Measurement of the Z boson cross-section in - Harvard University ...

Chapter 2: The Accelerator and the Experiment 41
2.2.2 The Inner Detector (ID)
The primary purpose of the inner detector is to perform pattern recognition for
track finding in a very dense environment. At full design luminosity, O(10 3 ) particles
will be produced per bunch crossing at 25 ns intervals. To successfully reconstruct
charged particle tracks in this environment, the ID must use high-granularity detector
elements with a fast response time close to the interaction region. To accurately
estimate the momenta of these tracks, the magnetic field inside the ID must be well-
mapped 1 . In addition, the ID must be capable of precise primary and secondary
vertex reconstruction to enable tagging of heavy flavor and τ leptons.
To realize these goals, the ATLAS inner detector has been designed with three
components: a silicon pixel detector closest to the beampipe, a silicon microstrip
tracker (SCT), and a transition radiation tracker (TRT). The pseudorapidity coverage
of the ID is |η| < 2.5. Figure 2.4 shows a schematic of the ID. The three subdetectors
are briefly described below.
The Pixel Detector
The silicon pixel detector largely determines the impact parameter resolution and
vertex reconstruction capabilities of ATLAS. The pixel detector contains ≈ 80.4 mil-
lion pixels, each of dimension 50 × 400 µm 2 , covering an area of 1.7 m 2 . In the barrel
region, there are three pixel layers, with distances from the nominal beam axis of 50.5
mm, 88.5 mm and 122.5 mm. In each endcap region, there are three disks of pixels.
The axial coverage of the pixel detector is up to 650 mm on each side of the nominal
1 Since the axial length of the solenoid is smaller than that of the ID, the magnetic field in the
ID is non-uniform.