Commissioning of the LHCb preshower detector with cosmic rays and first LHC collisions

The LHCb calorimeter system consists of a Scintillating Pad Detector (SPD), a PreShower detector (PS) followed by electromagnetic (ECAL) and hadronic (HCAL) calorimeters. The LHCb preshower detector provides the longitudinal segmentation of the electromagnetic shower detection. It is located immediately upstream from the ECAL, with one-to-one correspondence between ECAL towers, SPD and preshower cells. The PS detector is part of the first level of the LHCb trigger system (called Level-0). It is used in conjunction with the SPD, ECAL and HCAL to search for clusters of 2 x 2 cells and to identify e, γ, π0, hadron of highest ET. This contribution discusses the requirements for the synchronisation and calibration of the PS detector, as well as the methods that were used to determine the first settings before the LHC start-up. The methods used to perform the fine synchronisation and calibration once LHC collisions were available are introduced, and the precisions that were obtained are given.