Study of the LHCb Muon Detector performance using 2010 beam data

The LHCb muon detector is used online in level-0 (L0) and high-level (HLT) triggers and for the offline muon identification. All these applications require a very high efficiency in the very short LHC time gate. The most stringent requirement is dictated by the L0 trigger, which, in order to effectively reduce the background, asks the muon system to give a signal in all the five stations. To efficiently satisfy such requirement the muon chambers were designed to have a detection efficiency larger than 99% within the 25 ns LHC gate. Cosmic data and collision data acquired in 2009 were used for the first calibrations with physics signals and the first evaluation of performance, but the precisions was limited by statistical and systematic uncertaintites. The large p-p collisions data sample made available by the 2010 LHC run allowed for improving these results, and in particular for a precise determination of the chamber efficiency. In this note, after summarizing the main features of detector operations during 2010, we first evaluate the average response to muon tracks in terms of cluster size, to evaluate the cross-talk among adjacent detector channles. Paricle rates are measured, verifying their scaling with luminosity over the wide range of beam intensity spanned during the 2010 data taking. We then descirbe the improvements on timing calibration and the resulting time resolution. Finally, an accurate measurement of the chamber efficiency is presented. After discussing the method for background substraction and a check of the procedure, results are reported, including an estimate of the systematic uncertainties.