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

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 → γγ decays. Different photon identification methods are discussed and their corresponding selection efficiencies in data are compared with those found in simulated events.The central feature of the CMS apparatus is a superconducting solenoid of 6 m internal diameter, providing a magnetic field of 3.8 T. Within the superconducting solenoid volume are a silicon pixel and strip tracker, a lead tungstate crystal electromagnetic calorimeter, and a brass/scintillator hadron calorimeter (HCAL), each one composed of a barrel and two endcap sections. Muons are measured in gas-ionization detectors embedded in the steel flux-return yoke outside the solenoid. Extensive forward calorimetry complements the coverage provided by the barrel and endcap detectors. A more detailed description of the CMS detector can be found in Ref. [1].The pseudorapidity coordinates, η, of detector elements are measured with respect to the coordinate system origin at the centre of the detector, whereas the pseudorapidity of reconstructed particles and jets is measured with respect to the interaction vertex from which they originate. Photon reconstruction Photon reconstructionPhotons for use as signals or signatures in measurements and searches, rather than for use in the construction of jets or missing transverse energy, are reconstructed from energy deposits in the ECAL using algorithms that constrain the clusters to the size and shape expected for electrons and photons with p T 15 GeV. The algorithms do not use any hypothesis as to whether the particle originating from the interaction point is a photon or an electron, consequently electrons from Z → e + e − events, for which pure samples with a well defined invariant mass can be selected, can provide excellent measurements of the photon trigger, reconstruction, and identification efficiencies, and of the photon energy scale and resolution. The reconstructed showers are generally limited to a fiducial region excluding the last two crystals at each end of the barrel (|η| < 1.4442). The outer circumferences of the endcaps are obscured by services passing between the barrel and the endcaps, and this area is removed from the fiducial region by excluding the first ring of trigger towers of the endcaps (|η| > 1.566). The fiducial region terminates at |η| = 2.5...

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“…Since there is excellent agreement between data and simulation for reconstructed photons [120], the energy resolution of photons in simulated events provides an accurate estimate of their resolution in data. Fig.…”

“…The data-to-simulation scale factors are shown below the main panels [116]. (Right) The distribution of measured over true energy, E meas /E true , for photons in simulated H → γγ events, in a narrow η range in the barrel, 0.2 < |η| < 0.3 for photons with R 9 ≥ 0.94 [120].…”

“…The event selection is described in Ref. [152,153,120,154]. The ratio of the electron energy, E, measured in the ECAL, to the electron momentum, p, measured in the tracker, is computed in each event, and a reference E/p distribution is obtained from the entire data set after applying laser corrections.…”

“…They are identified using selections on the hadronic to electromagnetic energy ratio (H/E) and the transverse shower width σ ηη , measured in terms of the energy weighted spread within the 5 × 5 crystal matrix centered on the crystal with the largest energy deposit in the supercluster [120],…”

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“…It is noted that the CMS ECAL has a resolution of ∆η × ∆φ = 0.0174 × 0.0174 and has radius R= 1.3 meters [18] while for ATLAS has a resolution of ∆η × ∆φ = 0.025 × 0.025 and has radius R=1.5 meters [19,20]. If we assume the s particle flies transversely and decays to two photon.…”