BESⅢ主漂移室径迹重建软件的设计与开发
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摘要
北京正负电子对撞机BEPCⅡ正在建造之中，并且已经于2007年3月实现了正负电子束对撞。北京谱仪Ⅲ(BESⅢ)是BEPCⅡ上的一个全新的采用现代探测技术建造的高精度探测器，其主要物理目标是在τ-粲能区进行弱电相互作用和强相互作用研究以及新物理的寻找，其实验结果将极大地促进粒子物理的发展。BESⅢ将在夸克和轻子两个方面对电弱相互作用理论进行精确的检验：通过D和D_s介子的纯轻子衰变可极大地提高对CKM矩阵元的测量精度；对τ~+τ~-产生截面的测量结果可用于验证非相对论QED(NRQED)理论对该截面的计算结果，这将有助于我们更好地理解τ~+τ~-在阈值附近的相互作用机制。在τ-粲能区对QCD的研究包括以下几个方面：基本QCD參数的测量，如强相互作用耦合常数α_s，粲夸克质量m_c等；轻强子谱的精确测量；寻找QCD预言的各种含胶子的态，如胶子球、混杂态等；通过测量粲偶素的产生和衰变研究粲偶素的性质以检验和发展QCD理论的计算。在τ-粲能区也存在着寻找新物理的可能性，如D或D_s的产生、在J/ψ衰变过程中的轻子和重子数破坏过程、D~0(?)混合、味道改变的中性流(FCNC)等。
主漂移室(Main Drift Chamber，MDC)是BESⅢ的重要的子探测器，其主要功能是精确测量带电粒子的动量和能量损失(用于粒子鉴别)。主漂移室共有43个信号丝层，每4个信号丝层被组合成为一个超层(最后一个超层包含3个信号丝层)。带电粒子穿过漂移室时，与室内气体介质发生相互作用，在信号丝上产生脉冲信号，从而探测到带电粒子的空间位置。43个信号丝层可对带电粒子的径迹进行最多43次的测量。最外层信号丝所覆盖的极角达到cosθ=±0.93。信号丝层分为轴丝层和斜丝层两种类型，斜丝层用于测量带电粒子的纵向位置。整个MDC位于场强为1.0T的超导磁铁内。主漂移室的单丝空间分辨率设计值为130μm；动量为1GeV/c时，动量分辨率的设计值为σ_(pt)/Pt=0.5％。
带电粒子的径迹重建是BESⅢ离线数据处理过程中的重要环节，其主要任务是利用MDC所记录的带电粒子的空间位置信息精确地重建带电粒子的径迹和动量。径迹重建的步骤是：1)采用适当的模式识别方法将MDC所记录的带电粒子的击中组合成径迹；2)利用带电粒子在磁场中的运动方程进行径迹拟和，从而得到带电粒子的动量。由于在e~+e~-对撞过程中一般会产生多个带电粒子且MDC也会产生一些噪声信号，因此，带电粒子径迹重建是离线数据处理中较为复杂的过程。本论文的主要目的就是为BESⅢ实验设计开发高效、实用和精确的主漂移室带电粒子重建程序。
本论文的主要工作是：在BESⅢ离线数据处理框架(BesⅢOffline Software System，BOSS)下，利用面向对象的程序设计技术，并使用C++语言开发了主漂移室重建软件包MdcPatRec，并利用蒙特卡罗数据对其性能进行了严格的检验，证明该软件包可以很好地满足BESⅢ离线数据分析的需要。
在MdcPatRec中，径迹重建算法分为径迹寻找和径迹拟合两部分，使用基于模式匹配的方法进行径迹寻找。径迹寻找算法事先把径迹穿过超层留下的击中信号保存为模式字典，在寻迹时把击中邻居组与模式字典中的模板进行匹配，匹配成功的击中组合成为径迹段。模式字典共有4击中模式8种，3击中模式20种。径迹段连接算法把轴丝超层的径迹段连接成二维圆径迹，并把圆径迹和斜丝超层的径迹段连接成三维螺旋线径迹。圆径迹和螺旋线径迹都使用最小二乘法分别作3参数和5参数拟合，得到重建径迹。重建后的径迹要使用击中判别算法分配属于多条径迹的击中。此外，我们还针对次级顶点径迹的特点开发了MdcxReco算法用于重建非原点出发和低横动量径迹的重建。MdcxReco也是基于模式匹配方法进行径迹寻找的算法。为了适应低动量和短径迹寻迹的要求，MdcxReco的模式字典共有4击中模式14种，3击中模式20种。径迹段连接算法将每三个相邻超层的径迹段连接成一组，并以该组径迹段为种子，向内向外连接其它径迹段成为长径迹。连接后的长径迹使用最小二乘法进行径迹拟合。MdcPatRec基于面向对象技术开发，特点是模块化强，具有良好的重用性和扩展性，程序的控制参数可以在运行时通过job Option文件调整。
