1. Calibration of the CMS Electromagnetic Calorimeter with LHC collision data Maria Margherita Obertino on behalf of the CMS Collaboration Introduction The electromagnetic calorimeter (ECAL) of the CMS experiment is an homogeneous, hermetic detector with high granularity. It is made of 75,848 lead-tungstate (PbWO 4 ) crystals. The central barrel calorimeter (EB) is organized into 36 supermodules (SM) and it is closed at each end by an endcap calorimeter (EE) consisting of two “dees”. For the light collection it is equipped with avalanche photodiodes (APD) in the barrel part and vacuum phototriodes (VPT) in the endcaps. A silicon/lead pre-shower detector (ES) is installed in front of the crystal calorimeter in the endcaps in order to improve the  0 discrimination and the vertex reconstruction for photons. The CMS ECAL is one of the highest resolution electromagnetic calorimeters ever constructed, but relies upon precision calibration in order to achieve and maintain its design performance. Variations in light collected from the lead tungstate crystals, due to intrinsic differences in crystals/photodetectors, as well as variations with time due to radiation damage for example, need to be taken into account. Sophisticated and effective methods of inter-crystal and absolute calibration have been devised, using collision data and a dedicated light injection system. For inter- calibration, low mass particle decays (p0 and eta) to two photons are exploited, as well as the azimuthal symmetry of the average energy deposition at a given pseudorapidity. Absolute calibration has been performed using Z decays into electron-positron pairs. The light injection system monitors the transparency of the crystals in real-time and enables the re- calibration of the measured energies over time. This is cross- checked by the comparison of E/p measurements of electrons from W decays (where the momentum is measured in the CMS tracker) with/without these re-calibrations applied. Introduction The electromagnetic calorimeter (ECAL) of the CMS experiment is an homogeneous, hermetic detector with high granularity. It is made of 75,848 lead-tungstate (PbWO 4 ) crystals. The central barrel calorimeter (EB) is organized into 36 supermodules (SM) and it is closed at each end by an endcap calorimeter (EE) consisting of two “dees”. For the light collection it is equipped with avalanche photodiodes (APD) in the barrel part and vacuum phototriodes (VPT) in the endcaps. A silicon/lead pre-shower detector (ES) is installed in front of the crystal calorimeter in the endcaps in order to improve the  0 discrimination and the vertex reconstruction for photons. The CMS ECAL is one of the highest resolution electromagnetic calorimeters ever constructed, but relies upon precision calibration in order to achieve and maintain its design performance. Variations in light collected from the lead tungstate crystals, due to intrinsic differences in crystals/photodetectors, as well as variations with time due to radiation damage for example, need to be taken into account. Sophisticated and effective methods of inter-crystal and absolute calibration have been devised, using collision data and a dedicated light injection system. For inter- calibration, low mass particle decays (p0 and eta) to two photons are exploited, as well as the azimuthal symmetry of the average energy deposition at a given pseudorapidity. Absolute calibration has been performed using Z decays into electron-positron pairs. The light injection system monitors the transparency of the crystals in real-time and enables the re- calibration of the measured energies over time. This is cross- checked by the comparison of E/p measurements of electrons from W decays (where the momentum is measured in the CMS tracker) with/without these re-calibrations applied. References CMS Collaboration, “The CMS experiment at the CERN LHC”, J. Inst. 3 S08004 (2008) CMS Collaboration, “The Electromagnetic Calorimeter Technical Design Report”, CERN/LHCC 97-33 (1997) M. Bonesini et al., “Intercalibration of the electromagnetic calorimeter with cosmic rays”, CMS NOTE 2005/023 L. Agostino et al., “Inter-calibration of the CMS electromagnetic calorimeter with isolated electrons”, J. Phys. G 33(2007) N67-N84 P. Adzic et al., “Energy resolution of the barrel of the CMS electromagnetic calorimeter”, J. Inst. 2 P04004 (2007) P. Adzic et al., “Intercalibration of the barrel electromagnetic calorimeter of the CMS experiment at start-up”, J. Inst. 3 P10007 (2008) CMS Collaboration, “Performance and operation of the CMS electromagnetic calorimeter”, J. Inst. 5 T03010 (2010) CMS Collaboration, “Electromagnetic calorimeter calibration with 7 TeV data”, CMS PAS EGM-10-003 (2010) ϕ η Summary At start-up the calibration precision of the electromagnetic calorimeter in the CMS experiment was between 1.5% and 2.5% in the barrel and of about 5% in the endcaps. The level of precision reached is such that Z width measurement is not affected in EB, and is already almost insensitive to residual mis-calibration even in EE Using the first 250 nb -1 collected with the CMS detector in 2010 at a center of mass energy of 7 TeV a channel-by-channel calibration is achieved with a precision of about 0.6% in the central barrel. The global energy response scale of the ECAL is also studied, and found to agree with the expectation to within about 1% in the barrel and 3% in the endcaps. The calibration of the CMS ECAL will continue with the upcoming LHC data, with collection of large samples of neutral pion decays as well as W and Z bosons decaying into energetic and isolated electrons. Summary At start-up the calibration precision of the