A: Gaseous tracking chambers based on the Gas Electron Multiplier (GEM) technology will be used for the first two stations of the Muon Chamber system of the Compressed Baryonic Matter (CBM) experiment at FAIR. Large size trapezoidal modules each having 2D pad-readout with ∼ 2.2k pads were designed and fabricated for data taking in the mCBM (mini-CBM) experiment set-up at the SIS18 facility of GSI as a part of the FAIR Phase-0 program. In this contribution, we report on the testing of two such GEM modules within nucleus + nucleus collisions at 1 -2A GeV beam energies in the mCBM experiment. Data with the first version of a self-triggered readout ASIC has been used for the first time. The response of the GEM modules in terms of spill structure, cluster-size characteristics, time correlation between detector sub-systems, among others, have been studied. Preliminary results obtained during the mCBM data campaign in March 2019, displaying the detector characteristics, which include time-based event reconstruction, are discussed.

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Large area triple GEM chambers will be employed in the first two stations of the MuCh system of the CBM experiment at the upcoming Facility for Antiproton and Ion Research FAIR in Darmstadt/Germany. The GEM detectors have been designed to take data at an unprecedented interaction rate (up to 10 MHz) in nucleus-nucleus collisions in CBM at FAIR. Real-size trapezoidal modules have been installed in the mCBM experiment and tested in nucleus-nucleus collisions at the SIS18 beamline of GSI as a part of the FAIR Phase-0 program. In this report, we discuss the design, installation, commissioning, and response of these GEM modules in detail. The response has been studied using the self-triggered readout electronics. In free-streaming data, the first attempt on an event building based on the timestamps of hits has been carried out, resulting in the observation of clear spatial correlations between the GEM modules in the mCBM setup for the first time. Accordingly, a time resolution of ∼15 ns have been obtained for the GEM detectors. Gain uniformity & cluster characteristics have also been studied.

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