文中我们使用蒙特卡罗模拟数据，在BOSS离线软件版本6．1．0下，对BESⅢ主漂移室离线重建软件的结果进行了检查，并对重建性能指标进行了分析。文中对各种不同事例的重建速度进行了测量，平均重建单径迹μ的重建速度约为每条径迹14ms，对J/ψ→anything事例的重建速度为每条径迹150ms。此外，还给出程序各部分所占运行时间，径迹拟合占了重建的60％时间。MdcPatRec运行J/ψ→μ~+μ~-事例占用内存约为50MB，并且运行大量数据没有内存泄漏，可以满足BESⅢ离线重建长时间运行的要求。
·重建效率是主漂移室的重要性能之一。对于单径迹e~-，μ~-，π~-3种粒子，当横动量大于0.12GeV/c时，寻迹效率都在90％以上。而且通过对不同极角下的径迹重建效率的考察，验证了MdcPatRec在各个极角上的重建是均匀的，对于大角度出射的粒子同样具有较高的重建效率。
·主漂移室径迹重建采用5参数(d_0，Φ_0，k，z_0，tanλ)描述重建径迹。对于1GeV/c的单径迹μ~-事例，将重建径迹参数与蒙特卡罗模拟真值进行了比较，Φ_0与其真值之差的分布经过加权双高斯拟合得到的分辨为σ(△Φ_0)=2.47mRad；横动量分辨达到σ(△pT)=5.31MeV/c；z_0与其真值之差的分布σ(z_0)=0.79mm。另外我们还使用了不同橫动量下的数据检察了径迹参数分辨与橫动量的关系和径迹参数间的相关性，并给出了其它重建径迹参量的分布，各种性能都基本满足要求。
·动量分辨和空间分辨也是体现算法性能的重要指标。对于J/ψ→μ~+μ~-事例样本，重建径迹动量分辨为σ(p)/p=0.60％；对于横动量为1GeV/c的μ~-径迹，动量分辨σ(p)/p=0.48％，这与BESⅢ主漂移室的设计要求一致。当输入空间分辨的平均值为120μm时，J/ψ→μ~+μ~-事例空间分辨为118.8μm。
·主漂移室噪声的模拟使用基于BESⅡ噪声分布的噪声模型。为检查噪声对重建的影响，模拟产生了不同噪声水平上动量分别为1GeV/c和200MeV/c的单电子事例，在噪声水平90％以内，重建性能会随噪声水平的增加而缓慢变差，但即使在高噪声水平下各项性能指标均满足要求。通过与模拟数据的比对，发现在加入20％的噪声后重建击中数大约会减少2个。
·我们还研究了单元效率、顶点晃动、事例起始时间对重建结果的影响。在98％的单元效率下，重建性能不会有太大影响，但是最后一个超层会丟失一部分效率。三个束团和噪声的加入会使重建效率降低；动量分辨和空间分辨主要受噪声的影响；束团晃动对顶点分辨的影响较大。更进一步的研究将在以后的工作中继续进行。
·为了验证MdcPatRec在物理分析工作中的性能，使用蒙特卡罗数据对初步的物理结果进行了检验。主要包括ψ(2S)→π~+π~-，J/ψ→π~+π~-l~+l~-衰变道进行了轻子的重建效率、轻子鉴别效率的检查；并使用MdcxReco次级定点粒子寻迹软件包，考察了K_S~O→π~+π~-非原点出发的K_S~O粒子的寻迹效率；同时，次级顶点粒子的重建质量和寿命也根据K_S~O→π~+π~-和(?)~-→∧π~-，∧→pπ~-衰变道进行了分析；各项结果均与预期符合。通过物理分析的检查可以看到，对于理想的蒙特卡罗数据，MdcPatRec主漂移室径迹重建结果进行的物理分析工作状态良好结果正常，MdcxReco对次级顶点径迹的寻迹作了很好的补充。
主漂移室径迹重建软件MdcPatRec在BOSS框架下开发完成，已经用于BESⅢ数据分析的准备工作。我们使用了大量不同类型的数据来检查MdcPatRec的重建性能，从各方面的指标来看，主漂移重建软件MdcPatRec处于较好的状态，具有效率高，各项分辨好，速度快的优点，为其它子探测器重建和物理分析提供了较好的重建径迹参数，基本能够满足BESⅢ实验的使用要求。
