Study On Vertex And Resonance Particle Reconstruction At BESIII

The Beijing Electron Positron Collider(BEPC) and Beijing Spectrometer (BES) have been upgraded to BEPCⅡ/BESⅢ.The raw collision data have been collected at a high luminosity since July,2008.Event reconstruction,and especially vertex reconstruction,is an indispensable and important part of BESⅢdata analysis system,and also a precondition for subsequent physics analysis.BESⅢdoes not include a vertex detector in its overall design.The hit information of tracks that closest to the origin locates at the Main Drift Chamber(MDC) of BESⅢ,thus the decay vertices of resonance particles cannot be detected directly. The key problems of vertex reconstruction are:how to optimize track parameters of daughter particles based on the fact that they derive from a common decay point;how to precisely reconstruct the mass and lifetime of relatively long lifetime resonance particles;how to determine strict primary vertex positions and effectively remove the ghost tracks generated from track reconstruction,to suppress backgrounds so as to analysis signal events.High-precision primary vertex reconstruction will be able to determine the average of beam profile.In order to solve the above problems on vertex and resonance particle reconstruction at BESⅢ,The research works of this dissertation are:(1) Reconstruction of relatively long lifetime resonance particlesThe relatively long lifetime resonance particles,such as K_s~0 and∧/(?),etc,are abundantly produced atτ-charm energy region at BESⅢ.The reconstruction of resonance particles is actually reconstruction of decay vertices and kinematic fitting between the production and decay vertices.In this thesis,a global vertex fitting algorithm based on the Least Squares Method is presented,in which we also study the effects of vertex constraints,kinematic equations and the calculation of track parameters of resonance particles.A global vertex reconstruction program is designed and implemented using C++ language under the framework of BESⅢ offline Software System,BOSS.The results of resonance particle reconstruction based on Monte Carlo simulation are given.The cut on decay length is studied in order to suppress the backgrounds in physics analysis,and the reconstruction efficiency for their lifetime measurements is discussed.In addition,the reconstruction of cascading decay particleΞ,is also studied.It further verifies the applicability and feasibility of the algorithm.(2) Vertex reconstruction based on Kalman filter technique and vertex finding algorithmThe purpose of primary vertex reconstruction is to determine the interaction points of events,and then further determine the average of beam profile in high energy physics experiments.The information of a general interaction point is a critical parameter for reconstruction of secondary particles decayed from that point.In the thesis,a BESⅢvertex fitting algorithm based on Kalman filter technique is developed for handling the problems of ghost tracks and badly fitted tracks.The corresponding program is discussed,and its algorithm parameters are optimized using simulation data.A vertex finding algorithm which is to sort a set of tracks into subsets that share a common point is also proposed.(3) Analysis of vertex reconstruction using raw collision data at BE-SⅢFor checking the performance of vertex reconstruction algorithms,the beam bunch positions reconstructed from raw collision data collected at BESⅢare discussed.The procedure of raw collision data processing is described in detail, and for physics analysis,an optimized condition of selecting fitted tracks based on the results of primary vertex reconstruction is proposed.The preliminary results of resonance particle reconstruction from raw data is also given and analyzed.In order to achieve the time requirements in BESⅢData Quality Monitoring(DQM) system,a fast vertex fitting algorithm is developed and its results are compared with those of the global vertex fitting using simulation data.