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Improving CP Measurement with THEIA and Muon Decay at Rest

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