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 10~(33)cm~(-2)s~(-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 measuredτ~+τ~- 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 theτ~+τ~- 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, D~0D|-~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~-,π~- 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 (d_0,φ_0,κ, z_0, tanλ). For singleμ~- at 1 GeV/c , check the reconstructed parameter with its true value in MC. The resolution of△d_0 isσ(△d_0) = 0.17mm; the resolution of△φ_0 isσ(△φ_0) = 2.47 mRad; transverse momentum resolutionσ(△p_T) = 5.31 MeV/c ; vertex resolution of Z directionσ(z_0) = 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~+π~- J/ψ→π~+π~- l~+l~- and secondary vertex tracking result given by MdcxReco package. K_s~0 reconstruction efficiency from K_s~0→π~+π~- has been checked. Decay form K_s~0→π~+π~- and (?)~-→∧π~- ,∨→pπ~- 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.
主漂移室(Main Drift Chamber，MDC)是BESⅢ的重要的子探测器，其主要功能是精确测量带电粒子的动量和能量损失(用于粒子鉴别)。主漂移室共有43个信号丝层，每4个信号丝层被组合成为一个超层(最后一个超层包含3个信号丝层)。带电粒子穿过漂移室时，与室内气体介质发生相互作用，在信号丝上产生脉冲信号，从而探测到带电粒子的空间位置。43个信号丝层可对带电粒子的径迹进行最多43次的测量。最外层信号丝所覆盖的极角达到cosθ=±0.93。信号丝层分为轴丝层和斜丝层两种类型，斜丝层用于测量带电粒子的纵向位置。整个MDC位于场强为1.0T的超导磁铁内。主漂移室的单丝空间分辨率设计值为130μm；动量为1GeV/c时，动量分辨率的设计值为σ_(pt)/Pt=0.5％。
带电粒子的径迹重建是BESⅢ离线数据处理过程中的重要环节，其主要任务是利用MDC所记录的带电粒子的空间位置信息精确地重建带电粒子的径迹和动量。径迹重建的步骤是：1)采用适当的模式识别方法将MDC所记录的带电粒子的击中组合成径迹；2)利用带电粒子在磁场中的运动方程进行径迹拟和，从而得到带电粒子的动量。由于在e~+e~-对撞过程中一般会产生多个带电粒子且MDC也会产生一些噪声信号，因此，带电粒子径迹重建是离线数据处理中较为复杂的过程。本论文的主要目的就是为BESⅢ实验设计开发高效、实用和精确的主漂移室带电粒子重建程序。
本论文的主要工作是：在BESⅢ离线数据处理框架(BesⅢOffline Software System，BOSS)下，利用面向对象的程序设计技术，并使用C++语言开发了主漂移室重建软件包MdcPatRec，并利用蒙特卡罗数据对其性能进行了严格的检验，证明该软件包可以很好地满足BESⅢ离线数据分析的需要。
在MdcPatRec中，径迹重建算法分为径迹寻找和径迹拟合两部分，使用基于模式匹配的方法进行径迹寻找。径迹寻找算法事先把径迹穿过超层留下的击中信号保存为模式字典，在寻迹时把击中邻居组与模式字典中的模板进行匹配，匹配成功的击中组合成为径迹段。模式字典共有4击中模式8种，3击中模式20种。径迹段连接算法把轴丝超层的径迹段连接成二维圆径迹，并把圆径迹和斜丝超层的径迹段连接成三维螺旋线径迹。圆径迹和螺旋线径迹都使用最小二乘法分别作3参数和5参数拟合，得到重建径迹。重建后的径迹要使用击中判别算法分配属于多条径迹的击中。此外，我们还针对次级顶点径迹的特点开发了MdcxReco算法用于重建非原点出发和低横动量径迹的重建。MdcxReco也是基于模式匹配方法进行径迹寻找的算法。为了适应低动量和短径迹寻迹的要求，MdcxReco的模式字典共有4击中模式14种，3击中模式20种。径迹段连接算法将每三个相邻超层的径迹段连接成一组，并以该组径迹段为种子，向内向外连接其它径迹段成为长径迹。连接后的长径迹使用最小二乘法进行径迹拟合。MdcPatRec基于面向对象技术开发，特点是模块化强，具有良好的重用性和扩展性，程序的控制参数可以在运行时通过job Option文件调整。
