Chapter IV: The Compact Muon Solenoid
4.5 The muon system
The CMS muon system has three primary functions: muon identification, momen- tum measurement, and triggering [15,41–43]. The 3.8 T solenoidal magnet and its flux-return yoke enable precise muon momentum resolution and trigger capability.
Additionally, the yoke serves as a hadron absorber for the identification of muons.
The thickness of the material crossed by muons as a function of pseudorapidity is shown in Fig.4.16.
Figure 4.15: Results of Phase-I HE SiPM performance. Left: the reconstructed muon signal from collision events in a HE tower readout via SiPM (ieta=24, iphi=63) is compared with the signal from a tower readout via HPD (ieta=-24, iphi=63). The signal is divided by the muon track length in the active material. Right: Muon deposits in HE towers for different eta regions and depths. Muons from collision events are considered when their track traverses the HCAL while remaining within the same tower. The muon signal peak is fitted with the convolution of a Gaussian and a Landau. The Landau location parameter is divided by the number of scintillator layers in the considered depth [40].
Figure 4.16: Material thickness in interaction lengths at various depths, as a function of pseudorapidity [15].
The CMS muon system is composed of three types of gas ionization chambers: drift tube chambers (DTs), cathode strip chambers (CSCs), and resistive plate chambers (RPCs). These chambers are interspersed within the layers of the steel flux-return yoke, enabling detection of a muon at multiple points along its path. The DT and CSC chambers are situated in the barrel region with|[| < 1.2and in the endcaps with 0.9 < |[| < 2.4, respectively. The RPC detectors complement the DT and CSC chambers in both barrel and endcaps regions, with the maximum pseudorapidity coverage extending up to |[|= 1.9, as shown in Fig.4.17.
Figure 4.17: An R-z cross section of a quadrant of the CMS detector with the axis parallel to the beam (z) running horizontally and the radius (R) increasing upward. The interaction point is at the lower left corner. The locations of the various muon stations and the steel flux-return disks (dark areas) are shown. The drift tube stations (DTs) are labeled MB (“Muon Barrel”) and the cathode strip chambers (CSCs) are labeled ME (“Muon Endcap”).
Resistive plate chambers (RPCs) are mounted in both the barrel and endcaps of CMS, where they are labeled RB and RE, respectively. [41].
muon, the drift time to the anode wire in the middle of the cell is measured with an optimally shaped electric field. The high spatial resolution of about 100 `m per 8-layer chamber is achieved thanks to the resolution per cell of 250 `m or better, making it possible to use drift chambers as the tracking detectors for the barrel muon system. Figure4.18displays the layout of one wheel of muon DT chambers, a single drift cell, and a photograph of several DT muon chambers inside the CMS magnet yoke.
The CMS endcap muon detector is composed of 540 cathode strip chambers (CSCs), which are multiwire proportional chambers comprised of 6 anode wire planes (gas gaps) interleaved among 7 cathode panels. The overall gas volume is larger than 50
<3, and the number of wires is about 2 million. The strips run radially outward to measure the muon position (z), while the anode wires run azimuthally and provide the radial measurement (R). The precise muon coordinate along the wires (q) can be reconstructed by interpolating the charges read out on the strips. The CSCs provide good position resolution (50–140 `m, depending on chamber type) and time resolution (3 ns per chamber). They can operate at high particle rates and in strong and non-uniform magnetic fields. Fig. 4.19 illustrates the operation of the cathode strip chambers and shows some of the trapezoidal CSC chambers during installation in the CMS detector.
Figure 4.18: The CMS barrel muon detector (DTs). Left: Layout of the CMS barrel muon DT chambers in one of the 5 wheels. The chambers in each wheel are identical with the exception of wheels –1 and +1 where the presence of cryogenic chimneys for the magnet shortens the chambers in 2 sectors. Note that in sectors 4 (top) and 10 (bottom) the MB4 chambers are cut in half to simplify the mechanical assembly and the global chamber layout. Middle: Sketch of a cell showing drift lines and isochrones. The plates at the top and bottom of the cell are at ground potential. The voltages applied to the electrodes are +3600V for wires, +1800V for strips, and 1200V for cathodes. Right: DT chambers (aluminum) sandwiched between steel plates of the yoke (red), during installation. [15,43]
Figure 4.19: The CMS endcap muon detector (CSCs). Left: Layout of a CSC made of 7 trapezoidal panels. The panels form 6 gas gaps withplanes of sensitive anode wires. The cut-out in the top panel reveals anode wires and cathode strips. Only a few wires are shown to indicate their azimuthal direction. Strips of constant qrun lengthwise (radially). The 144 largest CSCs at ME2/2 and ME3/2 are 3.4 m long along the strip direction and up to 1.5 m wide along the wire direction. Middle: A schematic view of a single gap illustrating the principle of CSC operation. By interpolating charges induced on cathode strips by avalanche positive ions near a wire, one can obtain a precise localisation of an avalanche along the wire direction. Right: Outer CSC chambers ME4/2 during installation. [15,43].
mode at high electric field. The RPCs provide accurate timing and fast triggering, with an excellent intrinsic timing resolution of around 1.5 ns for a double-gap chamber. This allows the RPC to tag the time of an ionizing event in a much shorter time than the 25 ns between two consecutive LHC bunch crossings (BX).
An RPC trigger can provide the BX assignment to track candidates and estimate the transverse momenta with high efficiency in a high-rate environment at LHC.
Fig.4.20illustrates the schematic layout and displays a photo of the endcap RPCs.
Figure 4.20: The CMS RPCs. Left: Layout of a double-gap RPC. Middle: Working principle of the double gap RPCs in CMS. Right: Outer CSC chambers ME4/2 during installation. [15,43].