Improving CP Measurement with THEIA and Muon Decay at Rest

TL;DR

This paper investigates using the THEIA detector with a muon decay at rest source to measure leptonic CP violation more precisely, especially when combined with DUNE, by leveraging low-energy neutrino beams that are unaffected by matter effects.

Contribution

It introduces a novel experimental setup combining THEIA with a muon decay at rest source to improve CP phase measurement accuracy, distinct from previous high-energy beam approaches.

Findings

Combining THEIA with DUNE reduces CP phase uncertainty significantly.

The muon decay at rest source provides a clean measurement of the CP phase without matter effect contamination.

For DUNE + μTHEIA-100, the CP uncertainty can be better than 8 degrees.

Abstract

We explore the possibility of using the recently proposed THEIA detector to measure the $\bar \nu_\mu \rightarrow \bar \nu_e$ oscillation with neutrinos from a muon decay at rest ($\mu$DAR) source to improve the leptonic CP phase measurement. Due to its intrinsic low-energy beam, this $\mu$THEIA configuration ($\mu$DAR neutrinos at THEIA) is only sensitive to the genuine leptonic CP phase $\delta_D$ and not contaminated by the matter effect. With detailed study of neutrino energy reconstruction and backgrounds at the THEIA detector, we find that the combination with the high-energy DUNE can significantly reduce the CP uncertainty, especially around the maximal CP violation cases $\delta_D = \pm 90^\circ$. Both the $\mu$THEIA-25 with 17kt and $\mu$THEIA-100 with 70kt fiducial volumes are considered. For DUNE + $\mu$THEIA-100, the CP uncertainty can be better than $8^\circ$.