Bringing the ATLAS muon spectrometer to life with cosmic rays

The muon spectrometer of the ATLAS experiment at the Large Hadron Collider is the largest device ever built to track high energy particles. It has been designed to provide muon identification and measurement in the hard environment of proton-proton collisions at high energy and high luminosity at the LHC. Three toroid magnets, one in the barrel and two in the end-caps, host the particle detectors for the trigger and for precision tracking. The bending power, which ranges between 1 and 7.5 T-m, depending on the pseudorapidity, and the low amount of material crossed by the muons in the spectrometer, allow the precise determination of the transverse momentum over a wide pseudorapidity interval, |eta|<2.4, with a resolution better than 10% up to 1 TeV for |eta|<1.1. Four different types of detectors, two devoted to triggering and two to the precision momentum measurement, cover an area of 10,000 square meters and are read out by 1 million channels of electronics. Many advanced technological components are used in both the hardware and the software to control the detector status, monitor the data quality. This impressive number of elements, spread over the large volume of the spectrometer, were commissioned for many months with cosmic rays and were ready to take data when the first beam was circulated in the LHC. A systematic study of the detector’s performance was done in the following months. More than 200 million of cosmi c ray triggers were taken in different conditions, out of them 92% triggered by the muon system itself. These data have been used to map all detector elements, and to improve the understanding of the trigger, pattern recognition, and tracking, as well as to commission the calibration and alignment procedures and the data analysis software. We present the status of the muon detectors and the main results from the reconstruction of this event sample, showing that the ATLAS muon spectrometer is well advanced towards physics data taking.