Muon system

Muon triggering and offline muon identification are fundamental requirements of the LHCb experiment. Muons are present in the final states of many CP-sensitive B decays and play a major role in CP asymmetry and oscillation measurements, as muons from semi-leptonic b decays provide a tag of the initial state flavour of the accompanying neutral B mesons.

The muon system provides fast information for the high-pT muon trigger at the earliest level (Level-0) and muon identification for the high-level trigger (HLT) and offline analysis.

The system is composed of five stations (M1-M5) of rectangular shape, covering an acceptance of ±300 mrad (horizontally) and ±250 mrad (vertically). M1 is placed in front of the scintillating pad detector/pre-shower. M2-M5 follow the hadron calorimeter (HCAL) and are separated by iron filters. The stations cover an area of 435 m².

Each station is divided into four regions, R1 to R4, with increasing distance from the beam axis. All the regions have approximately the same acceptance, and their granularity is shaped according to the particle density in that region in order to keep occupancy roughly constant over the detector. The granularity of the readout is higher in the horizontal plane, in order to give an accurate measurement of the track momentum and Pt.

Information must be gathered within 20 nanoseconds, so the detectors are optimized for speed. The system is therefore equipped with Multi Wire Proportional Chambers (MWPC) with 2 mm wire spacing and a small gas gap (5 mm). Triple-GEM detectors are used in the innermost region (R1) of Station M1, where the rate is highest. This choice was dictated by the better ageing properties of this type of detector. There are 1380 chambers in the muon system, of 20 different sizes.

The detectors provide space point measurements of the tracks, giving a binary (yes/no) information. In order to provide this information, different readout methods are employed in the various parts of the detector: anode wire readout, cathode pad readout, or both. A total of 126,000 front-end readout channels are used. The electronics is based on custom radiation-hard chips developed for the muon system.