Measurement Of Sin²(2θ₁₃) Via Neutron Captured On Hydrogen At Daya Bay Reactor Neutrino Experiment

Neutrino is the elementary particle to c:onstitute the universe.However,neu?trino is t,he least under-stood particle iup to now.In 1998,Super-Kainiokande discovered the compelling evidence in favour of neutrino oscillations,and the"phenomenon of neutrino oscillations" is the only one experimental evidence be-yond the Standard Model of particle physics.Neutrino oscillation can be described in terms of six parameters in three neu-tron framework:three mixing angles(θ₁₂,θ₂₃,θ₁₃,),two mass-squared splitting(Δm₂₁²,Δm322)and CP violating phase(δ_CP).Before 2012,θ₁₃,δ_CP and the sign of Δm322 were unknown.The θ₁₃ is the key to determine the CP violation and the next generation of neutrino experiments.Therefore,Daya Bay reactor neutrino experiment aimed at measuring sin²(2θ₁₃)to 0.01 at 90%at C.L..Daya Bay Collaboration reported the the first result after 55 days data-taking,sin²(2θ₁₃)=0.092 ± 0.016（stat.）± 0.005（syst,.）,neutrino mixing angle θ₁₃ is non-zero with 5.2 standard deviation,and it is the world’s first discovery of this new oscillation.Daya Bay reactor neutrino experiment detects neutrino via inverse beta decay reaction（IBD）,vc,+ p →7 e+ +n.The positron annihilates quickly with electron,which is tagged as prompt signal,and cdelayed signal from the neutron thermal-ization and then captured on gadolinium（nGd）or hydrogen（nH）,releasing-8 MeV γ-cascade or 2.22 MeV γ.The prompt and delayed signal forms a correlat-ed signals.Neutrinos can be detected through selectiing these correlated signals based on criteria of energy and time.This thesis adopts nH sample to measure the mixing angle 013 by comparing the rate of neutrinos in near and far sites.The level of backgrounds is much higher in nH sample comparing with nGd sample since its longer neutron capture time（～200 μis）and lower energy γ ray（～2.2MeV）.However,nH sample is also important since it can provide an independent measurement of sin²(2θ₁₃)due to its different data sample and systematic uncertainties.In order to reduce accidental backgrounds,different selection criteria is used.The energy of prompt signal starts at 1.5 MeV,the distance between prompt and delayed signal is less than 50 centimeter.Firstly,the gives an overview of selection criteria and nH sample,then shows the results and uncertainties of estimated backgrounds（accidental,fast neutron,⁹Li/⁸He,AmC）respectively,the accidental backgrounds is the largest backgrounds among all kinds of backgrounds,it is about 12%in near sites and 54%in far site.Secondly,it provides an introduction to the efficiencies of selecting nH IBD and uncorrelated uncertainties,and also updates prompt energy cut efficiency since the neutrino oscillation effect at 1.5 MeV.Finally,617 days physical data of Daya Bay reactor neutrino experiment are analyzed and the rate analysis yields sin²(2θ₁₃)= 0.071 ± 0.011.