Feasibility evaluation of the straw tube detector option for the CBM muon system ∗

The emission of lepton pairs out of the hot and dense collision zone of heavy ion reactions is a promising probe to study the electromagnetic structure of hadrons under extreme conditions. The reconstruction of vector mesons (ρ, ω, φ, J/ψ, ψ) is one of the prime tasks of the CBM experiment. To perform such a study using a dimuon decay mode, a muon system consisting of a set of absorbers and detector stations will be built. Since the muon absorber should efficiently suppress the particle flux, the downstream detector stations can be made using detector technologies different from those in the first stations. The straw tube option looks quite promising due to a good coordinate resolution of such detectors (∼200μm in the drift direction) and the fact that they have been widely used for more than a decade and this detector technology has shown to be rather simple and reliable[1]. A possible muon system configuration is shown in Fig. 1, where the last 3 detector stations (behind absorbers 4-6) are built from the straw tubes. Each station consists of 3 double layers (doublets) with different doublets rotated around the beam axis to create stereo views. Three doublets of the same station, separated in Z by some distance, can be used to make track vectors necessary for efficient suppression of the background. In order to check the ability of the straw detectors to work in the CBM experimental conditions, the simulation and reconstruction software has been modified to properly handle the new detector type. The Monte Carlo simulation has shown that the detector occupancy (the most limiting factor of the tubes) can stay within acceptable limit even with the maximum tube lengths covering the entire absorber diameter (Fig. 2). In addition, it is possible to further reduce the occupancy by making the anode wires segmented[2, 3]. It is planned to further proceed in the direction of evaluating the muon system performance with straw tube stations and optimizing the detector geometry (in particular, tube diameter and stereo angle). The foreseen software changes aim at the possibility to use the same Monte Carlo event samples for different detector options by simulating detector specific features at the hit production level.