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 $${\bar{\nu }}_\mu \rightarrow {\bar{\nu }}_e$$ ν ¯ μ → ν ¯ e oscillation with neutrinos from a muon decay at rest ($$\mu $$ μ DAR) source to improve the leptonic CP phase measurement. Due to its intrinsic low-energy beam, this $$\mu $$ μ THEIA configuration ($$\mu $$ μ DAR neutrinos at THEIA) is only sensitive to the genuine leptonic CP phase $$\delta _D$$ 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 $$\delta _D = \pm 90^\circ $$ δ D = ± 90 ∘ . Both the $$\mu $$ μ THEIA-25 with 17 kt and $$\mu $$ μ THEIA-100 with 70 kt fiducial volumes are considered. For DUNE + $$\mu $$ μ THEIA-100, the CP uncertainty can be better than $$8^\circ $$ 8 ∘ .