A Study Of The Reactor Antineutrino Anomaly At Daya Bay

The existence of neutrino oscillations indicates that neutrinos are massive,implying that the Standard Model is incomplete,or there is new physics beyond it.The precision measurement of neutrino oscillations is significant for revealing the nature and property of neutrinos.The intensive flux of electron antineutrino generated by the nulcear fis-sions inside the nuclear reactor is an ideal source for observing the neutrino oscillations.However,the measurements of reactor neutrino flux at a short baseline deviate from the predictions of present reactor model in the framework of three-flavor neutrino oscillations,with an anomalous deficit?referred to as the“reactor antineutrino anomaly”?.Utilizing the near detectors of the Daya Bay reactor neutrino experiment with the short baseline varying from 300 to 500 meters away from the reactor,we conducted a precision abso-lute measurement on the reactor antineutrino flux and spectrum,and studied the reactor antineutrino anomaly.This thesis presents the studies in the following three aspects.1)We analyzed the data sample of 1230 days in the Daya Bay near detectors and successfully observed more than two millions of reactor antineutrino events.The signal has a signature of a prompt signal of positron and a delayed signal of neutron capture on nuclei from the inverse beta decay?IBD?reaction.We used the²⁴¹Am-¹³C and²⁴¹Am-⁹Be sources to generate neutrons with different kinetic energy and independently simulate the IBD delayed signals inside the detector.The study of the correlations between the delayed spectrum and the neutron captures on various isotopes eventually enable us to obtain precision Monte Carlo simulation for the processes of neutron propagation in different detector materials and the de-excitation of nuclei after capturing the neutrons,improving the entire agreement between simulations and real detector performance.This led to a reduction of the systematic uncertainty by 40%on the absolute efficiency of IBD detection,hence improved the measurement precision of antineutrino flux.2)We did a precision measurement on the flux of reactor antineutrinos in the Daya Bay near detectors and compared it with the prediction from the theoretical model,with a measurement to prediction ratio as R=0.952±0.014?exp.?±0.023?theo.?,in which the theoretical uncertainty dominates.This confirmed the reactor antineutrino anomaly in Daya Bay as those observed in the other short-baseline reactor neutrino experiments.Furthermore,we measured the correlations of changes in antineutrino flux and the reactor nuclear fuel evolution,and studied the possible reasons for reactor antineutrino anomaly in the two aspects of existence of sterile neutrino and the bias in model prediction of antineutrino yields from fissionable isotopes.3)We did a precision measurement of the reactor antineutrino spectrum,observing an anomalous structure deviating from the reactor model prediction.We studied the correlations of changes in antineutrino spectra and the reactor nuclear fuel evolution.The observation disfavored the hypothesis that the anomalous structure is caused by only one fissionable isotope of²³⁵U or²³⁹Pu.