CMS silicon tracker layout

y y z r ϕ θ/η

Figure 3.5.: Coordinates in the silicon tracker: On the left side, the detector is cut along the beam axis. On the right side, the detector is cut perpendicular to the beam axis.

is used to measure the angle on the rz-plane. Pseudorapidity has the advantage that within its intervals the flux of particles is approximately constant.

Due to the magnetic field within the CMS tracker, charged particles start to spiral and their track is a helix. Thus, a track is characterized by five param- eters, all of them visualized in Figure 3.6. The collision point of the particles and origin of the track is specified by d0 and z0. Whereas z0 indicates the loca-

tion on the beam axis around the interaction point, and d0 denotes the closest

distance of the helix from the beam axis. The starting angle of the particle track on the rφ-plane is indicated by φ0 and the angle between the track and

the particle beam by θ. The fifth parameter is the curvature R; it denotes the radius of the helix. Via the curvature, the transverse momentum of the particle can be determined.

3.4. CMS silicon tracker layout

The building blocks of the CMS tracker are the detector modules: silicon detec- tors combined with the readout electronics and the power supply. The tracker layout describes the arrangement of these modules to form the silicon tracker. As the tracks originate from a small volume in the center of the detector a spherical arrangement would be ideal but building a spherical structure is com- plicated. The compromise is to arrange the modules in a cylindrical structure. The region around the interaction point where the modules are placed on the

x y P C ϕ0 d0 R (a) z r P z0 θ0 (b)

Figure 3.6.: Parameters of a particle track: The left side shows the transversal plane on which the path of the particle is bent. The right side shows the path of the particle on a plane parallel to the beam direction. Source: [71]. curved surface of the cylinder is called the barrel section. Endcaps complement the detector in the front and the back of the cylinder, as shown in Figure 3.7. At CERN, a tool has been developed that is used to simulate proposed tracker layouts; the result is published in [72], and data are available at [56]. In the same paper, Bianchi presented the new baseline layout, which is called the tilted barrel geometry and is shown in Figure 3.7. Its name originates from the fact that some modules in the barrel are tilted towards the interaction point. Thus, the detector modules are more perpendicular to the path of the particles, and fewer modules are needed to detect all particles. This leads to less material in the detector that deflects the particles. The drawback is the more complicated construction of the tracker.

The tracker consists of two parts: the inner tracker built of pixel sensors and the outer tracker built of stacked modules with strip and macro-pixel sensors. The inner tracker consists of four layers and a number of modules in the endcap regions. Due to the usage of pixel sensors, the spatial resolution of the inner tracker is very high, and the number of readout channels is high too. Therefore, it is not feasible to process the data of the inner tracker by the L1 trigger at the full LHC rate of 40 MHz. The inner tracker data are processed at the high-level trigger only. The outer tracker consists of detector modules, which are composed of two stacked silicon detectors that allow distinguishing the

3.4. CMS silicon tracker layout e Interaction Point Barrel Endcap La yer 1-4 5 6 7 8 9 10 2S Module PS Module Pixel Module

Figure 3.7.: Quarter of the CMS silicon tracker at the HL-LHC. The tracker layout used is Baseline2015_tilted_Pixel_V1_1. Source: [56]

transverse momentum of a particle at module level. Two different types of detector modules are used: the Strip-Strip (2S) modules consist of two strip sensors and the Pixel-Strip (PS) modules consist of a strip and a macro-pixel sensor. The detector modules are explained in more detail in Section 3.5. As compiled in Table 3.1, the silicon tracker consists of 3316 pixel detector modules in the inner tracker and 14 172 stacked modules in the outer tracker. The total active area is around 200 m2_{, which corresponds roughly to the area}

of a tennis court [73].

Strip detectors are used in the outer layers because the track location in the φ-direction is more interesting than in the θ-direction. The reason is that the magnetic field of the detector is parallel to the z-axis and deflects the particles in the φ-direction. The physical property related in φ-direction is the momentum that should be measured with high precision. In contrast, the information obtained in θ-direction serves to separate particles and interaction vertices. Therefore, the resolution in θ-direction may be reduced in the outer layers where the particle density is lower than close to the interaction point.

Table 3.1.: Detector modules according to the Baseline2015_tilted_Pixel_V1_1 lay- out. Source: [56]

Modules Active Area (m2)

Channels (million)

2S modules 8424 154.8 34.2

Outer Tracker PS modules 5748 51.0 187.6

Total 14172 205.8 221.8

Inner Tracker Pixel modules 3316 3.2 434.6