DE/dx Calibration In BESâ…¢ And Monte Carlo Study Of J/ψâ†'γη_c Branch

Beijing Electron Positron Collider II (BEPCII) and Beijing Spectrometer III (BESâ…¢) are facilities with high luminosity operating at ι-charm energy area, for investigating the third generation lepton ι and the third generation charm quark c. Two works based on BEPCII/BESIII are discussed in this thesis:one is the improvement of dE/dx calibration; the other is the Monte Carlo study on the measurement of the branching ratio of J/ψâ†'γ ηcradiation decay.At BESâ…¢, the variance of dE/dx information between different particles are employed to identify particles. Furthermore, the process of dE/dx calibration and the correction is important to get unbiased dE/dx values. The relevant algorithm libraries of BESâ…¢ of ffline software are stable and well running. In this thesis the distribution of dE/dx gain of run 29677-30523 with or without correction are shown and analyzed. Rad-Bhabha event is usually used as sample data of doca-angle correcting and space charge effect correction. On the basis of the existing algorithms, some amelioration has been done to optimize the rad-Bhabha event selection criteria. These criteria can ensure large statistical magnitude, meanwhile, eliminate more background. On the other hand, uneven distribution of momentum and cos(0) are damaging factors to dE/dx calibration. To tackle this problem, the method so-called data filter is tested with real data. It accelerates course of dE/dx calibration, improves the stability of the calibration result, increases the universality of the algorithm code. Furthermore, it provides a good foundation for the next exquisite calibration.Study of J/ψâ†'γηc decay process will help people to understand the charmonium decay mechanism and to test Quantum Color Dynamics. The branching ratio of this decay is studied by Monte Carlo in this thesis. Two steps are adopted to select signal events here. At first, about twenty thousand signal MC data of J/ψ-> γηc is generated by BESâ…¢ offline software. Combining these data sample with the existing theory, several event selection criteria, such as photon tag, energy deposition cut, etc are studied. At second, the Ï€0 and η veto criteria are identified by analysis of the topology information of J/ψ inclusive MC data in 2009 and the decay information from the Particle Data Group (PDG). Finally, these selection criteria are applied to the J/ψ inclusive MC data in 2009 in order to get the photon energy distribution and the ηc invariant mass distribution. Finally, a function is employed to fit ηc invariant mass distribution, and several parameters are calculated accurately. In the fitting histograms, a small signal peak is observable. This result is very consistent with the theoretical expectation. This study provides the research direction for the real data analysis in the near future.