About Matter Effects on Neutrino Mixing Matrix Phases

TL;DR

This paper investigates how matter effects influence the phases in the neutrino mixing matrix, revealing that the commonly used PMNS matrix may not fully diagonalize the system during propagation in matter.

Contribution

It highlights a previously overlooked aspect of matter effects on the neutrino mixing matrix phases and provides analytical and numerical analysis of this phenomenon.

Findings

Matter effects alter the diagonalization of the neutrino system.

The standard PMNS matrix may not be sufficient in matter environments.

Scenarios are discussed where these effects can be experimentally probed.

Abstract

As is widely known, the flavour and mass eigenbasis in neutrino sector can be related by a reduced unitary matrix, described by 3 real parameters and one phase, for Dirac neutrinos, and 2 extra phases if neutrinos are Majorana particles. Neutrino flavour oscillation experiments are insensitive to Majorana phases, which leads to using the PMNS matrix with only one phase to describe flavour transitions. In this paper, we point out one feature of such choice that is commonly ignored: when neutrinos propagate in matter, the reduced unitary matrix PMNS does not properly diagonalize the neutrino system. We present some numerical calculations and analytical estimations to illustrate such statement, and discuss some scenarios where this effect can be probed.