Commissioning of the CMS electromagnetic calorimeter and operational experience

The barrel part of the CMS electromagnetic calorimeter consists of 61,200 lead-tungstate crystals optically coupled to twice as many avalanche photodiodes. The on-detector read-out electronics comprises about 20,000 printed circuit boards and 5000 optical links. The integration and commissioning phase of the 36 supermodules, 1700 crystals each, is flowing steadily at CERN in compliance with the experiment installation schedule. Given the high complexity of the apparatus, each integration step is validated by specific quality controls. It follows a week of pedestal, test-pulse and laser light monitoring runs, and 10 days of cosmic ray calibration. An automatic analysis of the data provides immediate feedback for possible intervention and verifies the stability and performance of the detector. In addition, a subset of supermodules is undergoing an electron beam calibration while two supermodules are being operated, within a segment of the CMS detector, in a CMS-like environment during the magnet test. The strategy and procedure aimed to ensure a zero-fault tolerance in the commissioning of such a highly sophisticated and performing apparatus will be described, along with the operational experience accumulated so far.