文中我们使用蒙特卡罗模拟数据，在BOSS离线软件版本6．1．0下，对BESⅢ主漂移室离线重建软件的结果进行了检查，并对重建性能指标进行了分析。文中对各种不同事例的重建速度进行了测量，平均重建单径迹μ的重建速度约为每条径迹14ms，对J/ψ→anything事例的重建速度为每条径迹150ms。此外，还给出程序各部分所占运行时间，径迹拟合占了重建的60％时间。MdcPatRec运行J/ψ→μ~+μ~-事例占用内存约为50MB，并且运行大量数据没有内存泄漏，可以满足BESⅢ离线重建长时间运行的要求。
·重建效率是主漂移室的重要性能之一。对于单径迹e~-，μ~-，π~-3种粒子，当横动量大于0.12GeV/c时，寻迹效率都在90％以上。而且通过对不同极角下的径迹重建效率的考察，验证了MdcPatRec在各个极角上的重建是均匀的，对于大角度出射的粒子同样具有较高的重建效率。
·主漂移室径迹重建采用5参数(d_0，Φ_0，k，z_0，tanλ)描述重建径迹。对于1GeV/c的单径迹μ~-事例，将重建径迹参数与蒙特卡罗模拟真值进行了比较，Φ_0与其真值之差的分布经过加权双高斯拟合得到的分辨为σ(△Φ_0)=2.47mRad；横动量分辨达到σ(△pT)=5.31MeV/c；z_0与其真值之差的分布σ(z_0)=0.79mm。另外我们还使用了不同橫动量下的数据检察了径迹参数分辨与橫动量的关系和径迹参数间的相关性，并给出了其它重建径迹参量的分布，各种性能都基本满足要求。
·动量分辨和空间分辨也是体现算法性能的重要指标。对于J/ψ→μ~+μ~-事例样本，重建径迹动量分辨为σ(p)/p=0.60％；对于横动量为1GeV/c的μ~-径迹，动量分辨σ(p)/p=0.48％，这与BESⅢ主漂移室的设计要求一致。当输入空间分辨的平均值为120μm时，J/ψ→μ~+μ~-事例空间分辨为118.8μm。
·主漂移室噪声的模拟使用基于BESⅡ噪声分布的噪声模型。为检查噪声对重建的影响，模拟产生了不同噪声水平上动量分别为1GeV/c和200MeV/c的单电子事例，在噪声水平90％以内，重建性能会随噪声水平的增加而缓慢变差，但即使在高噪声水平下各项性能指标均满足要求。通过与模拟数据的比对，发现在加入20％的噪声后重建击中数大约会减少2个。
·我们还研究了单元效率、顶点晃动、事例起始时间对重建结果的影响。在98％的单元效率下，重建性能不会有太大影响，但是最后一个超层会丟失一部分效率。三个束团和噪声的加入会使重建效率降低；动量分辨和空间分辨主要受噪声的影响；束团晃动对顶点分辨的影响较大。更进一步的研究将在以后的工作中继续进行。
·为了验证MdcPatRec在物理分析工作中的性能，使用蒙特卡罗数据对初步的物理结果进行了检验。主要包括ψ(2S)→π~+π~-，J/ψ→π~+π~-l~+l~-衰变道进行了轻子的重建效率、轻子鉴别效率的检查；并使用MdcxReco次级定点粒子寻迹软件包，考察了K_S~O→π~+π~-非原点出发的K_S~O粒子的寻迹效率；同时，次级顶点粒子的重建质量和寿命也根据K_S~O→π~+π~-和(?)~-→∧π~-，∧→pπ~-衰变道进行了分析；各项结果均与预期符合。通过物理分析的检查可以看到，对于理想的蒙特卡罗数据，MdcPatRec主漂移室径迹重建结果进行的物理分析工作状态良好结果正常，MdcxReco对次级顶点径迹的寻迹作了很好的补充。
主漂移室径迹重建软件MdcPatRec在BOSS框架下开发完成，已经用于BESⅢ数据分析的准备工作。我们使用了大量不同类型的数据来检查MdcPatRec的重建性能，从各方面的指标来看，主漂移重建软件MdcPatRec处于较好的状态，具有效率高，各项分辨好，速度快的优点，为其它子探测器重建和物理分析提供了较好的重建径迹参数，基本能够满足BESⅢ实验的使用要求。
