How Broad is a Neutrino?

TL;DR

This paper introduces a flexible framework using K"allén-Lehmann propagator forms to explore potential new physics effects in neutrino propagation, including additional light states, and demonstrates how upcoming experiments like JUNO can improve constraints.

Contribution

It proposes a simple model spectrum of broadened neutrino states within a general propagator framework, enabling systematic probing of deviations from standard neutrino oscillations.

Findings

Current data constrains broadened neutrino models.

JUNO will significantly improve constraints on the heaviest neutrino.

The framework accommodates models with additional light neutrino states.

Abstract

Canonical neutrino oscillations arise due to the propagation of three mass eigenstates from production to detection. We aspire to capture, in one simple framework, a broad range of new physics effects on neutrino propagation beyond this canonical picture - this can be done by promoting the neutrino propagators to the general K\"all\'en-Lehmann form. In this work we demonstrate how models predicting additional light propagating species of neutrino are naturally accommodated in this language and propose a simple model spectrum composed of just three `broadened' states as a flexible ansatz by which to explore the phenomenology of new physics in neutrino propagation. Reinterpreting existing neutrino oscillation measurements, we illustrate how this framework provides the capacity to probe deviations from the standard three-neutrino scenario systematically and generally. Whilst current data allows for relatively strong constraints on broadened neutrinos, we find the upcoming JUNO experiment will yield significant improvements, particularly for the heaviest neutrino, paving the way to a clearer understanding of how neutrinos propagate in vacuum.