The Study Of The BESⅢ Track Fitting And The Simulation Of CGEM Detector

The Beijing Electron Positron Collider(BEPCII) and Beijing Spectrometer III(BESIII) are experimental apparatus which is running stably at the tau-charm energy region in high energy physics with the highest luminosity. As the center tracking detector of BESIII, the main function of main drift chamber(MDC) is to measure the vertex, momentum and energy loss of charged particles.The MDC offline data processing system is an important component of the BESIII Offline Software System(BOSS). It consists of calibration, track finding, track fitting and track matching. The track fitting is based on Kalman Filter and it’s a least-square residual fit. The recursive property enables it to handle the effects from multiple scattering, energy loss and nonuniform magnetic field. The algorithm will give a more accurate track parameters and covariant error matrices. But the failure rate of the track fitting is high for low momentum tracks. In this paper the reasons for the failure and the improvement solutions will be studied.The significant loss of hits is the main reason of the failure of the track fitting after analyzing the reconstruction results. There are three kinds of improvement projects:try to enlarge the threshold of the layers in which the hits are discarded continuously; use the "reference track" offered by track finding as the initial states of the system to filter; Gauss_sum filter will be used instead of the linear Kalman filter. Compared with the original algorithm, these three methods can reduce the failure rate of the track fitting, but the momentum resolution will become worse. Since the improvement projects cannot take the failure rate and the spatial momentum resolution into account at the same time, we need a deeper understanding of the reasons for the fitting failure in order to make a better improvement in the track fitting.The inner chamber is encountering a serious aging problem. The gain and the spatial resolution decreased year by year, the hit efficiency was reduced at the same time. So it is urgent to update the inner chamber. As one of the important candidate for the update of the inner chamber, the signal acquisition and readout of a CGEM detector are different from the MDC. Therefore it is necessary to develop a new data processing software, including simulation, reconstruction and calibration. The building of digital model depends on some parameters, such as the gain, the Lorenz angle and the size and position of the multiplied electrons cluster in the anode. So a simulation study of the response of CGEM detector based on Garfield++ software is another aspect of this paper.The preliminary results of the simulation study of CGEM are received. According to the current design, the gain of CGEM detector is about 103～105; the Lorentz angle of electrons is about 20.2 degree when the magnet field is IT; The simulation of a π- with 1GeV/c momentum traversing through the CGEM detector displays that the offset of the center of the multiplied electrons cluster is 0.24cm in φ-direction and the width of the cluster is 0.027cm while the offset is extremely small and the width is 7.4×10-4 cm in z-direction; the incident angle of track has little influence on the offset of the cluster; the greater the entrance angle, the smaller the cluster is.The results above help us to understand the specific response performance, and they can provide the reference basis for the building of digital model of CGEM detector and the study of reconstruction.