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CMS tracker sees first cosmic muons

2 May 2006

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.

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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.

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