Research Of The Beam And Detector For The COMET Phase-I Experiment

As of now,the neutrino oscillation phenomenon is the only new physics beyond the standard model(SM)confirmed by experiments in a laboratory.It rejects the con-servation of family lepton number in the SM.The origin of neutrino masses remains as a mystery.However many new physics models attempt to work for an explanation,and many of them predict a sizable effect of charged lepton flavour violation(CLFV).In the minimal extension of the SM with the consideration of neutrino masses,the rate of CLFV processes is practically negligible.While in new physics models,the predic-tion of CLFV processes can reach the level to be searched using current experimental techniques.This makes CLFV processes an effective probe to new physics beyond the SM.The COMET experiment is aimed to search for a special CLFV process,the?^-N?e^-N process.This paper contains research works about the design and optimization of the muon beam line and the detector for the COMET Phase-I experiment,the performance study of the detector,the prototype test of the drift chamber,and the study of the sensitivity and background for the search of?^-N?e^-N in COMET Phase-I experiment.The muon beam in COMET Phase-I experiment is expected to be the most inten-sive one in the world.The proton target system,with a superconducting solenoid as the capture system and a thick target as the proton target,is the state-of-the-art technique.The property of the muon beam from such a target system still has much to be under-stood.This work covers the Monte Carlo study starting from simulation the proton on target and muon beam transportation physics,and an optimization of the proton target,the muon stopping target.It is the first time the muon beam from a proton target made of carbon using thick target system was studied.According to the study result,the collimator system and shielding systems along the muon beam line were designed,and the radiation level in the detector region was reduced to an acceptable level.The COMET Phase-I detector is a dedicated detector designed for searching for the?^-N?e^-N process with COMET Phase-I beam line.It consists of a drift chamber and a trigger detector.This work includes the studies starting from the conceptual design of the Phase-I detector based on the physical requirements.The geometry of the detector was carefully optimized to control the acceptance of the signal electron at the level of 30%.Combining with the muon beam study results,this work covers studies of the beam irradiation effect to the detector for the first time.To overcome the high hit rate in the trigger detector,a special shielding layer was designed,and a new triggering scheme using four-fold coincidence was proposed.At last,the noise occupancy level in the drift chamber and trigger rate from the detector were estimated as 18%and 2k Hz respectively,fulfilling the requirements from the experiment program.The physics goal of the COMET Phase-I experiment is to search for the?^-N?e^-N process with a single event sensitivity of 3.1×10^-15.Such a high sensitivity proposed a stringent requirement to the momentum resolution of the detector.Specif-ically,the tail component of the momentum resolution will be the key factor in the performance of the detector.This work includes a study of trajectory reconstruction in the drift chamber.A series of quality cuts were proposed to control the fitting quality while keeping the reconstruction efficiency of the signal electrons.The tail component was controlled within the acceptable level.Besides,this work also contains a prelimi-nary study of the track finding method,especially in the cases where the signal electron leaves multiple turns in the detector.The drift chamber for the COMET Phase-I experiment was a unique design with a new combination of cell structure,wire size and the gas mixture.The performance study of such a drift chamber requires a prototype test.The author participated the pro-totype test on-site work,and took charge of the calibration,reconstruction,and analysis work in the off-line state.The analysis result shows that with the baseline choice of the gas mixture,the spatial resolution and hit efficiency of the drift chamber are 105?m and 99%respectively,and the energy deposit resolution can reach 4%,which all meet the requirement from the experiment.This work also includes the performance study of another two types of gas mixtures,as backup plans for the experiment.At last,with the consideration of the above studies,a study of the physics sen-sitivity and background for the COMET Phase-I experiment was completed.All pos-sible background sources were examined,with specific methods to control the back-ground contamination proposed.The study result suggests that using the 3.2 k W proton beam,the COMET Phase-I experiment can reach its targeted single event sensitivity of3.1×10^-15for the search of the?^-N?e^-N process.Within the whole physics run,the number of background events were estimated as around 0.03.All the above studies have been included in the Technical Design Report of the COMET Phase-I experiment.