Performance of photon reconstruction and identification with the CMS detector in proton-proton collisions at sqrt(s) = 8 TeV

TL;DR

This study assesses CMS photon reconstruction and identification performance in 8 TeV proton–proton collisions, focusing on optimizing photon energy estimation and its simulation modelling for Higgs boson decays to two photons. It combines detailed ECAL calibration, sophisticated clustering and regression-based energy corrections, and both sequential and multivariate photon identification approaches, validated against data using Z→ee and Z→μμγ samples. The results show excellent data–MC agreement, achieve about 1% energy resolution in the ECAL barrel for unconverted/late-converting photons, and quantify energy-scale uncertainties that feed into the Higgs mass measurement. The work underpins robust photon-based analyses at the LHC, including H→γγ, by delivering precise energy scale/resolution, reliable conversion handling, and efficient, well-characterized photon identification.

Abstract

A description is provided of the performance of the CMS detector for photon reconstruction and identification in proton-proton collisions at a centre-of-mass energy of 8 TeV at the CERN LHC. Details are given on the reconstruction of photons from energy deposits in the electromagnetic calorimeter (ECAL) and the extraction of photon energy estimates. The reconstruction of electron tracks from photons that convert to electrons in the CMS tracker is also described, as is the optimization of the photon energy reconstruction and its accurate modelling in simulation, in the analysis of the Higgs boson decay into two photons. In the barrel section of the ECAL, an energy resolution of about 1% is achieved for unconverted or late-converting photons from H to gamma gamma decays. Different photon identification methods are discussed and their corresponding selection efficiencies in data are compared with those found in simulated events.

Figures (25)

• Figure 1: Distributions of the $R_\mathrm{9}$ variable for photons in the ECAL barrel that convert in the material of the tracker before a radius of 85$\,\text{cm}$ (solid filled histogram), and those that convert later, or do not convert at all before reaching the ECAL (outlined histogram).
• Figure 2: Comparison of the distribution of the inverse response, $E_\text{true}/E_\text{raw}$, in simulated events (points with error bars) with the sum of the pdfs predicted by the regression (curve). The comparison is made using a set of simulated photons independent of the training sample, in the (left) ECAL barrel and (right) endcap.
• Figure 3: Ratio of the energy measured by the ECAL over the momentum measured by the tracker, $E/p$, for electrons selected from $\mathrm{W}\to\mathrm{e}\nu\xspace$ decays, as a function of the date at which they were recorded. The ratio is shown both before (red points), and after (green points), the application of transparency corrections obtained from the laser monitoring system, and for both the barrel (upper plot) and the endcaps (lower plot). Histograms of the values of the measured points, together with their mean and RMS values are shown beside the main plots.
• Figure 4: Residual discrepancies in the photon energy scale obtained for the barrel in the final step of the fine-tuning procedure, as a function of $E_{\mathrm{T}}\xspace$, for different $\eta$ and $R_\mathrm{9}$ categories. The statistical uncertainties in these values are negligible. The horizontal error bars indicate the ranges of the $E_{\mathrm{T}}\xspace$ bins. The reciprocals of these values are applied as corrections to the energy scale. Some of the error bars have been deflected vertically to avoid overlap with others.
• Figure 5: Reconstructed invariant mass distribution of electron pairs in ${Z}\to\mathrm{e}^+\mathrm{e}^-\xspace$ events in data (points) and in simulation (histogram). The electrons are reconstructed as photons and the full set of photon corrections and smearings are applied. The comparison is shown for (left) events with both showers in the barrel and (right) the remaining events. For each bin, the ratio of the number of events in data to the number of simulated events is shown in the panels beneath the main plot. The band shows the systematic uncertainty in the ratio originating in the systematic uncertainty in the simulated energy resolution, and in the data energy scale.