CMS Preshower in situ absolute calibration with physics events

This paper describes the in situ absolute calibration of the Preshower (ES) detector of CMS. Energy deposited in the lead of the Preshower is measured by silicon sensors, allowing a re-scaling of the energy measured by the endcap crystals. Measurement of the energy deposited in the lead absorbers to 5% accuracy is required over a very large dynamic range (1–400 MIPs equivalent); a maximum accuracy of 1% on the measurement of the charge deposited in the silicon will be sufficient. Radiation damage to the sensors (decrease in charge collection efficiency by up to 17% over 10 years) and the electronics (decrease in gain by up to 2% over 10 years) will need to be assessed by periodic in situ calibrations. A precise in situ absolute calibration using particle signals from physics events is examined. It is shown that sufficient calibration accuracy can be obtained by using muon or pion events, and that the time required for the calibration is of the order of a few days at initial LHC luminosity and at least a factor of two less for nominal LHC luminosity.