Detectors of the CMS experiment

A single type of detector that can locate and measure all kinds of particles does not exist. Therefore, the typical particle collider experiment consists of several different detectors for different particles and physical quantities. In the case of the CMS experiment, the detectors are arranged onion-like around the collision point of the particle beams [7]. Figure 2.5 shows a cutaway of the experiment, so the location of the different sub-detectors is visible.

All detectors are split into two parts. Firstly, the barrel section where the sensors are arranged cylindrically around the interaction point. Secondly, the

Figure 2.5.: Overview of the different sub-detectors of the CMS experiment. Courtesy of CERN.

endcaps consisting of discs of sensors that close the barrels in the front and the back. Because much ionizing radiation is created by the colliding beams, radiation-hardness is a common issue that must be considered during the devel- opment of the detectors. This is especially important for the detectors closest to the interaction point.

A crucial part of the detector is the 4 T solenoid magnet [35]. With an inner diameter of 6 m, it is the biggest superconducting magnet ever built. This large inner diameter has the advantage that both calorimeters can be placed within the magnet. The result is a higher energy resolution of these sub-detectors because the particles do not have to pass the magnet material. The purpose of the magnet is to bend the paths of charged particles, such as electrons and muons. Thereby the transverse momentum and indirectly the energy of charged particles can be determined.

The innermost detector of the CMS experiment is the silicon tracker. It consists of thin sheets of silicon that are able to detect the passage of charged particles.

2.3. The CMS experiment

More on the functionality of silicon trackers can be found in Section 3.1. Mul- tiple layers of these silicon detectors are used to track the path of a charged particle. The CMS silicon tracker is split into an inner and an outer tracker. The inner tracker consists of pixel modules that allow a fine spatial resolution. The closest modules are mounted just about 4.4 cm from the collision point of the particles. In total 1440 modules with 66 million pixels form the inner tracker. The outer tracker is built of strip silicon sensors to save money and reduce the power dissipation within the detector. This optimization is possible because the resolution requirements are lower in modules further away from the interaction point. The outer tracker is built of more than 15 000 modules on 10 layers with an active area of about 198 m2_{. As little material as possible}

is used to build the silicon tracker so that multiple scattering is minimized for optimum momentum measurement.

Then the particles reach the next detector—the Electromagnetic Calorimeter (ECAL) [36]. Calorimeters are used to measure the energy of particles; the ECAL in particular measures the energy of photons and electrons. The CMS ECAL consists of about 76 000 lead tungstate (P bW O4) crystals. When a

high-energetic photon or electron interacts with a crystal, a shower of low- energetic photons is generated [37]. A shower means that out of a high-energetic particle many particles with lower energy are generated. These low-energetic photons are eventually absorbed by the crystal, and they cause the crystal to scintillate, i.e. to reproduce a flash of light. The energy produced in this light flash is proportional to the energy of the original particle. Avalanche photodiodes (barrel section) and vacuum photo triodes (endcaps) glued to the crystals detect the light pulses. The CMS ECAL has been designed to have a high energy resolution, to be fast and to be radiation resistant.

The last detector inside of the superconducting solenoid is the Hadron Calorime- ter (HCAL) [38]. It measures the energy of hadrons, e.g. protons, neutrons, pions, kaons. Also, the HCAL is very important to indirectly detect neutri- nos or unknown particles that are not interacting with any of the detectors. Missing energy provides evidence of these types of particles. Therefore, precise measurements from the HCAL are essential.

The HCAL consists of alternating layers of brass or steel absorbers with plastic scintillators. Hadrons interact with the matter of the brass and steel absorbers, and secondary particles are created. The secondary particles may interact again, and so a multitude of particles is created in a so-called particle shower. These showers cause the plastic scintillators to emit light, which is read out by optical

fibers and detected by photodiodes. Seventeen of these metal absorber, scintil- lator combinations are stacked into towers. The whole HCAL consists of 4300 of such towers.

As muons do not interact much with matter, the muon system [39] can be located outside of the superconducting solenoid. The muon system is complex and consists of 1400 muon chambers of three different types. The main detection system consists of Drift Tube (DT) wire chambers in the barrel section and Cathode Strip Chambers (CSCs) in the endcaps. Both have a similar working principle but differ in their properties. A wire chamber is a box filled with gas in which an array of wires is placed [37]. The wires are biased by a high voltage to produce an electric field in the chamber. If a particle passes through the detector, the particle ionizes the gas. The charge of the produced electrons and ions is then collected by the wires and readout by electronics.

The wire chambers are arranged in layers interleaved with the steel return yoke of the solenoid. This allows to reconstruct the tracks of the muons and measure their transverse momentum.

The third detector type of the muon system is the Resistive Plate Chambers (RPCs) that are built of two conducting plates with gas between them. They have a coarser position resolution but are faster than the other two detector types. The RPC have been added in the same areas as the wire chambers and serve as an independent system for the trigger.