1. CMS ECAL Laser Monitoring System Toyoko J. Orimoto, California Institute of Technology, on behalf of the CMS ECAL Group High-resolution, high-granularity scintillating crystal calorimeter 75,848 lead-tungstate (PbWO 4 ) crystals Crystals of the short radiation length, small Molière radius, and fast speed as a scintillator. Gap Events FilterFarm/HLT Laser Farm Disk Buffer OMDS DB ORCOFF DB Offline Reconstruction CMS Point 5 Offline CMS DAQ GT LASER Raw APD/PN Corrected APD/PN Repackage Laser Data ORCON DB Laser Monitoring Dataflow: Laser monitoring data will be taken during the LHC “abort gap” events, 3 μs every 90μs. 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 database located at Point 5, and then transferred to the ORCON/ORCOFF database. 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 harsh radiation environment (dose-rates of 15 rad/hour at 10 34 cm -2 s -1 ). The PbWO 4 crystals are radiation hard, but suffer from dose-rate dependent radiation damage. 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, calibration and monitoring of the crystals must be performed in situ at the LHC. The monitoring light source consists of three pairs of lasers (Nd:YLF pump laser and Ti:Sapphire laser), with diagnostics, a 3x1 optical switches, a 1x88 optical switch, a monitor and a PC based controller. Commissioning at CMS Point 5 Two laser systems (blue/green and IR/red) have been installed and commissioned in the CMS underground cavern at Point 5, and first laser data has been collected. Radiation causes a degradation 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. The system has been commissioned in-situ and is now running at CMS Point 5. APD VPT The wavelength of the Ti:S laser is tunable, and two wavelengths are available from each laser. Four wavelengths, 440, 495, 709, and 796 nm, are available using the 3x1 optical switch. Laser pulses of the selected wavelength is sent to each ECAL element using the 1x88 optical switch. Laser Specifications: 2 wavelengths per laser Pulse width, FWHM < 40ns to match ECAL readout Pulse jitter< 3ns for synchronization with LHC Pulse rate~100 Hz, scan of full ECAL in 20min Pulse intensity instability~few% Pulse energy1 mJ/pulse at monitoring wavelength (equivalent to 1.3 TeV in dynamic range) In 2006, a software feedback system was implemented so that the stability of the pulse intensity and FWHM are maintained at the level of a few percent, with a pulse timing jitter of less than 2 ns observed in laser runs lasting for over 2,000 hours during beam tests. Laser Stability Performance from Test Beam Results The laser monitoring system has been commissioned at a test beam 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. Test Beam Data Recovery Damage Electron Response Laser Response E(t)/E(t 0 ) = [L(t)/L(t 0 )]  Energy Resolution Before and After Correction APD/PN RMS Before and After Correction RMS(APD/PN) Pulse ShapeAPD Amplitude Event Time sample ADC counts Data from the laser monitoring is also used for ECAL commissioning. For instance, the laser is used to understand the timing of the ECAL signal as shown here. The plot on the right shows the crystal-by-crystal timing variations from laser data. The lower plot shows the ECAL signal timing from LHC beam shots (from, after laser timing corrections. ii ii ii Timing (1 Unit =25 ns) ii