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 10~(33)cm~(-2)s~(-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 measuredτ~+τ~- 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 theτ~+τ~- 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, D~0D|-~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~-,π~- 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 (d_0,φ_0,κ, z_0, tanλ). For singleμ~- at 1 GeV/c , check the reconstructed parameter with its true value in MC. The resolution of△d_0 isσ(△d_0) = 0.17mm; the resolution of△φ_0 isσ(△φ_0) = 2.47 mRad; transverse momentum resolutionσ(△p_T) = 5.31 MeV/c ; vertex resolution of Z directionσ(z_0) = 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~+π~- J/ψ→π~+π~- l~+l~- and secondary vertex tracking result given by MdcxReco package. K_s~0 reconstruction efficiency from K_s~0→π~+π~- has been checked. Decay form K_s~0→π~+π~- and (?)~-→∧π~- ,∨→pπ~- 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.
引文
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[43] Ma Xiang et al., Event Start Time Determination in BESⅢ by Using TOF And MDC Information
[44] 臧石磊，B~-→J／psi的寻找和BES Ⅲ漂移室重建软件的研究，中科院高能物理研究所博士论文，2005．
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[46] 王大勇，BES Ⅲ漂移室径迹拟合与dE／dx离线软件的研究，中科院高能物理研究所博士论文，2006．
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[2] 李政道，粒子物理和场论简引，科学出版社，第一版，1984；L．赖得，基本粒子与对称性，科学出版社，第一版，1983．
[3] 郑志鹏等，北京谱仪正负电子物理，广西科学计数出版社，第一版，1998．
[4] 谢一冈等，粒子探测器与数据获取，科学出版社，2003年7月．
[5] PEPII-An Asymmetric B Factory, Conceptual Design Report. SLAC-418, LBL-5379,1993.
[6] BaBar Collaboration. BaBar Technical Design Report. SLAC-R-457,1995.
[7] KEKB B-Factory Design Report. KEK Report 95-7,1995.
[8] Belle Collaboration, Belle Technical Design Report, KEK Report 95-1,1995.
