Studies Of Cosmic Ray Muon And Atmospheric Neutrino Backgrounds At Neutrino Experiment

The Daya Bay Reactor Neutrino Experiment is designed to measure neu-trino oscillation related to the mixing angle ?13 using anti-neutrinos produced by the reactors of the Daya Bay Nuclear Power Plant and the Ling Ao Nu-clear Power Plant.The major physics goal is a precise measurement of sin22?13 to 0.01 or better.Daya Bay is a large international experiment,including 230 collaborators from all around the world.In March 8,2012,the Daya Bay an-nounced the first discovery of a third neutrino oscillation and measured sin22?13 =0.092±0.016(stat.)±0.005(syst.).The result was selected as one of the ten major scientific progress of 2012 by Nature magazine.Daya Bay's goals require very low background rate.It's necessary to study the background clearly,particularly for productions induced by cosmic-ray muons.Neutrons induced by cosmic-ray muons are a significant background for un-derground experiments studying neutrino oscillations,neutrino-less double beta decay,dark matter and so on.The data from Daya Bay detectors allows to make a competitive measurement of neutron production by cosmogenic muons for three experimental sites.The neutron yield in Daya Bay's liquid scintilla-tor is measured to be Yn=(10.26 ± 0.86)x 10-5,(10.22 ± 0.87)x 10-5 and(17.03 ± 1.22)x 10-5?-1g-1cm2 at depths of 250,265,and 860 meters-water-equivalent.Using a power law function Yn = aE?b fits the measurements including the Daya Bay measurements,yielding b=(0.77 ± 0.03)for the dependence of the neutron yield on muon energy.It's meaning for prediction of corresponding neutron in the Jiangmen Underground Neutrino Observatory(JUNO).The com-parisons between other measurements and predictions from Geant4 and FLUKA are performed,showing some discrepancies between data and MC.The discrepan-cies indicate Geant4 and FLUKA are not complete to simulate neutron produc-tion process.For solving the issue,we use the accurate measurement of neutron yield of Daya Bay to discuss the physical process of neutron production in liq-uid scintillation detector in Geant4,filling the gap between comparative study of different physical processes and experimental measurements.The study can encourage the improvement of hadron physics models,providing more accurate muon simulation for the further experiments,such as JUNO.JUNO is another medium baseline reactor neutrino experiment after Daya Bay,which is a major basic scientific research project in China.The major physics goals are the determination of the neutrino mass hierarchy and precise measurement of mixing parameters of neutrino oscillation.In addition,JUNO detector is also sensitive to atmospheric neutrinos,supernova neutrino,diffuse supernova neutrino background(DSNB)and so on.For the studies of above various physical subjects,particularly for sensitivity study.how to effectively subtract and suppress background is a critical issue.Using the results at Daya Bay neutron background study and accumulated methods,we will continue to study the prediction of JUNO cosmic ray background,including neutron background induced by muon and atmospheric neutrino background.Fast neutron background induced by muon is one of significant backgrounds in precision measurements of neutrinos at JUNO,such as reactor neutrino,DSNB and so on.The simulated muon sample is large enough so that the statistical error is low and our study of prediction of fast neutron is more accurate.The fast neutron background rate for reactor neutrino and DSNB different detected energy ranges is obtained to be 2.2 and 3.4/year/18.3kton respectively.Mean-while,we also study the prediction of atmospheric neutrino background in JUNO.neutral current(NC)interaction of atmospheric neutrino with carbon atom in liquid scintillator is the most significant source of background for DSNB sensi-tivity study.The study discovers the ratio of NC background to signal is about 20.For DSNB detected energy range,the DSNB signal rate is predicted to be 0.14/kton/year.The NC background rate is(3.3 ± 0.7)/kton/year.And the charged current background rate is 0.02/kton/year.It is the first time at the collaboration that we have systematically calculated the NC and CC processes of low energy neutrino in the detector,and have predicted the background contri-bution to DSNB detection.For simulation-based models with average neutrino energy?14MeV and under 10 years data taking,the DSNB sensitivity can reach 3?.The higher average energies or higher fraction of black-hole forming SNe can obtain better sensitivity.