Measurement of the logical pad cluster size of the installed muon chambers with the 2008 cosmic runs

One of the main goals of the LHC detectors commissioning is to verify that the performance measured during beam tests is also obtained with the detectors installed in the pits. In the context of the LHCb muon system there are few important chamber parameters to be checked, such as the total efficiency, the time resolution, the pad multiplicity, the noise, the number of dead channels and shorts. This note deals with the measurement of the logical pad multiplicity, of shorts among channels and of uncon- nected channels of the installed muon chambers using cosmic rays from the 2008 runs. We present also a mea- surement of noise using a dedicated random trigger run. The logical pad multiplicity is found to be higher than measured during beam tests, even if in our analysis no time window cut was applied. We show that the noise is below an average of 100 Hz per region and station and that the number of shorts and of unconnected channels is very low in the outermost regions and stations, where we have enough statistics. We leave as a future development the related studies of time alignment of channels and the analysis a of a thresh- old scan run, not yet performed.