Observing muon neutrino to electron neutrino oscillations in the NOnuA Experiment.

摘要

Neutrino oscillations offers an insight on new physics beyond the Standard Model. The three mixing angles (theta12, theta13 and theta23) and the two mass splittings (Deltam 122 and Deltam23 2) have been measured by different neutrino oscillation experiments. Some other parameters including the mass ordering of different neutrino mass eigenstates and the CP violation phase are still unknown.;NOnuA is a long-baseline accelerator neutrino experiment, using neutrinos from the NuMI beam at Fermilab. The experiment is equipped with two functionally identical detectors about 810 kilometers apart and 14 mrad off the beam axis. In this configuration, the muon neutrinos from the NuMI beam reach the disappearance maximum in the far detector and a small fraction of that oscillates into electron neutrinos. The sensitivity to the mass ordering and CP violation phase determination is greatly enhanced. This thesis presents the nue appearance analysis using the neutrino data collected with the NOnuA experiment between February 2014 and May 2015, which corresponds to 3.45 x 10 20 protons-on-target (POT). The nue appearance analysis is performed by comparing the observed nu e CC-like events to the estimated background at the far detector. The total background is predicted to be 0.95 events with 0.89 originated from beam events and 0.06 from cosmic ray events. The beam background is obtained by extrapolating near detector data through different oscillation channels, while the cosmic ray background is calculated based on out-of-time NuMI trigger data. A total of 6 electron neutrino candidates are observed in the end at the far detector which represents 3.3 sigma excess over the predicted background. The NOnuA result disfavors inverted mass hierarchy for delta cp ∈ [0,0.6pi] at 90% C.L.