CMS Detector Layout

Fig.  shows the full CMS detector [5,6], which is $15\,\rm m$ in diameter, $21.6\,\rm m$ long and weighs $12500\,\rm t$. It relies on four principal sub-systems: A high-quality central tracking system (pink and yellow), surrounded by an electromagnetic calorimeter (green), a hermetic hadron calorimeter (purple) and finally a muon detector (red and yellow).

Figure: The CMS experiment at CERN.

All subsystems of the experiment are divided into a cylindrical barrel part and the two facing endcap sections. The ``forward'' region, shown on the scaffold to the right, is further away from the interaction point at very small angles. This arrangement gives a good coverage of almost everything arising from a collision, which is important for the reconstruction of events.

Figure: Longitudinal view of one quadrant of CMS.

A short introduction will be given to the components of the CMS experiment from the innermost to the outermost detectors. Fig.  shows a longitudinal view of CMS, where the origin denotes the interaction point. The angle specifications on top and left are given in units of pseudorapidity $\eta$, which is defined by

$$\eta = -\ln\left(\tan\frac{\alpha}{2}\right)\quad {\rm and}$$ (1.3)

$$\frac{r}{z} = \tan \alpha \quad ,$$

where $\alpha=90^{\circ}$ is perpendicular to and $\alpha=0^{\circ}$ coincides with the beam axis.

The central tracker consists of three pixel layers and ten strip layers. Its task is to measure the tracks of charged particles with a minimum of interaction. Originally, Micro-Strip Gas Chambers (MSGCs) were planned for its outer part. However, problems were repeatedly reported concerning aging and high voltage (HV) stability. Thus, the CMS Tracker community decided to build an all-silicon tracker [7,8] instead, which now covers a sensitive area of $206\,\rm m^2$. The Tracker, as it is the main topic of this thesis, will be discussed in detail in chapter .

The electromagnetic calorimeter (ECAL) consists of approximately 76000 scintillating
$\rm PbWO_4$ crystals with a depth of $23\,\rm cm$ (corresponding to 25 radiation lengths $X_0$) and a cross-section of $2.2\times 2.2\,\rm cm^2$. Electrons and photons are converted to light pulses, which are read out by silicon avalanche photodiodes. A small loss in attenuation of a few percent due to color center formation is observed from radiation. This effect can be calibrated with light injection into the crystal.

The main part of the hadron calorimeter (HCAL) is located inside the magnet, which is surrounded by a an additional small part in the central region (``tail catcher''). The central HCAL consists of a brass/scintillator sampling calorimeter. Its scintillation light is captured, wavelength shifted and guided to hybrid photodiodes. The active depth of the HCAL exceeds nine nuclear interaction lengths $\lambda_I$, corresponding to more than $99\%$ containment of hadronic cascades. The forward part of the HCAL consists of a steel absorber with quartz fibers. Traversing charged particles produce Cherenkov light in the fibers which is guided to photomultipliers.

The calorimeters are intended for energy measurement and triggering. They are surrounded by a superconducting coil providing a solenoidal magnetic field of $4\,\rm T$. The tracks of charged particles bend in this magnetic field $B$ which allows to measure the polarity of their charge $q$ and, assuming elementary charge, their momentum $p$ using the relation

$$p=q\,B\,r \quad,$$ (1.4)

where $r$ is the bend radius. This equation is given here in a non-relativistic form; it has to be adapted for velocities close to the speed of light like those which occur in the LHC experiments. Moreover, the polarity can be obtained by the bend orientation.

The Muon System consists of four stations in both barrel ( $\rm MB1\ldots MB4$) and endcap ( $\rm ME1\ldots ME4$) parts, which are integrated in the iron return yoke of the magnet. In the barrel part, each station consists of twelve layers of Drift Tube Chambers (DT). Resistive Plate Chambers (RPC) are used for bunch crossing identification and provide a cut on the muon transverse momentum at the first level trigger. In the endcap region, each station consists of six stations of Cathode Strip Chambers (CSC).

Four logic blocks make up the Trigger and Data Acquisition System. The first two stages, front-end detector electronics and first level trigger processors, are synchronous and pipelined. The first level trigger has to reduce the $40\,\rm MHz$ bunch crossing rate to an event rate of $100\,\rm kHz$ by filtering only interesting events. This trigger decision has a delay of $3.2\,\rm\mu s$ in relation to the corresponding bunch crossing. To avoid dead time, the data collected within this period must be stored in the front-end in order to pass it on after a trigger request.

The two later stages are a large switching network (``event builder'') with a throughput of $500\,\rm Gbit/s$ and an on-line event filtering system implemented in a computer farm. These stages are made of commercial components and thus can be upgraded as technology develops.

The combined information of all detector subsystems is used for the total event reconstruction and quantification. While the silicon tracker is intended for momentum and polarity identification, the energy is measured by the calorimeters, and penetrating muons are detected in the outermost layer. The triggering information is derived from calorimeters and the muon system.