Muon measurements at different beam energies in CBM

The CBM muon detection system is designed to measure muon pairs from the decay of vector mesons (ρ, ω, φ, J/ψ) produced in heavy-ion collisions. At FAIR energies the muon momenta can be rather low, and, therefore, we developed a muon detection concept which a dynamical definition of absorber thickness according to the muon momentum. The actual design of the muon detector system consists of 6 hadron absorber layers (iron plates of thickness 20, 20, 20, 30, 35, 100 cm) and 15-18 gaseous tracking chambers located in triplets behind each iron slab. The absorber/detector system is placed downstream of the Silicon Tracking System (STS) which determines the particle momentum. The definition of a muon depends on its momentum which varies with the mass of the vector mesons and with beam energy. For example, for beam energies above 15 AGeV muons from the decay of J/ψ mesons have to pass all 6 absorber layers with a total iron thickness of 225 cm corresponding to 13.4 interaction length λI . The muons from the decay of low-mass vector mesons (ρ, ω, φ) only have to penetrate through 5 iron absorber layers with a total thickness of 125 cm (corresponding to 7.5 λI ). The multiplicity of vector mesons is calculated with the HSD event generator, whereas the dimuon decay kinematics is computed with the PLUTO code. The signals are embedded in a heavy-ion collision which is simulated with the UrQMD event generator. Both signal and background tracks are transported through the detector setup using the TGEANT3 code within the cbmroot simulation framework. The L1 tracking procedure [1] is used for the track finding and for momentum reconstruction in the STS. LIT tracking [2] is used for track finding in MuCh. Tracks reconstructed in the STS are extrapolated through the full hadron absorber system, and then are accepted as muons. For the track reconstruction we assume that the detector layers are segmented into pads according to an occupancy of 5%. The efficiency for vector meson detection and the signalto-background ratio, calculated in a ±2σ window around the signal peaks for Au+Au collisions at 15, 25 and 35 AGeV are presented in table 1. The signal-to-background (S/B) ratio for low-mass vector mesons does not vary significantly with the beam energy, whereas for J/ψ mesons it increases strongly with beam energy due to the steeply rising charm production excitation function close to threshold. The signal efficiency decreases with decreasing beam energy due to muon absorption: muons with momenta below 1.6 GeV/c are stopped in 125 cm of iron. The efficiency for muons from low-mass vector mesons can be improved by accepting tracks as muons which pass only 4 iron absorber layers with