In-Situ Calibration of the CMS Electromagnetic Calorimeter

The in-situ intercalibration of the lead tungstate crystals of the CMS electromagnetic calorimeter will be performed using 3 techniques: An energy flow method will be used at startup to intercalibrate to a precision of around 2% within about 3 hours. The energy/momentum measurement of isolated electrons from W decay will then be used to obtain the design goal precision of 0.5% within about 2 months. Global intercalibration of different regions of the calorimeter and the determination of the absolute energy scale will be performed using energetic electrons from Z->ee events.