A Review of the Tension between the T2K and NOνA Appearance Data and Hints to New Physics

Abstract

In this article, we review the status of the tension between the long-baseline accelerator neutrino experiments T2K and NOνA. The tension arises mostly due to the mismatch in the apappearance data of the two experiments. We explain how this tension arises based on νμ→νe and ν¯μ→ν¯e oscillation probabilities. We define the reference point of vacuum oscillation, maximal θ23 and δCP and compute the νe/ν¯e appearance events for each experiment. We then study the effects of deviating the unknown parameters from the reference point and the compatibility of any given set of values of unknown parameters with the data from T2K and NOνA. T2K observes a large excess in the νe appearance event sample compared to the expected νe events at the reference point, whereas NOνA observes a moderate excess. The large excess in T2K dictates that δCP be anchored at −90° and that θ23 > π/4 with a preference for normal hierarchy. The moderate excess at NOνA leads to two degenerate solutions: (a) NH, 0 < δCP < 180°, and θ23 > π/4; (b) IH, −180° < δCP < 0, and θ23 > π/4. This is the main cause of tension between the two experiments. We review the status of three beyond standard model (BSM) physics scenarios, (a) non-unitary mixing, (b) Lorentz invariance violation, and (c) non-standard neutrino interactions, to resolve the tension.

Discrete Flavor Symmetries and Lepton Masses and Mixings

“…In contrast to the breaking pattern mentioned in Eqs. (15,16), if S 4 is considered to be broken spontaneously into Z 3 = {1, T, T 2 } (for the charged lepton sector) Z 2 = {1, SU } (for the neutrino sector)…”

“…This implies values of the CP-violating phase in the vicinity of δ CP = π/2, disfavored by 3σ for IH, and values around δ CP = 3π/2 in NH, disfavored at 2σ for IH. For a recent review, see [15]. Let us note that the T2K, Super-Kamiokande and the NOνA collaborations are working towards joint analyses.…”

Scalar-singlet assisted leptogenesis with CP violation from the vacuum

“…Currently, while some tension between NOνA and T2K results exist regarding the Dirac CP phase for a normal neutrino-mass ordering, maximal CP violation seems to be preferred for an inverted spectrum (for a recent review see ref. [6]). In the next decades, long-baseline neutrino experiments like DUNE [7] and Hyper-Kamiokande [8] will provide much more information on leptonic CP violation (LCPV) [9].…”

Vector leptoquark $U_3$ and CP violation at T2K, NOvA experiments

“…Recently, two independent groups have shown in their studies that presence of eµ and eτ can resolve the current NOvA and T2K tension in the measurement of CP violating phase δ CP [34,35]. In [36], it has been shown that this tension arises primarily from the ν e appearance data, while a review on the possible role of new physics leading to this tension has been discussed in [37].…”