[9] G. Vignola and DAFNE Project Team. DAFNE, The First Phi-Factory. LNF-96/033,1996.
[10] KLOE Collaboration. The KLOE Detector Technical Proposal. LNF-93/2,1993.
[11] 北京正负电子对撞机重大改造工程BEPC Ⅱ初步设计，2003年11月．
[12] Preliminary Design Report of the BES Ⅲ detector, Jan, 2004.
[13] 裘宗燕等译，C++程序设计语言(特别版)，第一版，北京机械工业出版社，2002．
[14] Gaudi project homepage:http://proj-gaudi.web.cem.ch/proj-gaudi/.
[15] Root - an object-oriented data analysis framework, http://root.cern.ch/.
[16] http://www.cern.ch/clhep.
[17] Li Wei-Guo, BEPCIIBES III Upgrade and the Prospective Physics,
[18] 王运永，多丝正比室与漂移室，科学出版社，1982
[19] 刘建北，BES Ⅲ漂移室性能的实验研究，中科院高能物理研究所博士论文，2005．
[20] 郑志鹏，朱永生主编，北京正负电子物理，广西科学出版社，1998
[21] Preliminary Design Report,2004
[22] Preliminary Design Report: The BES Ⅲ Detector. 2004:62-83.
[23] Preliminary Design Report: The BES Ⅲ Detector. 2004:84-118.
[24] Preliminary Design Report: The BES Ⅲ Detector. 2004:119-133.
[25] Preliminary Design Report: The BES Ⅲ Detector. 2004:134-169.
[26] Preliminary Design Report: The BES Ⅲ Detector. 2004:170-192.
[27] Preliminary Design Report: The BES Ⅲ Detector. 2004:193-208.
[28] Preliminary Design Report: The BES Ⅲ Detector. 2004:209-268.
[29] Preliminary Design Report: The BES Ⅲ Detector. 2004:269-290.
[30] Preliminary Design Report: The BES Ⅲ Detector. 2004:291-300.
[31] Agostinelli, S. and Allison em et al GEANT4-a simulation toolkit, Nucl. Instrum. Methods A, 2003, 506: 250-303.
[32] http://www.nongnu.org/cvs/
[33] Weidong Li, Huaimin Liu et al. The Offline Software for the BES Ⅲ Experiment, Proceeding of CHEP06, Mumbai, 2006.
[34] Deng Zi-Yan et al.HEP&NP, 2006, 30 (05):371-377(in Chinese)(邓子艳等，高能物理核物理，2006，30(05)：371-377)
[35] You Zheng-Yun, BES Ⅲ Detector Description and Event Display, BES Ⅲ Collaboration Meeting,2006.
[36] R. K. Bock et al., Data Analysis Techniques for High-Energy Physics, 2nd ed. 2000
[37] W. Allison. Relativistic Charged Particle Identification by Energy Loss. Ann. Rev., Part Sci., 1980,30:253
[38] WANG Da-Yong et al., Nuclear Electronics & Detection Technology, Kalman track fitting for BES Ⅲ, 2006, (Accecpted).
[39] 朱永生著，实验物理中的概率和统计，科学出版社，1991 李惕碚，“实验的数学处理’'(第1版)，科学出版社，1980年2月．
[40] Kalman, Rudolph Emil et al., A New Approach to Linear Filtering and Prediction Problems, 1960,82:35-45.
[41] 伍灵慧等，BESⅢ漂移室单元结构和丝层安排的设计与优化，高能物理与核物理，2005，29(05)：276
[42] 张晓梅等，北京谱仪BESⅢ主漂移室快速径迹重建系统，核电子学与探测技术，2005，25：638-643．
[43] Ma Xiang et al., Event Start Time Determination in BESⅢ by Using TOF And MDC Information
[44] 臧石磊，B~-→J／psi的寻找和BES Ⅲ漂移室重建软件的研究，中科院高能物理研究所博士论文，2005．
[45] Zhang Yao, MdcPatRec Note, BES Ⅲ technical note, Institute of High Energy Physics, CAS, 2006.
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