Combined Analysis of Neutrino and Antineutrino Oscillations at T2K

Neutrino mass hierarchy, CP-violation, and octant of θ 23 are the fundamental unknowns in neutrino oscillations. In order to address all these three unknowns, we study the physics reach of a setup, where we replace the antineutrino run of T2HK with antineutrinos from muon decay at rest (µ-DAR). This approach has the advantages of having higher statistics in both neutrino and antineutrino modes, and lower beam-on backgrounds for antineutrino run with reduced systematics. We find that a hybrid setup consisting of T2HK (ν) and µ-DAR (ν) in conjunction with full exposure from T2K and NOνA can resolve the issue of mass hierarchy at greater than 3σ C.L. irrespective of the choices of hierarchy, δ CP , and θ 23 . This hybrid setup can also establish the CP-violation at 5σ C.L. for ∼ 55% choices of δ CP , whereas the same for conventional T2HK (ν +ν) setup along with T2K and NOνA is around 30%. As far as the octant of θ 23 is concerned, this hybrid setup can exclude the wrong octant at 5σ C.L. if θ 23 is at least 3 • away from maximal mixing for any δ CP .

Abstract:We study the consequences of invisible decay of neutrinos in the context of the DUNE experiment. We assume that the third mass eigenstate is unstable and decays to a light sterile neutrino and a scalar or a pseudo-scalar. We consider DUNE running in 5 years neutrino and 5 years antineutrino mode and a detector volume of 40 kt. We obtain the expected sensitivity on the rest-frame life-time τ 3 normalized to the mass m 3 as τ 3 /m 3 > 4.50 × 10 −11 s/eV at 90% C.L. for a normal hierarchical mass spectrum. We also find that DUNE can discover neutrino decay for τ 3 /m 3 > 4.27 × 10 −11 s/eV at 90% C.L. In addition, for an unstable ν 3 with an illustrative value of τ 3 /m 3 = 1.2 × 10 −11 s/eV, the no decay case could get disfavoured at the 3σ C.L. At 90% C.L. the expected precision range for this true value is obtained as 1.71 × 10 −11 > τ 3 /m 3 > 9.29 × 10 −12 in units of s/eV. We also study the correlation between a non-zero τ 3 /m 3 and standard oscillation parameters and find an interesting correlation in the appearance and disappearance channels with the mixing angle θ 23 . This alters the octant sensitivity of DUNE, favorably (unfavorably) for true θ 23 in the lower (higher) octant. The effect of a decaying neutrino does not alter the hierarchy or CP violation discovery sensitivity of DUNE in a discernible way.

We propose a complex extension of µτ permutation antisymmetry in the neutrino Majorana matrix M ν . The latter can be realized for the Lagrangian by appropriate CP transformations on the neutrino fields. The resultant form of M ν is shown to be simply related to that with a complex (CP) extension of µτ permutation symmetry, with identical phenomenological consequences, though their group theoretic origins are quite different. We investigate those consequences in detail for the minimal seesaw induced by two strongly hierarchical right-chiral neutrinos N 1 and N 2 with the result that the Dirac phase is maximal while the two Majorana phases are either 0 or π. We further provide an uptodate discussion of the ββ0ν process vis-a-vis ongoing and forthcoming experiments. Finally, a thorough treatment is given of baryogenesis via leptogenesis in this scenario, primarily with the assumption that the lepton asymmetry produced by the decays of N 1 only matters here with the asymmetry produced by N 2 being washed out. Tight upper and lower bounds on the mass of N 1 are obtained from the constraint of obtaining the correct observed range of the baryon asymmetry parameter and the role played by N 2 is elucidated thereafter. The mildly hierarchical right-chiral neutrino case (including the quasidegenerate possibility) is discussed in an appendix.