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

Chapter 2: The Accelerator and the Experiment 43
temperatures of -5 to −10 o C. Notwithstanding, the innermost layer (the ‘B-layer’)
must be changed after three years of operation at design luminosity.
The Semiconductor Detector (SCT)
The SCT consists of four double-sided layers of silicon strips in the barrel, and
nine double-sided disks in each endcap region. Each strip is 6.4 cm in length and
80 µm in width. The radial extent of the SCT is up to r = 560 mm, and the axial
coverage is up to |z| = 2735 mm. The SCT has ≈ 6.3 million readout channels. The
signal generation process is the same as that for the pixels and, as with the pixels,
the SCT is operated at a low temperature in order to maintain a low noise level in
spite of radiation damage.
A track typically traverses four SCT layers. In each SCT layer in the barrel region,
the strips on one side run parallel to the beamline and provide r − φ coordinate
measurement. The strips on the other side are set at a 40 mrad stereo angle. Because
of this relative angle, measurements from strips on the two sides can be combined
to obtain z coordinate information. The endcap disks have one set of strips running
radially and the other set at a 40 mrad stereo angle to measure the r coordinate. In
both regions, the r − φ resolution is 17 µm. The z coordinate is measured to 580 µm
in the barrel, and the r coordinate to the same accuracy in the endcap.
The Transition Radiation Tracker (TRT)
The TRT consists of 4 mm diameter straw tubes containing a Xe/CO2/O2 gas
mixture. The body of the tubes is Al-layered polyimide, and the anode is a gold-
layered tungsten wire of diameter 31 µm. There are 73 planes of tubes in the barrel,