CMS-PAS EGM-11-001
Energy calibration and resolution of the CMS electromagnetic calorimeter in pp collisions at sqrt(s) = 7 TeV

Abstract: The CMS electromagnetic calorimeter (ECAL) has been operating in different data taking conditions since its full integration in the 2008. This paper describes how the proton-proton collision data at sqrt s = 7 TeV center-of-mass energy recorded in the year 2010 have been used to improve and assess the calorimeter performance. Signals from pure samples of electrons and photons are used as a benchmark: in the paper the algorithms used for reconstruction and selection of the electromagnetic objects in CMS are presented together with their performance. Previous results reported by the CMS collaboration on the commissioning and performance of the electromagnetic calorimeter using cosmic rays data and first LHC beam splash events are extended in this paper.

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Click image for larger png version Click here for .pdf version		Figure 1: Layout of the CMS ECAL, showing the barrel supermodules, the two endcaps and the preshower detectors. The ECAL barrel coverage is up to abs(η)=1.48; the endcaps extend the coverage to abs(η)=3.0; the preshower detector fiducial area is approximately 1.65<abs(η)<2.6.
Click image for larger png version Click here for .pdf version		Figure 2: Relative response to laser light during 2011, normalized to data at the start of 2011. An average is shown for each pseudorapidity range. The bottom plot shows the corresponding instantaneous luminosity. After the last LHC technical stop, a recovery of crystal transparency is observed during the low luminosity heavy-ion data-taking at the end of 2011.
Click image for larger png version Click here for .pdf version	Click image for larger png version Click here for .pdf version	Figure 3: Left: An example of the eta-meson peak reconstructed from the invariant mass of photon pairs in EB, with the result of the fit with a Gaussian distribution (continuous line) and a polynomial function (dotted line); Right: Stability of the eta0→gamma gamma mass measurement in EB as a function of time, over a period of 60 hours, for data recorded in September 2011. The plot shows the data with (green points) and without (red points) LM corrections applied.
Click image for larger png version Click here for .pdf version	Click image for larger png version Click here for .pdf version	Figure 4 (top). Relative energy response variation for EB determined from the E/p analysis of electrons in W-boson decays. Left: examples of fits to the E=p distributions before (red) and after (green) LM corrections. Middle: response stability during the 2011 pp data-taking period before (red open circles) and after (green points) response corrections; the blue line shows the inverse of the average LM corrections. Right: distribution of the projected relative energy scales.
Click image for larger png version Click here for .pdf version	Click image for larger png version Click here for .pdf version	Figure 4 (bottom). Relative energy response variation for EE determined from the E/p analysis of electrons in W-boson decays. Left: examples of fits to the E=p distributions before (red) and after (green) LM corrections. Middle: response stability during the 2011 pp data-taking period before (red open circles) and after (green points) response corrections; the blue line shows the inverse of the average LM corrections. Right: distribution of the projected relative energy scales.
Click image for larger png version Click here for .pdf version	Click image for larger png version Click here for .pdf version	Figure 5: Mass resolution for the reconstructed Z-boson peak, from Z→e+e- decays, as a function of time for EB (left) and EE (right) before (red dots) and after (green dots) LM corrections are applied.
Click image for larger png version Click here for .pdf version	Click image for larger png version Click here for .pdf version	Figure 6: The intercalibration precision obtained in 2011 using ϕ symmetry, the E/p ratio with electrons, pi0\eta decays, and the resultant precision, with its uncertainty, for the combination of the methods, in EB (left) and EE (right).
Click image for larger png version Click here for .pdf version		Figure 7: Energy scale factors from the E/p analysis of isolated electrons as a function of η for data and MC simulation. The shaded regions, corresponding to the EB/EE interface regions, are usually excluded from the acceptance for physics analyses. The relative differences between MC simulation and data, as a function of η, are used to derive a set of relative energy scale calibrations to be applied to data.
Click image for larger png version Click here for .pdf version	Click image for larger png version Click here for .pdf version	Figure 8: Reconstructed invariant mass from Z→e+e- decays, for single-channel corrections set to unity (blue), for final intercalibration (red), and for both final intercalibration and LM corrections (black), in the EB (left) and the EE (right).
Click image for larger png version Click here for .pdf version	Click image for larger png version Click here for .pdf version	Figure 9: Left: Average values of the F\Pe supercluster correction function plotted as a function of pseudorapidity for electrons from W-boson decays with R9≥0.94 and R9<0.94, respectively. The steep increase at abs(η)≈1 is predominantly due to tracker material. Local structures correlate with the detector geometry (see text for details). Right: Material budget of the different components of the CMS tracker in front of the ECAL as a function of abs(η). The components are added to give the total tracker material budget. Notations in the legend correspond to TOB: tracker outer barrel, TIB: tracker inner barrel, TID: tracker inner discs, TEC: tracker endcaps.
