Unitarity Tests of the Neutrino Mixing Matrix
We discuss unitarity tests of the neutrino mixing (PMNS) matrix. We show that the combination of solar neutrino experiments, medium-baseline and short-baseline reactor antineutrino experiments make it possible to perform the first direct unitarity test of the PMNS matrix. In particular, the measurements of Daya Bay and JUNO (a next generation medium-baseline reactor experiment) will lay the foundation of a precise unitarity test of |Ue1| 2 + |Ue2| 2 + |Ue3| 2 = 1. Furthermore, the precision measurement of sin 2 2�13 in both thee disappearance and thee appearance (from a �µ beam) channels will provide an indirect unitarity test of the PMNS matrix. Together with the search for appearance/disappearance at very short distances, these tests could provide important information about the possible new physics beyond the three-neutrino model. Introduction: In the past decades our understand- ing of neutrinos has advanced dramatically. Initially, neutrinos were thought to be massless, since only left- handed neutrinos and right-handed antineutrinos were detected in experiments (1). The existence of non-zero neutrino masses and the neutrino mixing were then suc- cessfully established through the observation of neutrino flavor oscillations. Recent reviews can be found e.g. in Ref. (2, 3). In the three-neutrino framework, the oscil- lations are characterized by the neutrino mixing (com- monly referred to as the Pontecorvo-Maki-Nakagawa- Sakata or PMNS in short) matrix (4-6) and two neu- trino mass-squared differences (�m 2 = m 2 − m 2 and �m 2 = m 2 − m 2 ). The PMNS matrix UPMNS (or U in short),