Muon System

The muon spectrometer includes some precision tracking chambers for accurate momentum resolution and an effective trigger system based on fast response chambers. The Barrel chambers (|η| < 1) are arranged in three cylindrical layers, while the End-Cap chambers (|η| ≤ 2.7) are mounted on wheels normal to the detector axis. The size of the muon spectrometer is about 22 m in diameter and 44 m in length.

The muon trigger looks for high pT muons by reconstructing tracks that point roughly at the interaction point, both in the r−φ and the r-zprojection. Two thresholds are applied (about 6 and 20 GeV/c) with the trigger acceptance extending to |η| = 2.4. Because of the different rates in the barrel and the end-caps, two different kinds of detectors are employed.

Resistive Plate Chambers (RPCs), operated in avalanche mode, are used in the barrel. Each chamber uses two gas volumes, Bakelite plates, and four planes of read-out strips. Two layers of chambers are installed in the middle station to provide the low pT trigger. A third layer is mounted on the outer chamber station and is used, together with the other layers, for the high pT trigger. The time resolution of the RPCs has been measured to be below 2 ns, corresponding to trigger resolution better than 3 ns.

Thin Gap Chambers (TGCs), multi-wire chambers, operated in saturated mode, are used in the end-caps. Their anode-to-anode pitch is 1.8 mm with the anode to cathode gap being 1.4 mm. The cathode is coated with graphite. External pick-up strips provide the coordinate along the sense wires. The chamber gas is a mixture (55%-45%) of carbon dioxide and n-pentane. Three multi-layers of chambers (one triplet and two doublets) are located in the middle tracking station. Additional TGCs are part of the inner station and are used to increase the tracking ability. Tests performed at a high rate have shown single-plane time resolution of about 4 ns rms, with 98% efficiency, corresponding to a trigger efficiency of 99.6%.

The precision chambers are used for high resolution tracking and momentum measurement of the muons.

Monitored Drift Tubes (MDTs) are used in the pseudorapidity region of |η| < 2. These are drift chambers formed by aluminum tubes with 3 cm diameter and lengths ranging from 0.9 to 6.2 meters. On each chamber the tubes are arranged in two multi-layers, each formed by three or four layers of tubes. They use a gas mixture of 93% Ar and 7% CO2, kept at 3 bar(a), and are operated at a gas gain of 2 × 104. They can thus sustain high rates without aging and with very little sensitivity to space-charge. The single-tube resolution is 100 μm for most of the range in drift distance, and the multi-layer resolution is ∼50μm. To exploit such tracking accuracy on chambers covering up to 10 m2, an extremely acurate mechanical construction is needed. In addition to a sophisticated mounting structure, the aluminum frames supporting the multi-layers are equipped with straightness monitors, temperature sensors (needed to correct for thermal expansion of the tubes) and with magnetic field sensors to predict the ~E × ~B effect on drift time. In total 1174 MDTs are used.

Cathode Strip Chambers (CSCs) are used in the region of 2 <|η| < 2.7. These multi-wire proportional chambers MWPCs, with a sense wire pitch of 2.54 mm, are read out by 5.08 mm pitch readout strips. The track resolution in the bending plane is 60 μm. 32 CSCs are employed.