Recent testbeam results of the CMS electromagnetic calorimeter

Precision calorimetry plays an essential role in the Higgs search at LHC. The CMS electromagnetic calorimeter (ECAL) will be made of about 76000 PbWO 4 scintillating crystals, readout by Avalanche Photodiodes in the Barrel and Vacuum Phototriodes in the Endcaps. In order to exploit fully the potential of the CMS electromagnetic calorimeter, it is important to establish suitable procedures for its calibration and monitoring. Here some of the most relevant results from the testbeam campaigns in 2002 and 2003 are illustrated. In particular the energy and position resolution and the stability of the monitoring and cooling systems are described in detail