Research Collection Journal Article Performance of Photon Reconstruction and Identification with the CMS Detector in Proton-Proton Collisions at √s = 8 TeV Author(s): CMS Collaboration; Khachatryan, Vardan; Bachmair, Felix; Bäni, Lukas; Bianchini, Lorenzo; Buchmann, Marco A.; Casal, Bruno; Chanon, Nicolas; Dissertori, Günther; Dittmar, Michael; Donegà, Mauro; Dünser, Marc; Eller, Philipp; Grab, Christoph; Hits, Dmitry; Hoss, Jan; Kasieczka, Gregor; Lustermann, Werner; Mangano, Boris; Marini, Andrea C.; Marionneau, Matthieu; Martinez Ruiz del Arbol, Pablo; Masciovecchio, Mario; Meister, Daniel; Mohr, Niklas; Musella, Pasquale; Nägeli, Christoph; Nessi-Tedaldi, Francesca; Pandolfi, Francesco; Pauss, Felicitas; Perrozzi, Luca; Peruzzi, Marco; Quittnat, Milena; Rebane, Liis; Rossini, Marco; Starodumov, Andrey; Takahashi, Maiko; Theofilatos, Konstantinos; Wallny, Rainer; Weber, Hannsjörg A.; et al. Publication Date: 2015 Permanent Link: https://doi.org/10.3929/ethz-b-000105311 Originally published in: Journal of Instrumentation 10(8), http://doi.org/10.1088/1748-0221/10/08/P08010 Rights / License: Creative Commons Attribution 3.0 Unported This page was generated automatically upon download from the ETH Zurich Research Collection. For more information please consult the Terms of use. ETH Library Journal of Instrumentation OPEN ACCESS Related content - The Search and Discovery of the Higgs Performance of photon reconstruction and Boson: The ATLAS and CMS detectors L R F Castillo identification with the CMS detector in proton- - Performance of electron reconstruction and selection with the CMS detector in proton collisions at √s = 8 TeV proton-proton collisions at s = 8 TeV - Overview of Energy Reconstruction, and To cite this article: 2015 JINST 10 P08010 Electron and Photon Performances with the CMS ECAL in Run II Rafael Teixeira de Lima and CMS Collaboration View the article online for updates and enhancements. Recent citations - Searching for additional Higgs bosons via Higgs cascades Christina Gao et al - Hadron collider searches for diboson resonances Tommaso Dorigo - Multiboson interactions at the LHC D. R. Green et al This content was downloaded from IP address 129.132.109.227 on 03/05/2018 at 15:14 PUBLISHED BY IOP PUBLISHING FOR SISSA MEDIALAB RECEIVED: February 9, 2015 REVISED: April 21, 2015 ACCEPTED: July 9, 2015 PUBLISHED: August 18, 2015 Performance of photon reconstruction and 2015 JINST 10 P08010 identification with the CMS detector in proton-proton p collisions at s = 8TeV The CMS collaboration E-mail: [email protected] ABSTRACT: A description is provided of the performance of the CMS detector for photon recon- struction 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 elec- tromagnetic 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 ! gg decays. Different photon identification methods are discussed and their corresponding selection efficiencies in data are compared with those found in simulated events. KEYWORDS: Pattern recognition, cluster finding, calibration and fitting methods; Performance of High Energy Physics Detectors ARXIV EPRINT: 1502.02702 c CERN 2015 for the benefit of the CMS collaboration, published under the terms of the Creative Commons Attribution 3.0 License by IOP Publishing Ltd and Sissa Medialab srl. Any further distribution of this work must maintain attribution to the author(s) and the doi:10.1088/1748-0221/10/08/P08010 published article’s title, journal citation and DOI. Contents 1 Introduction1 2 CMS detector2 3 Data and simulated event samples3 4 Photon reconstruction4 2015 JINST 10 P08010 4.1 Calibration of individual ECAL channels5 4.2 Clustering5 4.3 Correction of cluster energy6 4.4 Fine tuning of calibration and simulated resolution9 4.5 Photon energy resolution 12 4.6 Energy scale uncertainty 16 5 Conversion track reconstruction 21 6 Photon identification 24 6.1 Electron rejection 25 6.2 Photon identification variables 26 6.3 Photon identification based on sequential requirements 29 6.4 Multivariate photon identification 31 7 Summary 36 The CMS collaboration 43 1 Introduction This paper describes the reconstruction and identification of