electromagnetic calorimeter in the CMS experiment was between 1.5% and 2.5% in the barrel and of about 5% in the endcaps. The level of precision reached is such that Z width measurement is not affected in EB, and is already almost insensitive to residual mis-calibration even in EE Using the first 250 nb -1 collected with the CMS detector in 2010 at a center of mass energy of 7 TeV a channel-by-channel calibration is achieved with a precision of about 0.6% in the central barrel. The global energy response scale of the ECAL is also studied, and found to agree with the expectation to within about 1% in the barrel and 3% in the endcaps. The calibration of the CMS ECAL will continue with the upcoming LHC data, with collection of large samples of neutral pion decays as well as W and Z bosons decaying into energetic and isolated electrons. The strategy to intercalibrate The ϕ -symmetry method is based on the assumption that for a large number of minimum bias events the total transverse energy (E T ) deposited should be the same for all crystals in a ring at fixed pseudorapidity (η). Inter-calibration in ϕ can be performed by comparing the total transverse energy (ΣE T ) deposited in one crystal with the mean of the total ΣE T collected by crystals at the same absolute value of η. In the barrel, crystals are arranged in 85 pairs of rings (positive and negative sides) with 360 crystals in each ring. In the determination of the transverse energy sum, only deposits lying between a low and high thresholds are considered. The former is applied to remove the noise contribution, the latter is meant to avoid that a few very high E T hits bias significantly the value of the summed transverse energy. The ϕ inhomogeneities of the detector are taken into account introducing a data-driven correction. In the  0 /  method, the invariant mass of photon pairs from π 0 /   γγ is used to obtain the inter-calibration constants. The photon candidates are reconstructed using a simple 3 ✕ 3 window clustering algorithm. The cluster energy is computed as the sum of crystal energies S 9 = Σ 3 ✕ 3 c i  E i where c i denotes the calibration constant and E i the energy deposited in each i-th crystal. The shape of the cluster energy deposition should be consistent with that of an electromagnetic shower produced by a photon. The selected sample contains about 1.4  10 7 signal π 0 decays. Single crystal intercalibration Laser Monitoring system During LHC cycles the ECAL response varies depending on the irradiation conditions, which modify the transparency of the PbWO 4 crystals. This effect takes place on a time scale of hours and can cause transparency changes of a few percent during LHC fills/interfills periods, dependent on the instantaneous and integrated luminosity. To maintain the ECAL design performance a laser monitoring (LM) system was designed to accurately monitor response changes for each crystal at the level of 0.2%, with one measurement per crystal every 20 to 30 minutes. The light monitoring system [XX]consists of two different lasers: a blue laser with a wavelength (λ=440 nm) close to the PbWO 4 emission peak, and an infra-red laser (λ=796 nm). The blue laser is used to monitor crystal transparency at the scintillation peak whereas the infra-red laser is used to disentangle effects due to irradiation from other possible effects such as photodetector or electronics gain variations. For the endcaps, the monitoring system is complemented by an LED pulser system. To proved corrections with the required precision a very large effort has been put in the laser data analysis. The signal is corrected for laser pulse width and Absolute energy scale with  decays The absolute scale of the ECAL, the ADC to GeV conversion factor, has been measured during the test beam campaign for EB and EE separately. In-situ, the ECAL energy scale will be determined by reconstructing di- electronand di-photon invariant mass peaks. A first measurement can be done using      and     decays. To be considered in the combinatorial selection of  pairs, a photon candidate is required to have transverse momentum above 0.4 GeV. The   candidate are then selected by requiring its transverse momentum above 1 GeV. Right plots show the invariant mass distribution of the selected   candidates both for the collected and simulated events. SM amplitude change in order to cope with laser maintenance interventions. The response is normalized to the signal of a PN installed in the same optical fiber fan-out, whose stability and linearity have been studied with dedicated laser amplitude scans. The plot shows the relative response to laser light (440 nm) measured by the ECAL laser monitoring system, averaged over all crystals in bins of pseudo rapidity  for the 2011 data taking. The observed transparency loss has an exponential behaviour and reaches a saturation level which depends on the dose rate. The average response change is about 2–3% in the Barrel and reaches 40% for |η | = 2.7 in endcap, in agreement with the expectations for the achieved instantaneous luminosity. The spontaneous recovery of the crystals in periods without irradiation is clearly visible. The crystal-by-crystal calibration constants obtained with ϕ -symmetry and π 0 methods independently are combined together with the beam dump constants. The precision of the resulting inter-calibration constants is shown in the lower plot as a function of crystal pseudorapidity. The 0.5% test beam precision is not negligible with respect to the combination precision and therefore subtracted in quadrature. For the central barrel (|crystal η index| ≤ 45) the combined inter-calibration precision is found to be 0.6%.