Intrinsic flavor violation in neutrinos produced through decays

TL;DR

This paper demonstrates that within the Standard Model extended with known neutrino properties, there is an intrinsic, non-zero probability of producing neutrinos of the 'wrong' flavor during decays, leading to flavor violation at creation.

Contribution

It reveals an intrinsic flavor violation effect in neutrino production processes, quantifying its dependence on the neutrino state and spectrum, which was previously unrecognized.

Findings

Flavor violation can reach up to 1% in muon decay channels.

The effect is more pronounced at low energies in a continuous spectrum.

Flavor violation is larger than other indirect lepton flavor violation processes.

Abstract

We show there is a non-null probability to produce neutrinos of the ``wrong'' type in general decays, within the SM augmented by the known three massive neutrinos and nontrivial mixing. Such effect is equivalent to an intrinsic flavor violation (lepton flavor violation) at creation without requiring neutrino propagation. The exact amount of flavor violation depends on the neutrino flavor state to be detected. For realistic conditions, the violation is tiny but much larger than other indirect lepton flavor violation processes not involving neutrinos as final states. For neutrinos produced in a continuous spectrum, we show the effect is larger, in relative terms, for neutrinos produced in the low-energy portion of the spectrum. For muon decay, if the usual neutrino flavor state is assumed a flavor violation as large as 1\% is possible in the channel $\mu^+\to\bnu_\tau e^+ \nu_e$. We also discuss the relation between flavor violation and flavor indefiniteness for neutrino flavor states.