CMS Electromagnetic Calorimeter preparation towards the LHC data taking

The CMS lead tungstate crystal electromagnetic calorimeter (ECAL) is expected to reach excellent performances in order to guarantee the full CMS discovery potential of the Higgs boson. An extensive test beam calibration campaign has been carried out during 2006: 9 ECAL Barrel supermodules (each supermodule is composed of 1700 crystals) have been put on the beam. One of them has been exposed twice at one month interval in order to assess the reproducibility precision. This extensive amount of data has been used also for detailed linearity, energy and position resolution studies and to validate and tune the GEANT 4 based electromagnetic shower simulation. Another test beam has been conducted on a slice of the whole calorimeter (ECAL plus HCAL), exposing it to beams of electrons and pions with energies from 1 to 300 GeV. This test complements the previous one in understanding the different response of the whole CMS calorimetry to electrons and pions in view of the optimal combination of the two calorimeter components (ECAL and HCAL). Parallel to these tests, the cosmic ray commissioning of the barrel supermodules continued on a dedicated cosmic ray setup. This test not only represents an initial commissioning of each supermodule, but also gives the possibility to measure the intercalibration at a level of better than 2%. Thanks to these tests, the year 2006 represented a fundamental year to assess the readiness and integration of ECAL online and offline components: infact, the CMS standard data acquisition, data quality monitoring (DQM) architecture and offline reconstruction software have been used. In this respect, another milestone for the ECAL project has been achieved during the Magnet Test and Cosmic Challenge (MTCC) where two ECAL Barrel supermodules have acquired data in global runs with all the other subdetectors.