Design And Development Of BESⅢ Main Drift Chamber Track Reconstruction Software

The upgrading of Beijing electron-positron collider (BEPCII) has achieved important progress. The collision of electron-positron beams has been realized at March, 2007. The peak luminosity of the upgraded BEPCII will reach 10³³cm^-2s^-1. The Beijing Spectrometer III (BESIII) operating at BEPCII is a new high precision detector built with modern detecting techniques. Its main physics goal is to study the eletroweak and strong interactions and to search for new physics at tau-charm energy region. The outcome of BESIII would help to develop particle physics substantially. The BESIII experiment will test electroweak interactions with a very high precision in both quark and lepton sectors. From the pure leptonic decays of D and D_s will help a lot in improving the precision of CKM matrix elements measurements. The measuredÏ„⁺Ï„^- cross section can be used to test the theoretical calculation of the cross section, especially the non-relativistic QED (NRQED) calculation of the interaction. This will supply us a better understanding of theÏ„⁺Ï„^- interaction near the threshold. The study of QCD in the tau-charm energy region includes following aspects: determination of basic QCD parameters such as the strong coupling constantα_s, the mass of the charm quark m_c, the high precision measurement of the light hadron spectroscopy, searching for gluonic states such as glueballs and hybrids, and the study of channonium physics by measuring the production and decay properties of the channonium states to test and develop QCD calculations. At the tau-charm energy region, there are lots of possibilities to search for new physics effect such as D or D_s production, lepton and baryon number violation processes in J/ψdecays, D⁰D|-⁰ mixing and the flavor changing neutral current (FCNC) with D data.The Main Drift Chamber (MDC) is an important sub-detectors in BES III. Its main function is to provide precise momentum measurement and dE/dx measurement for charged particles. The dE/dx measurement could be used for particle identification. The MDC consists of 43 sense wire layers, grouped into 11 super-layers(4 sense wire layers in each super-layer, and the last super-layer has 3 sense wire layers). When charged particle passing through the MDC, it interacts with the gas medium within the MDC and produces pulses on the sense wires. In this way, the position of the charged particle is measured. The MDC can measure 43 points at most for a charged particle produced at interaction point. There are two kind of sense wire layers: axial and stereo wire layers. The stereo wire layers are used to provide longitudinal measurements of the charged particle positions. The outmost sense wire layer covers a polar angle of cos9 =±0.93. The whole MDC is placed within a solenoid super conducting magnet with a magnetic field of 1.0 T. The designed spacial resolution for a single sense wire is 130μm. The designed momentum resolution at 1 GeV/c isσ_Pt/Pt = 0.5%Charged particle track reconstruction is an important process in the BES III offline data analysis. Its main purpose is to precisely determine the trajectory and momentum of a charged particle using the space points recorded by MDC. The general procedure of the charged particle track reconstruction is: 1) combine the measured space points to tracks using suitable pattern recognition method; 2) fit the tracks to the equation of motion of charged particle in magnetic field to get the momentum of the particle. In general, several charged particles will be produced during e⁺e^- collision, and the MDC itself can produce noise hits, so track reconstruction is a complex process in the offline event reconstruction. The goal of this thesis is to design and develop a high efficiency, easy to use and high precision charged particle track reconstruction program.In this thesis, we designed and developed a charged particle track reconstruction program, MdcPatRec, using C++ language and Object-oriented techniques under the frame work of BESII offline Software System, BOSS. The performance of this program was strictly checked by using the Monte Carlo generated events. It was found that the program is satisfactory for BESIII offline event reconstruction.The track reconstruction algorithm consists of track finding and track fitting. Pattern matching method is