The tracker for the CMS experiment at CERN passed an important milestone in March when the first cosmic-muon tracks were observed in one of the end caps. CMS is one of the two large multi-purpose detectors being constructed at the Large Hadron Collider. Its tracker system, comprising a barrel detector and two end caps, contains 25,000 silicon-microstrip sensors covering 210 m2, with 9.6 million electronic readout channels. Its construction involves teams from the whole of Europe and the US, with the final assembly at CERN.
The two tracker end caps (TECs) feature silicon-strip modules mounted on wedge-shaped carbon-fibre support plates, or “petals”. Up to 28 modules are arranged in radial rings on both sides of these plates; one eighth of an end cap is populated with 18 petals and is called a “sector”.
One of the TECs, TEC+, is being constructed at the RWTH (Rheinisch-Westfälische Technische Hochschule) Aachen and testing began earlier this year. A total of 400 silicon-strip modules are read out simultaneously, using close-to-final readout and power-supply components and data-acquisition software. The first sector has already been thoroughly tested, demonstrating a channel inefficiency of less than 1% and common-mode noise of only 25% of the intrinsic noise.
To understand the behaviour of the TEC sector better, including the response to real particles, basic functionality testing was followed by a run with cosmic muons. Thousands of tracks have been recorded and will be used to study tracking performance and to exercise various track-alignment algorithms.
The next important step will be to test the first sector under CMS operating conditions, with the silicon modules working at a temperature of less than -10 °C. The remaining seven sectors will then be assembled and in autumn the TEC+ will be delivered to CERN.