Click image for larger png version Click here for .pdf version	Click image for larger png version Click here for .pdf version	Figure 10: Reconstructed dielectron invariant mass for electrons from Z→e+e- events, applying a fixed-matrix clustering of 5x5 crystals, applying the supercluster reconstruction to recover radiated energy, and applying the supercluster energy corrections. For the EE the effect of adding the preshower detector energy is shown.
Click image for larger png version Click here for .pdf version	Click image for larger png version Click here for .pdf version	Figure 11 (left column): The dielectron invariant mass distribution for Z-boson decays with both electrons in EB with R9≥0.94. Distributions in MC simulation (left) and data (right) are shown. The parameters listed in each panel are Δm - the difference between the CB mean and the true Z-boson mass, and σCB - the width of the Gaussian term of the CB function (see text for details).
Click image for larger png version Click here for .pdf version	Click image for larger png version Click here for .pdf version	Figure 11 (middle column): The dielectron invariant mass distribution for Z-boson decays with both electrons in the EB. Distributions in MC simulation (left) and data (right) are shown. The parameters listed in each panel are Δm - the difference between the CB mean and the true Z-boson mass, and σCB - the width of the Gaussian term of the CB function (see text for details).
Click image for larger png version Click here for .pdf version	Click image for larger png version Click here for .pdf version	Figure 11 (right column): The dielectron invariant mass distribution for Z-boson decays with both electrons in the EE. Distributions in MC simulation (left) and data (right) are shown. The parameters listed in each panel are Δm - the difference between the CB mean and the true Z-boson mass, and σCB - the width of the Gaussian term of the CB function (see text for details).
Click image for larger png version Click here for .pdf version	Click image for larger png version Click here for .pdf version	Figure 12 (left column): Invariant mass distribution of Z→mumugamma events. Plots show MC simulation (left) and data (right) for EB photons with R9≥0.94. The relative mean deviation of the reconstructed photon energy from that expected from the decay kinematics, δ, and the mean energy resolution of the selected events are listed (see text for details).
Click image for larger png version Click here for .pdf version	Click image for larger png version Click here for .pdf version	Figure 12 (middle column): Invariant mass distribution of Z→mumugamma events. Plots show MC simulation (left) and data (right) for inclusive EB photons. The relative mean deviation of the reconstructed photon energy from that expected from the decay kinematics, δ, and the mean energy resolution of the selected events are listed (see text for details).
Click image for larger png version Click here for .pdf version	Click image for larger png version Click here for .pdf version	Figure 12 (right column): Invariant mass distribution of Z→mumugamma events. Plots show MC simulation (left) and data (right) for inclusive EE photons. The relative mean deviation of the reconstructed photon energy from that expected from the decay kinematics, δ, and the mean energy resolution of the selected events are listed (see text for details).
Click image for larger png version Click here for .pdf version	Click image for larger png version Click here for .pdf version	Figure 13 (top row): Relative electron energy resolution in data and MC events unfolded in bins of pseudorapidity η for the barrel and the endcaps, using electrons from Z→e+e- decays. The resolution is shown separately for electrons with R9≥0.94. The resolution, σE, is extracted from a fit to Z→e+e- events, using a Breit-Wigner distribution convolved with a Gaussian function as the signal model.
Click image for larger png version Click here for .pdf version	Click image for larger png version Click here for .pdf version	Figure 13 (bottom row): Relative electron energy resolution in data and MC events unfolded in bins of pseudorapidity η for the barrel and the endcaps, using electrons from Z→e+e- decays. The resolution is shown separately for electrons with R9<0.94. The resolution, σE, is extracted from a fit to Z→e+e- events, using a Breit-Wigner distribution convolved with a Gaussian function as the signal model.
Click image for larger png version Click here for .pdf version	Click image for larger png version Click here for .pdf version	Figure 14: Distribution of the dielectron invariant mass for the default MC simulation (filled line), for the MC simulation with additional Gaussian smearing (green line), and for the data (dots). The distributions for events with both electrons in EB (left) and in EE (right) are displayed.
Click image for larger png version Click here for .pdf version	Click image for larger png version Click here for .pdf version	Figure 15 (top row): Photon energy resolution in bins of pseudorapidity abs(η) for the barrel (left) and the endcaps (right). The resolution is shown separately for photons having R9≥0.94. The energy resolution is plotted for the simulated H→gamma gamma events for the default MC simulation and for MC simulation with the addition of Gaussian smearing. The green band shows the uncertainty on the photon resolution calculated as the quadratic sum of the uncertainty on the smearing term and the statistical uncertainty in the photon resolution (shown by the vertical error bars).
Click image for larger png version Click here for .pdf version	Click image for larger png version Click here for .pdf version	Figure 15 (bottom row): Photon energy resolution in bins of pseudorapidity abs(η) for the barrel (left) and the endcaps (right). The resolution is shown separately for photons having R9<0.94. The energy resolution is plotted for the simulated H→gamma gamma events for the default MC simulation and for MC simulation with the addition of Gaussian smearing. The green band shows the uncertainty on the photon resolution calculated as the quadratic sum of the uncertainty on the smearing term and the statistical uncertainty in the photon resolution (shown by the vertical error bars).