Current and future neutrino oscillation constraints on leptonic unitarity

Abstract

The unitarity of the lepton mixing matrix is a critical assumption underlying the standard neutrino-mixing paradigm. However, many models seeking to explain the as-yet-unknown origin of neutrino masses predict deviations from unitarity in the mixing of the active neutrino states. Motivated by the prospect that future experiments may provide a precise measurement of the lepton mixing matrix, we revisit current constraints on unitarity violation from oscillation measurements and project how next-generation experiments will improve our current knowledge. With the next-generation data, the normalizations of all rows and columns of the lepton mixing matrix will be constrained to ≲10% precision, with the e-row best measured at ≲1% and the τ-row worst measured at ∼10% precision. The measurements of the mixing matrix elements themselves will be improved on average by a factor of 3. We highlight the complementarity of DUNE, T2HK, JUNO, and IceCube Upgrade for these improvements, as well as the importance of ντ appearance measurements and sterile neutrino searches for tests of leptonic unitarity.

Tau Neutrino Identification in Atmospheric Neutrino Oscillations Without Particle Identification or Unitarity

“…The unitarity framework is a relatively model independent framework to quantify deviations from the standard three flavor oscillation picture. Investigations into this framework have found that the electron neutrino row is fairly well constrained, the muon neutrino row is relatively well constrained, and that the tau neutrino row is largely unconstrained using a subset of the above data sets [11][12][13]. A new study focused on the tau neutrino row has showed that more data sets than previously used significantly improves the constraints on the tau neutrino row [14].…”

“…That is, while it is known from long-baseline accelerator experiments that since θ 23 ∼ 45 • [8-10], if one does not assume the 3 × 3 lepton mixing matrix is unitary, then on an event-by-event basis every tau neutrino would appear to be indistinguishable from an electron neutrino. These various data sets and their impact on the tau row of the lepton mixing matrix without the assumption of unitarity are discussed in [14] which significantly expands the tau neutrino input beyond previous unitarity analyses [11][12][13].…”

“…We parameterize this lack of tau neutrinos in a unitarity violating scheme described in section II. That is, the detection of tau neutrinos provides an important probe on the tau row unitarity, the least well constrained part of the lepton mixing matrix [11][12][13].…”

Flavor-dependent radiative corrections in coherent elastic neutrino-nucleus scattering

“…sin 2 2θ 23 would require at least a statistical uncertainty of a few %, which seems a bit too optimistic (that is, unrealistic) for next, or even next-to-next generation experiments. Nevertheless, studying tau neutrinos with this setup could provide nontrivial information on the unitarity of the PMNS matrix [207][208][209][210][211]. As a final remark, future dark matter direct detection experiments like DARWIN [212] will be able to observe CEνNS from the 8 B solar neutrino flux.…”

The Nonsymmetric Flavor Transition Matrix and the Apparent P violation

“…Any scenario beyond the assumption expressed by a different flavor transition matrix could manifest a class of new physics. Typical new physics includes the energy-dependent leptonic mixing matrix [13,14], neutrino-decay [15][16][17][18][19][20][21][22], non-unitary mixing [23][24][25][26][27][28][29][30], pseudo-Dirac neutrinos [31][32][33][34][35][36], neutrino secret interactions [37][38][39][40], etc [41]. These new-physics based transition matrices predict various of flavor ratios of the HAN at Earth.…”