The Leptonic Dirac CP Phase from Residual Symmetry and Measurement with Muon Decay at Rest
Shao-Feng Ge (MPIK)

With the 1-3 mixing angle measured at reator neutrino experiments, Daya Bay and RENO, there are still three unknown oscillation variables, the neutrino mass hierarchy, the octant of the atmospheric mixing angle, and the leptonic CP phase. Of these three, the CP phase is the most difficult to measure precisely and important for distinguishing flavor symmetries. In this talk I will first talk about residual symmetries, which takes the role as low energy effective field theory for neutrino mixing and is analogy of custodial symmetry in the electroweak sector, to predict the CP phase from the measured mixing angles and then introduce a new type of neutrino experiment muDAR (muon decay at rest) to measure the CP phase. Currently, T2K and NOvA are designed for this measurement. Nevertheless, they suffer from degeneracy and efficiency problems. The situation can be improved by adding a muDAR source. A combination TNT2K, with T2(H)K running in neutrino mode and muDAR in anti-neutrino mode using the same detector(s), can measure the CP phase more precisely and break the degeneracy between delta and 180^o - delta. The same configuration can also apply to next-generation medium baseline reactor neutrino experiments like JUNO and RENO-50, enhancing their physics potential from mass hierarchy to also CP phase. With only one source and no extra detector, this dessign is much better than DAEdLAS which requires 3 sources, but only 20% duty factor and 4 times higher luminosity for each. In addition, TNT2K can partially break the contamination from non-unitarity, which can make the CP sensitivity at T2K/NOvA to completely disappear, and a further addition of a near detector can fully retrieve the ability of CP measurement.