Performance simulation studies of a realistic model of the CBM Silicon Tracking System

1. Performance simulation studies of a realistic model of theCBM Silicon Tracking System Anna Kotynia Goethe University, Frankfurt, Germany for the CBM Collaboration http://www-cbm.gsi.de Silicon Tracking for CBM Reconstruction • ooo • Due to charge sharing between strips, the majority of ctracks is expected to spread signal across several strips. Therefore, a cluster algorithm has been applied to refine the reconstruction of the particles’ positions . • The cluster center is determined using the Center of Gravity (COG) equation. Challenge: • Au+Au collisions , 25 GeV/u • up to 1000 charged particles/event • interaction rates up to 10 MHz Reconstructed URQMD event: central Au+Au coll. @ 25AGeV • The reconstructed total cluster charge matches the expectation: • i.e. 23 ke- in a 300 µm thick silicon sensors. STS requirements: • radiation hard and low-mass • high position resolution • track reconstruction for all charged particles • momentum resolution of the order of 1.5% at 1GeV/c • primaryvertex resolution of about 50 µm STS Layout Illustration of the cluster finding method. The bottom part of the figure shows the charge divisionsampled at the horizontal line in the middle of the picture. Distribution of the number of strips per cluster. • 8 tracking stations: 30, 40, 50, 60, 70, 80, 90, 100 cm downstream of the target, in a 1 T dipole magnet; • 4 m2active area; • The number, positionsand segmentation of the layers are optimized for efficient track reconstruction and high momentum resolution. Occupancies and reconstruction results • The occupancy has been evaluated in the highluminosity scenario for minimum bias collisions, assuming an interaction rate of 10 MHz. • The chip occupancyis relevant to the data acquisition, while the sensor occupancy influences the tracking performance. Hot regions are observed in a small fraction of the detector, with more than 32MHz data rate and more than 20 hits/cm2/event. STS in dipole magnet • Sensor occupancy • Chip occupancy • The STS stations have a modular structure and are constructed from 4 types of 300µm thick double-sided silicon micro-strip sensors, arranged on 8 types of carbon-fiber supported ladder structures. Average hit density per sensor for the STS station closest to the target. • Distribution of data ratesproduced by thereadout chip of all eight STS stations. Realistic STS system in CbmRoot • The application of the realistic detector responce functions during digitization, of the COG algorithm for space point recontruction and the Cellular Automaton [2] for track finding and Kalman Filter for track fitting, results in: Simulation and signal digitization Hit Finding efficiency: Primary: 98.97 % Secondary: 91.69 % Point resolution: Vertical : 60µm Horizontal:8 µm • The simulations comprise the complete chain of physical processes for charged particles traversing the detector, from charge creation in silicon to the digital output signals [1]. During the digitization process realistic detector response functions are applied, such as: • signal sharing between strips; • charge collection inefficiency; • Lorentz shift due to the presence of the magnetic field; • channel dead time; • random noise added to the signal; References: [1] A.Kotynia, J.M.Heuser „ Performance simulations of the CBM Silicon Tracking System”, CBM Progress Report 2011 [2] I.Kisel, I.Kulakov, I.Vassiliev, „A standalone package for on-line event selection in the CBM experiment”, CBM Progress Report 2008 79 Simulated track of a minimum ionizing particle in the silicon detector