CMS Preshower in-situ Absolute Calibration with Physics Events

This note describes the in-situ absolute calibration of the Preshower detector of CMS. The Preshower is based on silicon strip sensors that will be installed in the endcaps of CMS in front of the crystal Calorimeter. Energy deposited in the lead of the Preshower is estimated by the 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), thus a maximum accuracy of 1% on the measurement of the charge deposited in the silicon will be sufficient. There are two principle sources of response variation at startup (sensor-to-sensor and channel-to-channel): sensor thickness (RMS of 1-2%) and gain uniformity of the electronics (RMS ~3%). These will be measured and thus taken into account. 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 minimum ionizing particle signals from physics events is examined. For the calibration method the full simulation framework of CMS has been used (CMSIM/CMKIN, OSCAR and ORCA). 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.