used in MdcPatRec for track finding. Firstly, the reconstruction algorithm searches for the segment in every super-layer through template matching method. The dictionary of MdcPatRec contains 8 4-hits template and 20 3-hits tem- plate. The successful matching hit-group is stored as segment which will be fitted to get the parameters. Secondly, the 2-dimension circle track finding implement with the combination of the axial segments. Circle track will be fitted with 3 parameter track fitting with least square method. Circle track and the stereo segments in the super-layers is combined to find the 3-dimension helical track followed with 5 parameter track fitting. MdcxReco is another MDC track-finding algorithm for secondary vertex particle tracking. It uses segments in three adjacent superlayer to form a trial helix. If the helix is of sufficient quality , it is extrapolated forward and backward, and segments in the same track are added. This algorithm is designed to find tracks not coming form the primary vertex and tracks with low p_T. The dictionary of MdcxReco contains 14 4-hits template and 20 3-hits template. MdcPatRec have the characteristic of modularization, reliability and robustness. The control parameters can be set during run time through jobOption file.Performances of MdcPatRec reconstruction are checked with various Monte Carlo data under BOSS release 6.1.0. The reconstruction speed is one of the most important performances. We measured the CPU time of one track is about 14 ms for single Muon event. The average reconstruction CPU time of J/ψ→anything event is 150ms. The process of track fitting occupies about 60% of reconstruction time. J/ψ→μ⁺μ^-event take about 50MB memory without memory leakage using large number of data sample.Tracking efficiency is one of the most important tracking performance. For single track of e^-,μ^-,Ï€^- at p_T greater than 0.12 GeV/c , tracking efficiency is above 90%. MdcPatRec gives high tracking efficiency at various dip angle even at large dip angle with J/ψ→μ⁺μ^- event.MdcPatRec describe helical track with 5 track parameter (d₀,φ₀,κ, z₀, tanλ). For singleμ^- at 1 GeV/c , check the reconstructed parameter with its true value in MC. The resolution ofâ–³d₀ isσ(â–³d₀) = 0.17mm; the resolution of△φ₀ isσ(△φ₀) = 2.47 mRad; transverse momentum resolutionσ(â–³p_T) = 5.31 MeV/c ; vertex resolution of Z directionσ(z₀) = 0.79 mm.We use momentum and spatial resolution to check the precision of the reconstruc- tion algorithm. The momentum resolution of J/ψ→μ⁺,μ^- event isσ(p)/p = 0.60% after the double Gaussian fitting average. For 1 GeV/c Muon, the momentum resolutionσ(p)/p = 0.48% which reach the design value of MDC. And for input spatial resolution of 120μm, the spatial resolution of J/ψ→n⁺n^- event is 118.8μm.The simulation of drift chamber noise is based on noise type of BESII. We use electron event with different noise level at 1 GeV/c and 200 MeV/c . When noise level below 90%, tracking performance of noise sample is satisfied even at high noise level. Noise sample with noise level 20% will reduce the hit number of reconstruction track about 2.Effect of cell efficiency, bunch smear and event start time also studied. With cell efficiency of 98% MdcPatRec gives almost same performance with 100% except lost hits in the last super-layer. Multi-bunch and noise will reduce the tracking efficiency; noise effect the momentum resolution and spatial resolution; and vertex resolution changes under bunch smear.Tracing performance with physic sample has been checked such as lepton tracking and PID efficiency withψ(2S)→π⁺Ï€^- J/ψ→π⁺Ï€^- l⁺l^- and secondary vertex tracking result given by MdcxReco package. K_s⁰ reconstruction efficiency from K_s⁰→π⁺Ï€^- has been checked. Decay form K_s⁰→π⁺Ï€^- and (?)^-→∧π^- ,∨→pÏ€^- also been studied. MdcxReco make a very good supplement to the tracking algorithm.The MDC track reconstruction algorithm MdcPatRec has been developed under BOSS environment. Performances are check with large number of MC data. With the simulation data, the tracking performances such as efficiencies and momentum resolution have been studied and results are consistent with parameters from detector design. The algorithm is also proved to be robust enough to process data with severe background expected by the BES III experiment.The preliminary physics result is satisfied from tracking point of view.