Operation and performance of the ATLAS Semiconductor Tracker

Abstract

The semiconductor tracker is a silicon microstrip detector forming part of the inner tracking system of the ATLAS experiment at the LHC. The operation and performance of the semiconductor tracker during the first years of LHC running are described. More than 99% of the detector modules were operational during this period, with an average intrinsic hit efficiency of (99.74±0.04)%. The evolution of the noise occupancy is discussed, and measurements of the Lorentz angle, δ -ray production and energy loss presented. The alignment of the detector is found to be stable at the few-micron level over long periods of time. Radiation damage measurements, which include the evolution of detector leakage currents, are found to be consistent with predictions and are used in the verification of radiation background simulations. A Estimation of the bulk leakage current using models 52The ATLAS collaboration 57 IntroductionThe ATLAS detector [1] is a multi-purpose apparatus designed to study a wide range of physics processes at the Large Hadron Collider (LHC) [2] at CERN. In addition to measurements of Standard Model processes such as vector-boson and top-quark production, the properties of the newly discovered Higgs boson [3,4] are being investigated and searches are being carried out for as yet undiscovered particles such as those predicted by theories including supersymmetry. All of these studies rely heavily on the excellent performance of the ATLAS inner detector tracking system. The semiconductor tracker (SCT) is a precision silicon microstrip detector which forms an integral part of this tracking system. The ATLAS detector is divided into three main components. A high-precision toroid-field muon spectrometer surrounds electromagnetic and hadronic calorimeters, which in turn surround the inner detector. This comprises three complementary subdetectors: a silicon pixel detector covering radial distances 1 between 50.5 mm and 150 mm, the SCT covering radial distances from 299 mm to 560 mm and a transition radiation tracker (TRT) covering radial distances from 563 mm to 1066 mm. These detectors are surrounded by a superconducting solenoid providing a 2 T axial magnetic field. The layout of the inner detector, showing the SCT together with the pixel detector and transition radiation tracker, is shown in figure 1. The inner detector measures the trajectories of charged particles within the pseudorapidity range |η| < 2.5. It has been designed to provide a transverse momentum resolution, in the plane perpendicular to the beam axis, of 1 ATLAS uses a right-handed coordinate system with its origin at the nominal interaction point in the centre of the detector and the z-axis along the beam-pipe. The x-axis points from the interaction point to the centre of the LHC ring and the y-axis points upwards. Cylindrical coordinates (r, φ ) are used in the transverse plane, φ being the azimuthal angle around the beam-pipe. The pseudorapidity η is defined in terms of the polar angle θ as η = − ln tan(θ /2).-1 -