Improving CP Measurement with THEIA and Muon Decay at Rest

Abstract

We explore the possibility of using the recently proposed THEIA detector to measure the $\bar{\hat{I}}{½}_{\hat{I}}¼\to \bar{\hat{I}}{½}_e$ oscillation with neutrinos from a muon decay at rest ($\hat{I}¼$DAR) source to improve the leptonic CP phase measurement. Due to its intrinsic low-energy beam, this $\hat{I}¼$THEIA configuration ($\hat{I}¼$DAR neutrinos at THEIA) is only sensitive to the genuine leptonic CP phase $\hat{I}{´}_D$ 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 $\hat{I}{´}_D=\pm 90^{\circ }$. Both the $\hat{I}¼$THEIA-25 with 17kt and $\hat{I}¼$THEIA-100 with 70kt fiducial volumes are considered. For DUNE + $\hat{I}¼$THEIA-100, the CP uncertainty can be better than $8^{\circ }$.

26 pages, 17 figures, and 1 table