure at 99% CL a new CP-violating phase δ3 for sin2 2θ34 ≥ 0.06.