MDC重建 袁野 张瑶 2013-07-04 2013年粒子物理实验计算软件与技术研讨会威海

BESIII&MDC BESII MDC xy= 250m sp/p = 0.6% @1GeV dE/dx=8.5% p @mip

MDC简介 BEPCII是一个高亮度、多束团的对撞机，这就要求有一个与其相配合的采用现代探测技术的高质量探测器。一方面，现有的BESII探测器面临严重的老化问题，其电子学和数据获取系统不能支持多束团模式；另一方面，100倍的统计增加也要求系统误差相应地减少。 主漂移室（MDC）的主要任务是: 精确测量从相互作用点产生的带电粒子动量和方向； 为带电粒子的粒子鉴别提供足够好的能量损失（dE/dx）测量； 对带电粒子的测量有尽可能大的立体角覆盖； 对低动量带电粒子径迹有尽可能大的重建效率，为带电粒子的一级硬件触发提供信号。

三、重建原理 Trajectory is a helix in 3D; a circle in the transverse plane B=1.5 T Collision Vertex Trajectory is a helix in 3D; a circle in the transverse plane Radius: R

Main Drift Chamber (MDC) geometry Interaction point Cosθ=0.83 Cosθ=0.93 43 sense wire layers group to 11 super-layers End-plates :ladder shape cosq from -0.93 to 0.93 Small cell geometry Newest & final design parameters from MDC hardware group are used in simulation and reconstruction

三、重建原理 带电粒子在磁场中运动时，所飞行的路径是一条空间螺旋线，它在与磁场方向垂直的平面上的投影是圆弧线。这是漂移室径迹重建的依据。 假设有一横动量较大的粒子通过某个超层，则会在该超层的信号丝上产生触发信号，生成径迹段，将各个超层的径迹段组合起来，即形成径迹。

Mdc Tracking Algorithm We have two algorithms for MDC track reconstruction: TrkReco & MdcPatRec TrkReco MdcPatRec Personpower S. L. Zang (IHEP) X. Y. Zhang (SDU) Y. Zhang (SDU) Segment finding Conformal transform Pattern matching Track fitting Least square method

TrkReco Segment Finding: Conformal Transform A circle or line which passes through the origin is transformed into a line A circle or line which doesn't pass through the origin is transformed into a circle pivot Axial hits After transform After fill histogram

MdcPatRec Segment Finding: Pattern matching Segment finder looks for patterns and ambiguity patterns of hit wires and calls the valid ones segments. Ambiguity patterns resolve the left-right ambiguity locally Here is an example of a cell and a valid pattern: A cell in one superlayers One segment pattern in cell (2,0) (2,1) (2,2) (2,3) (2,4) (2,5) (1,0) (1,1) (1,2) (1,3) (1,4) (1,5) (3,0) (3,1) (3,2) (3,3) (3,4) (3,5) (0,0) (0,1) (0,2) (0,3) (0,4) (0,5) clockwise We have 8 4-hit patterns and 20 3-hit patterns 4 3 2 1 7 6 5 1 1 2 3 4 5 6 7

Progress on low momentum tracking : TCurlFinder TCurlFinder is a package focusing on low momentum track finding. The work flow of TCurlFinder: Link consecutive axial hits into a segment Fit the best segment with IP as a circle 2D finder (r/phi) Expand the circle with axial segment in nearby superlayers Salvage unused axial hits Find close stereo hits near the 2Dcircle 3D finder (stereo) Set SZ for those close stereo hits,then fit them into a line Helix fitting

径迹拟合 径迹投影圆弧线和空间螺旋线使用最小二乘法对径迹进行拟合。 其中，drift为漂移距离，doca为最近距离 最小二乘法是以c2取最小值为目的，寻求使c2最小值的时的参数估计值。 博士论文答辩

Runge-Kutta method Motivation: To consider the effect of non-uniform magnetic field in the track fitting process. To improve the momentum resolution. The mean value and resolution of momentum are improved significantly. — runge-kutta — without NUMF corr. y’p+p-J/y; J/ye+e- Bhabha Momentum (GeV/c) Recoiling Mass of p+p- Invariant Mass of e+e-

Software Optimize -- Segment Dictionary in Pat The segment dictionary is designed based on symmetric 8-wire segment neighbors. MC simulation shown the dictionary in PAT can not cover all hit patterns.

Because the wire numbers of different layers in some super-layers are different, some segment shapes are not symmetric. To remedy this asymmetry, we tried to broaden 8-wire segment to 14-wire segment in PAT.

tracking efficiency at different transverse momentum. Black: default dictionary Red: expanded dictionary tracking efficiency at different transverse momentum. Could improve tracking efficiency of PAT at low transverse momentum Need more detailed analysis, especially tracking quality and speed under high level noise

Tuning tracking algorithm Optimize cuts in axial segment combine Start from the beginning, detailed study of codes and cuts, make each step reliability Wrong combination Red from wrong track combination Phi0:0.6->0.2 Correct combination single Psi(3770)->4pi Fix a bug in segment linking Delete continues fired hits in one layer Delete reconstructed track with gap 3d track candidate，add first 4 layer to fit

Tracking performance Bhabha from psi(3770) PAT+TSF+ Runge-Kutta + Kalman fitting Residual distribution Momentum distribution sP=15.8MeV/c s=127mm Bhabha from psi(3770)

Kalman filter拟合失败率下降一半 解决径迹劈裂 单径迹重建效率提高1% 物理事例产额明显提高 逐步分析核查，在数据筛选，模式字典， 径迹连接，击中打捞，拟合等方面进行 了多处改进和优化： Kalman filter拟合失败率下降一半 解决径迹劈裂 单径迹重建效率提高1% 物理事例产额明显提高 old new Increase(%) KPi 9991+/-106 10476+/-108 4.9+/-1.6 K2Pi 15065+/-132 15951+/-135 5.9+/-1.3 K3Pi 14130+/-142 14642+/-141 3.6+/-1.4 KsPi 1895+/-48 2036+/-50 7.4+/-3.8 Ks2Pi 2977+/-71 3159+/-70 6.1+/-3.5 Ks3Pi 1701+/-81 1781+/-77 4.7+/-6.7

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