Friese, V.

Kotynia, A.

GSI Repository

Performance simulations of the CBM-STS with realistic material budget∗V. Friese1 and A. Kotynia21GSI, Darmstadt, Germany; 2Goethe-Universita¨t, Frankfurt, GermanyThe task of the Silicon Tracking System (STS) of theCBM experiment is to reconstruct the trajectories of up to600 charged particles created in nucleus-nucleus collisions.For the performance of the system, its material budget is acrucial issue since multiple scattering in the detector mate-rial will lead to a decrease in both track finding efficiencyand momentum resolution. In order to assess these per-formance figures, a realistic implementation of active andpassive materials in the simulations is required.The system is composed of double-sided micro-stripsensors of 300 μm thickness, which was found to be thebest balance between signal-to-noise ratio and materialbudget. In addition to these active sensors, cables transport-ing the analog signals from the inner sensors to the read-outat the top or bottom of the system add to the material in theacceptance. Lately, a detailed design of the analog cableswas developed [1], which now allows to have a realistic de-scription of the material budget as input to the simulations.The signals will be transported by two staggered layersof Aluminum cables on polyimide carriers. Each modulecontributes two of these cable stacks, reading out the frontand the back side, respectively. In the simulation geometry,these cables are represented by a single volume of 100 μmsilicon (0.11 % X0) as equivalent material budget. At thevertical periphery of a STS station, up to four of such cablevolumes overlap. Consequently, the material budget withinone station varies with the vertical distance from the beam;its maximal value is about 0.8 %. As an example, the ma-terial budget distribution of station 4 at z = 60 cm is shownin Figure 1.Figure 1: Distribution of material budget in tracking station 4This model of the STS was implemented in theCBM software framework and subjected to simulations ofAu+Au collisions in the CBM detector setup. A realisticdetector response was applied as described previously [2].∗Supported by EU/FP7 HadronPhysics2 and Helmholtz InternationalCenter for FAIRThe tracks were reconstructed by the Cellullar Automatontrack finder algorithm; their parameters were determinedby the Kalman Filter. The results of these simulations areshown in Fig. 2. The average efficiency for primary tracksabove 1 GeV is 96 %, only 1 % less than obtained in pre-vious simulations without the cable materials. Similarly,the efficiency for secondary tracks is hardly affected by theadditional material. A more noticeable, but still moderateeffect of the cable material is seen in the momentum reso-lution. Its average value is found to be 0.98 % (Gaussianσ), compared to 0.87 % without cables.Figure 2: Track reconstruction efficiency (upper panel) and mo-mentum resolution (lower panel) in the STS as a function of themomentum for all tracks in central Au+Au collisions at 25 AGeVprojectile energyIn summary, the current simulations of the STS comprisea realistic material budget, including the analog read-outcables. The support structures made of carbon fibre are notyet included, but their contribution to the total material isminor. Within our simulations, we find the track recon-struction efficiency and the momentum resolution to matchthe CBM requirements. The module concept for the STScan thus be considered validated.References[1] C. J. Schmidt et al., An assembly concept for modules of theCBM Silicon Tracking System, this report[2] A. Kotynia and J. M. Heuser, GSI Scientific Report 2010, 22GSI SCIENTIFIC REPORT 2012 PHN-NQM-EXP-2857

Performance simulations of the CBM-STS with realistic material budget

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Performance simulations of the CBM-STS with realistic material budget

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