CMS ECAL Laser Monitoring System Christopher S. Rogan, 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. 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 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. 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. 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 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) Irradiation of PbWO 4 crystals results in the formation of color centers which absorb and scatter light, reducing the transparency of the crystal. Simultaneously, the scintillation mechanism of the crystal is unaffected by irradiation, resulting in the crystal’s response to the laser monitoring system being different than its electromagnetic shower response. According to radiation damage models, the relationship between the crystal response to the laser monitoring system ( ) and to electromagnetic showers ( ) can be described by a power law: The laser monitoring system has been commissioned at a test beam facility at CERN, where the performance was evaluated over the period of months. A stability on the order of 0.1% has been demonstrated for the monitoring system, allowing even small changes in transparency to be monitored with precision and the dynamics and characteristics of crystal transparency changes to be studied. Dispersion of  for 33 BTCP endcap crystals # Crystals  Traditionally, values of  have been measured by fitting plots like the one to the left (the fit method). Here, events incident in a single crystal are grouped in small time intervals and the crystal response is fit, determining a mean and spread. These data points are compared with the corresponding laser measurements. In the 2006 and 2007 test beams, a new method was considered, where the energy resolution of all events explicitly minimized as a function of  (the minimization method). In 2007, a wide beam-spot was used to irradiate four crystals at once with a mono-energetic electron beam, and the energy response resolution was minimized w.r.t. the four crystals’  values simultaneously. This approach can be generalized to collision data, where electromagnetic resonances can be used to measure alpha in situ. Test Beam Studies  four xtal minimization  fit method mean = 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. 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. Timing (1 Unit =25 ns)  measurements for 33 endcap (left) and 35 barrel (right) crystals. The mean values of the distributions are identical. Systematics for endcap measurements still under investigation. Comparison between  values extracted using the fit method and using the minimization method yield consistent values. Systematic uncertainties of minimization method are appreciably smaller. Results of correction procedure using measured  values for 120 GeV/c electron beam. The correction fully recovers the pre-irradiation resolution. Commissioning at CMS preliminary