The Silicon Tracking System of CBM: towards tests with heavy ion collisions

The Silicon Tracking System (STS) is the central detector for charged-particle identification and momentum determination in the future CBM experiment at the new FAIR accelerator facility. It is designed to measure up to 1000 particles in nucleus-nucleus collision at interaction rates up to 10 MHz, to achieve a momentum resolution better than 2% inside a 1 Tm magnetic field, and to be capable of identifying complex particle decays topologies, e.g., such with strangeness content. The STS comprises 8 tracking stations equipped with double-sided silicon microstrip sensors. Two million channels are read out with self-triggering electronics, matching the data streaming and online event analysis concept of the experiment. The STS functional building block is a module, consisting of a sensor, micro-cables and two front-end electronics boards. The modules are mounted on carbon fiber support ladders. The sensors provide double-sided segmentation at a strip pitch of 58 ^m and 7.5-degree stereo angle. Ultra-thin micro-cables with length up to 50 cm transfer the sensor signals to the electronics located out of the detector acceptance. The custom-developed readout ASIC has a self-triggering and radiation hard architecture that delivers time and amplitude information per channel. Particular design choices and features of the GBTx-based readout are motivated by the high data rate for which the experiment is designed. In mCBM, the FAIR phase 0 precursor setup of CBM, the STS deployed a prototype of one small tracking station with full readout chain. This allowed evaluating the performance in Ag+Au collisions at different interaction rates, up to 10⁶. Moreover, the detector was successfully integrated with other subsystems into a freestreaming DAQ. This paper aims to describe the main technical challenges and prospects for the STS and to present the results of the latest testing campaign.