1. CMS ECAL Laser Monitoring System Toyoko J. Orimoto, California Institute of Technology On behalf othe CMS ECAL Collaboration High-resolution, high-granularity scintillating crystal calorimeter 75,848 lead-tungstate (PbWO 4 ) crystals Crystals of the short radiation length, small Molière radius, and speed as a scintillator. Gap Events CMS FilterFarm/HLT Laser Farm Disk Buffer OMDS ORCOFF Offline Reconstruction P5 Offline Tier0 CAF DAQ GT LASER ? Raw APD/PN Corrected APD/PN Repackage Laser Data ORCON Laser Monitoring Dataflow: Laser monitoring data will be taken during the LHC “abort gap” events, 3ms every 90  s. Gap events will arrive at the Filter Farm, containing, among other data, the ECAL laser event data, which will be sorted and then analyzed in a PC farm to extract APD/PN values. The data is then inserted into the OMDS DB located at Point 5, and then transferred to the ORCON/ORCOFF DB. During the transfer procedure, corrections will be applied. The laser APD/PN ratios, reference values, and scale factors necessary to implement the transparency correction will be stored in the offline database, and the correction is applied in the offline reconstruction. The Laser Monitoring System Overview: At the LHC design luminosity, the CMS detector will be exposed to a very harsh radiation environment. The PbWO 4 crystals are radiation hard up to a high integrated dosage, but suffer from dose-rate dependent radiation damage. Exposure at the level of LHC luminosity causes a decrease in crystal transparency due to radiation induced absorption. Although the crystals will self-recover during periods in absence of radiation, this recovery takes places on the order of a week. Therefore, changes in crystal transparency, and therefore calorimeter response, due to radiation damage must be corrected for to maintain the energy resolution of the detector. The CMS ECAL utilizes a laser monitoring system to monitor the light output of the crystals. With this system, we can measure the change in transparency of each crystal continuously during LHC running, with very high precision. Compact Muon Solenoid (CMS) Electromagnetic Calorimeter (ECAL) The design energy resolution of the ECAL has a constant term of 0.5%, and to maintain this, in situ calibration and monitoring of the crystals must be performed. Performance from Testbeam Results The laser monitoring system has been commissioned at a testbeam facility at CERN, where the performance was evaluated over periods of months. The stability of the system has been exhibited to be on the order of 0.1%; with such performance, even small changes in transparency can be monitored with precision. The monitoring light source consists of 3 laser systems with diagnostics, two optical switches, a monitor and a PC based controller. All three lasers are model 572DQ-S Q-switched green Nd:YLF lasers, from Quantronix. Each laser provides frequency doubled laser pulses at 527 nm with pulse intensity up to 20 mJ at rep rates of up to 1kHz. The wavelength of the Ti:S laser is tunable, and two wavelengths are available from each Ti:S laser. Thus there are four wavelengths, 440, 495, 709, and 796 nm are available using the optical switch. The selected wavelength is sent to each ECAL supermodule using a 1x80 optical switch. The third laser is a spare to optimize availability, during maintenance. APD VPT Laser Specifications: 2 wavelengths Spectral contamination< 10 -3 Pulse width, FWHM < 40ns to match ECAL readout Pulse jitter< 3ns for sychronization with LHC Pulse rate~80 Hz, the max allowed by ECAL DAQ Pulse intensity instability< 10% Pulse energy1 mJ/pulse at monitoring wavelength (equivalent to 1.3 TeV in dynamic range) Laser Stability: Commissioning at Point 5 Blah. Test Beam Data Recovery Damage