The Inner Detector is the closest subsystem to the interaction point and therefore has high importance [42]. Its primary task is the precise reconstruction of the trajectories (tracks) of charged particles. Knowing the trajectory and the magnetic field in the Inner Detector, one can calculate the charge, the initial momentum, the direction of flight and the impact parameter of charged particles. The impact parameter describes the point of closest approach of the trajectory to the beam line.
The design of the Inner Detector must fulfill several requirements to allow an optimal search for rare physics processes. The track reconstruction efficiency of the Inner Detector must be larger than 90%. The design of the Inner Detector ensures a coverage in |η|-direction up to2.5 and a full φ-coverage. The transverse momentum resolution is supposed to be better than 30% for charged particles with a transverse momentum of 500GeV. Moreover, the Inner Detector must provide a precise primary and secondary vertex reconstruction, which is important for the identification of B-mesons and converted photons.
4.1. THE ATLAS DETECTOR 21 The resolution of the Inner Detector can be parameterized by [43]
∆pT
pT ≈
0.00036pT[GeV]⊕
0.013 √sinφ
The first term corresponds to the intrinsic resolution, while the second term parameterizes the multiple scattering effects due to more material in the forward region.
A rather large problem of the Inner Detector is the high multiplicity of charged particles per collision, which leads to many overlapping tracks and therefore introduces ambiguities in the track reconstruction. The idea to minimize this problem is the combination of a high precision measurement of few points and a nearly continuous low precision measurement of many points along the particle trajectory. The Inner Detector is built of three subsystems to achieve this combination.
• The pixel detector has a very high granularity and allows a high precision measurement of three dimensional interaction points along the particle trajectory
• The silicon strip detector, or semi conducting tracker (SCT), measures at least four three dimensional space-points along the trajectory also to high precision.
• The straw tracker, or transition radiation tracker (TRT), provides on average 36 mea- surements in the bending plane of the particle.
These three subsystems are discussed in the following.
Pixel Detector
The active material of the pixel detector is silicon, which is structured in rectangular cells with a size of 40×400µm2. These cells are called pixels and can be compared to the pixels of a usual digital camera. Charged particles which pass through silicon produce electron hole pairs. A bias voltage, which is applied to each cell, causes the electrons and holes to drift to the readout-side of the cell. The threshold on single cell-level is a charge corresponding to 3000e−. The amount of charges, which was deposited in one cell, is stored above this threshold.
The cells are placed in three cylindrical layers in the barrel region, with distances to the beam-line of r = 5.05cm, r = 8.85cm and r = 12.25cm. The endcap-region is covered by three disks of cells on each side. The pixel detector has in total 80 million cells, with an efficiency of nearly 100%, which was tested in the H8 test-beam setup [44]. The test-beam measurements revealed a resolution of 12µm in the rφ-plane and a resolution of 110µm in z-direction. This high precision of the pixel detector drives the measurement of the impact parameter of each reconstructed track.
Silicon Strip Detector
The SCT is responsible for the tracking at radii from 30cm to 60cm. It is important for the determination of the z-position of the vertex, the momentum resolution and also for the pattern recognition of the reconstruction algorithms.
Silicon was also used here as active material, but in contrast to the pixel detector, the silicon is not structured in cells but in strips with a width of80µm. A sensor is formed of 768 strips and covers an area of6×12cm2.
A SCT module is a combination of the readout-electronic and two sensors, which are glued together with a relative angle of40mrad. The readout-electronics for one module allows only
22 CHAPTER 4. THE ATLAS EXPERIMENT a binary information from each strip, in contrast to the pixel detector, where also the amount of charges is accessible. This limits the spatial resolution to23µmper module. The relative angle between the two sensors allows the measurement of the second coordinate of the sensor’s plane to a precision of 800µm. The 2112 SCT modules are placed in four cylindrical layers in the barrel region and 988 modules in four disks in each endcap-side.
Transition Radiation Tracker
The number of precision layers is constrained by the high cost per unit area of semiconductor layers and their relative high radiation length. Hence it was decided to use a third sub- detector type, for radii larger than 60cm, which consists of straw tubes with a diameter of
4mm. These tubes are filled with a gas mixture of 70 : 27 : 3 X e : CO2 : O2 and have a gold-plated tungsten wire in the middle. Charged particles, which traverse trough the tube, lead to a ionization of the gas mixture.
In addition, the walls of the straw tubes contain radiator material (polyethylene) which enhances the production of transition radiation photons. These photons can be detected in X e-gas. The number of produced photons by a particle is proportional to the relativistic correction factorγ = E
m of the particles [45]. Electrons produce most of these photons due to
their small mass. This allows an additional identification of electrons.
The roughly 50,000 tubes of the TRT, which are arranged in 73 cylindrical layers, provide roughly 36 track points for the track reconstruction. The expected occupancy of 50% of the TRT tubes is challenging for the pattern recognition. Nevertheless, the track points are rather important for the resolution of the Inner Detector, since they are positioned along a relative large level arm.