The CMS electromagnetic calorimeter

The CMS electromagnetic calorimeter aims at providing high precision calorimetry at the Large Hadron Collider (LHC). It consists of about 75.000 PbW0₄ crystals that have to operate reliably for at least 10 years, in a high radiation environment. In order to fully profit of the high intrinsic energy resolution of the crystals, severe requirements on their temperature stability must be satisfied. Another challenge is given by the high data rates to be sustained by the readout electronics. With more than a half of the barrel modules produced, the calorimeter is well into its production phase. A large effort was devoted to optimize the integration between mechanics, cooling and readout electronics. The performance of the first modules has been tested in an electron beam, to validate the monitoring system and the calibration strategy. An overview of the calorimeter design and of its construction status will be given, as well as the results from the test beam measurements and the predicted performance at the LHC.