Experimental Studies On Low Energy Electron Antineutrinos And Related Physics

Electron antineutrinos within the energy range of several to several hundreds of MeV are composed of those from supernovae or past supernovae,the Earth,reactors,and atmospheric neutrinos.They provide a unique probe into the dynamic of sources as well as new phenomena beyond the Standard Model of particle physics.One of the interactions with the largest cross section for neutrino detection in this energy range is inverse beta decay(IBD),which produces a positron and a neutron.The detection of neutron is crucial for the separation between signal and backgrounds.This thesis presents studies based on the analysis of low energy signal and neutron production from neutrino interactions.· We used LEDs to calibrate the water scattering and absorption properties for the water Cherenkov detector Super-Kamiokande(SK).We employed radioactive sources to correct the bias introduced by the long-term gain increase of PMTs,and implemented a new algorithm to the energy reconstruction process,all of which reduced the systematics as a consequence.· With SK-IV data of 2,778 days,a search for SRN signal has been performed by detecting the neutron capture signal.We improved the neutron tagging algorithm by reconstructions of the neutron capture vertex based on time-of-flight subtracted PMT hit time and the primary vertex respectively,and optimized the determination of multi-variate cut.The signal-background discrimination power of the algorithm was significantly enhanced,and the signal efficiency was increased from 8.8% to19.3%.The SRN candidates were compared with the expectations of backgrounds including reactor neutrinos,spallation backgrounds,atmospheric neutrino induced backgrounds,and accidental coincidence.The result was consistent with null signal hypothesis.An upperlimit was derived,giving the best limits for the SRN search.· Atmospheric neutrinos interact with oxygen nuclei in water,producing gammas and neutrons,which imposes an important background for SRN detection by the mis-identification of gammas as positrons.Based on the SRN sample except for Cherenkov angle cut,we obtained NCQE candidates with Cherenkov angle > 50?.The mean NCQE cross section from atmospheric neutrinos on oxygen was measured to be(0.95 ± 0.12(stat.)+0.49-0.32(sys.))× 10-38cm2,consistent with the theoretical prediction.· We performed a calculation on geoneutrinos fluxes generated in the Earth and analyzed the oscillation impact with both vacuum approximation and MSW effect.Considering the backgrounds of reactor neutrinos and(?,n)interaction,an prediction has been made for the geoneutrino and U/Th ratio sensitivities in a liquid scintillator detector at the Jinping underground lab in Sichuan,China.These measurements will provide powerful constraints on the radiogenic heat budget of the Earth as well as tests on Earth models.