Nonunitarity of the lepton mixing matrix at the European Spallation Source

TL;DR

This paper investigates how nonunitarity in the lepton mixing matrix, arising from neutrino mass mechanisms, affects CP violation measurements and explores how ESSnuSB and DUNE experiments can constrain related new physics parameters.

Contribution

It provides a detailed analysis of CP sensitivities and parameter constraints in the presence of nonunitarity at the ESSnuSB experiment, including combined sensitivities with DUNE.

Findings

Nonunitarity modifies neutrino oscillation probabilities.

ESSnuSB can constrain nonunitarity parameters.

Combining DUNE and ESSnuSB improves sensitivity.

Abstract

If neutrinos get mass through the exchange of lepton mediators, as in seesaw schemes, the neutrino appearance probabilities in oscillation experiments are modified due to effective nonunitarity of the lepton mixing matrix. This also leads to new CP phases and an ambiguity in underpinning the ''conventional'' phase of the three-neutrino paradigm. We study the CP sensitivities of various setups based at the European spallation source neutrino super-beam (ESSnuSB) experiment in the presence of nonunitarity. We also examine its potential in constraining the associated new physics parameters. Moreover, we show how the combination of DUNE and ESSnuSB can help further improve the sensitivities on the nonunitarity parameters.