Cosmics in the LHCb Outer Tracker

The LHCb experiment at the Large Hadron Collider studies the decay of B mesons to test the description of CP violation in the Standard Model and to search for new physics. The decay $B_s \to \mu^+ \mu^-$ has been identified as very promising in the search for new physics. An excellent invariant mass resolution is required to suppress backgrounds to this decay. This in turn requires a momentum resolution of dp/p = 0.4%. The Outer Tracker is part of the LHCb tracking system and has been commissioned with cosmic muons. The noise in the Outer Tracker is shown to be less than 0.05%. To use drift time information in the reconstruction of cosmic tracks, the event time must be known. Four methods to obtain the event time are studied and compared. It is shown that the event time can be obtained with a resolution better than 2.6 ns. Using drift time information, tracks are reconstructed with a resolution of 344 $\mu$m. Knowledge of the event time enables the calibration of electronic time offsets and the r(t)– relation. Application of obtained electronic time offsets improves the resolution of tracks to 334 $\mu$m, while calibration of the r(t)-relation is unsuccessful. A spatial resolution of 334 $\mu$m is sufficient to obtain the required momentum resolution.