Indirect unitarity violation entangled with matter effects in reactor antineutrino oscillations

TL;DR

This paper analyzes how tiny deviations from unitarity in the neutrino mixing matrix, caused by the seesaw mechanism, are entangled with matter effects in reactor antineutrino oscillations, concluding that matter effects dominate and unitarity violation has negligible impact.

Contribution

The study provides an analytical approximation of antineutrino oscillation probabilities considering unitarity violation and matter effects, showing their entanglement occurs only at next-to-leading order.

Findings

Unitarity violation effects are smaller than matter effects in oscillation probabilities.

Entanglement between unitarity violation and matter effects appears only at next-to-leading order.

Experimental sensitivities to neutrino parameters remain robust despite unitarity violation.

Abstract

If finite but tiny masses of the three active neutrinos are generated via the canonical seesaw mechanism with three heavy sterile neutrinos, the 3\times 3 Pontecorvo-Maki-Nakagawa-Sakata neutrino mixing matrix V will not be exactly unitary. This kind of indirect unitarity violation can be probed in a precision reactor antineutrino oscillation experiment, but it may be entangled with terrestrial matter effects as both of them are very small. We calculate the probability of \overline{\nu}_e \to \overline{\nu}_e oscillations in a good analytical approximation, and find that, besides the zero-distance effect, the effect of unitarity violation is always smaller than matter effects, and their entanglement does not appear until the next-to-leading-order oscillating terms are taken into account. Given a 20-kiloton JUNO-like liquid scintillator detector, we reaffirm that terrestrial matter effects should not be neglected but indirect unitarity violation makes no difference, and demonstrate that the experimental sensitivities to the neutrino mass ordering and a precision measurement of \theta_{12} and \Delta_{21} \equiv m^2_2 - m^2_1 are robust.