Wing Sheung Chan

The Large Hadron Collider and the ATLAS detector 37 Insertable b -layer The IBL is the innermost layer of the ID. It was added to the ID during the long shutdown of the LHC in 2014. The main reason for the addition is to minimise the risk of tracking efficiency lost due to radiation damage to the original innermost layer of the ID. The addition also allows more precise tracking and vertexing, which is especially beneficial to physics performance that depends on the precise measurements of displaced secondary vertices or impact parameters, such as the identification of b -quark-initiated jets or hadronically decaying τ leptons. The IBL is a cylinder of silicon pixel sensors. Each pixel has a size of (50 µ m , 250 µ m) in the ( φ, z ) direction and the centres of the sensors are at a radius of 33 . 25mm from the beam axis. Pixel detector The pixel detector is a precision tracking detector that covers the region | η | < 2 . 5 . It has three concentric cylindrical layers of silicon pixel sensors in the “barrel” region and three disks of the same sensors on each side in the “end-cap” region. In the barrel region, the three cylindrical layers are 50 . 5mm , 88 . 5mm and 122 . 5mm away from the beam axis respectively and cover the region | z | < 400 . 5 mm . In the end-cap region, the centres of the three disks are 495mm , 580mm and 650mm away from the nominal collision point respectively and cover the region 88 . 8 mm < R < 149 . 6 mm . Every pixel sensor is identical and the minimum pixel size is 50 µ m × 400 µ m . Arranged in a specific configuration, the layers of sensors together have an intrinsic accuracy of (10 µ m , 115 µ m) in the ( φ, z ) direction in the barrel region and in the ( φ, R ) direction in the end-cap region. Semiconductor tracker The SCT is another precision tracking detector but has slightly less granularity than the pixel detector. It relies on layers of silicon strip detectors placed in alternating orientations to measure every spatial coordinate. Each track is expected to cross eight layers, which marks four space points. In the barrel region, strips are placed parallel to the beam axis or along the φ direction and cover the region 299 mm < R < 514 mm and | z | < 749 mm . In the end-cap region, strips are placed running radially or along the φ direction and cover the region 275 mm < R < 560 mm and 839 mm < | z | < 2735 mm . The intrinsic accuracy of the SCT is (17 µ m , 580 µ m) in the ( φ, z ) direction in the barrel region and in the ( φ, R ) direction in the end-cap region. Transition radiation tracker The TRT is the outermost layer of the ID. The TRT registers hits using cylindrical drift tubes (straws) that extend up to | η | = 2 . 0 . Each straw is 4mm in diameter and filled with 70% xenon, 27% carbon dioxide and 3% oxygen. At the centre of each straw, a 30-µ m -diameter gold-plated tungsten wire serves as the anode for the drift field. The