Results of the Cosmic Run

The goal of the cosmic data taking with the CMS detector is the commissioning and calibration of the individual subdetectors, the alignment of the tracking detectors and the muon chambers and the testing of the DAQ system. 

The CMS experiment recorded about270 million cosmic-ray triggered events with the solenoid at a field strength of 3.8 T. More than 4 million tracks were reconstructed out of which approximately 85,000 did cross the pixel detector volume. An event display showing a cosmic muon traversing the pixel detector is shown in Fig. 8.20. 

The results of the alignment of the silicon strip and pixel detectors is detailed in [14], After performing a track-based alignment, the precision of the detector position with respect to particle trajectories had been derived from the distribution of the median of the cosmic muon track residuals measured in each module. A precision of 3 /rm in the direction and 4 /xm in the z direction has been achieved for the BPIX detector. 

In total about 257,000 hits were reconstructed in the pixel detector with an average number of 60 hits per ROC. In Fig. 8.21 the number of track-associated hits in each BPIX ROC is shown. The ROCs that did not have any hits were excluded from the readout for the reasons given in Sect. 8.5.2. 

Furthermore, the cosmic data were used to estimate the hit efficiency of the BPIX modules from data. This is done by extrapolating tracks reconstructed in the silicon tracker to the pixel detector and checking the presence of a compatible hit. The efficiency is then defined as the number of tracks with a compatible pixel hit divided by the total number of tracks. For the efficiency measurement only tracks to which one additional hit was associated in both the top and bottom half of the pixel detector were selected. A layer efficiency averaged over the operable modules of (97.1 1.4)%, (97.1 1.9)% and (96.4 2.6)% was measured for the first, second, and third barrel layers, respectively. 

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