photons with the CMS detector [1] in p data taken in proton-proton collisions at s = 8TeV during the 2012 CERN LHC running period. Particular emphasis is put on the use of photons in the observation and measurement of the dipho- ton decay of the Higgs boson [2]. For this decay mode, the energy resolution has significant impact on the sensitivity of the search and on the precision of measurements made in the analysis. The uncertainties related to the photon energy scale are the dominant contributions to the systematic uncertainty in the Higgs boson mass, mH = 124:70 ± 0:31(stat) ± 0:15(syst)GeV, measured in ref. [2]. The procedure employed to optimize the photon energy estimation and its accurate mod- elling in the simulation is described. This procedure relies on the large sample of recorded Z boson decays to dielectrons, whose showers are reconstructed as photons, and on simulation to model differences in detector response to electrons and photons. – 1 – The reconstruction of photons from the measured energy deposits in the electromagnetic cal- orimeter (ECAL) [3] and the extraction of a photon energy estimate is described, as well as the association of the electron tracks to clusters in the ECAL for photons that convert in the tracker. A large fraction of the energy deposited in the detector by all proton-proton interactions arises from photons originating in the decay of neutral mesons, and these electromagnetic showers provide a substantial background to signal photons. The use and interest of photons as signals or signatures in measurements and searches is therefore mainly focussed on those with high transverse momentum where this background is less severe. Photon selection methods used for the H ! gg channel and other analyses are described, together with measurements of the selection efficiency. The efficiency measured in data is compared with that found in simulated events. 2015 JINST 10 P08010 The paper starts with brief descriptions of the CMS detector (section2), paying particular attention to geometrical details of the electromagnetic calorimeter that are important for shower reconstruction, and of the data and simulated event samples used (section3). Section4 describes photon reconstruction in CMS: clustering of the shower energy deposited in the ECAL crystals, correction of the cluster energy and fine tuning of the calibration, photon energy resolution, and uncertainties in the photon energy scale. Section5 describes the reconstruction of the electron tracks resulting from photons that undergo conversion before reaching the ECAL. Section6 dis- cusses the separation of prompt photons from energy deposits originating from the decay of neutral mesons, describing two identification algorithms, and giving results on their performance. The main results are summarized in section7. 2 CMS detector 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 detec- tors. A more detailed description of the CMS detector can be found in ref. [1]. The pseudorapidity coordinates, h, 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. The transverse energy, denoted by ET, is defined as the product of energy and sinq, with q being measured with respect to the origin of the coordinate system. Charged-particle trajectories are measured by the silicon pixel and strip tracker, with full az- imuthal coverage within jhj < 2:5. Consisting of 1 440 silicon pixel detector modules and 15 148 silicon strip detector modules, totalling about 10 million silicon strips and 60 million pixels, the silicon tracker provides an impact parameter resolution of ≈15 mm and a transverse momentum, pT, resolution of about 1.5% for charged particles with pT = 100GeV [4]. The total amount of material between the interaction point and the ECAL, in terms of radiation lengths (X0), raises from 0:4X0 close to h = 0 to almost 2X0 near jhj = 1:4, before falling to about 1:3X0 around jhj = 2:5. The probability of photon conversion before reaching the ECAL is – 2 – thus large and, since the resulting electrons (e+e− pairs) emit bremsstrahlung in the material, the electromagnetic shower of some photons starts to develop in the tracker. The