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Measurement of the Isolated Prompt Photon Production Cross Section in pp Collisions at s=7TeV

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The differential cross section for the inclusive production of isolated prompt photons has been measured as a function of the photon transverse energy ETγ in pp collisions at s=7TeV using data recorded by the CMS detector at the LHC. The data sample
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  EUROPEAN ORGANISATION FOR NUCLEAR RESEARCH CERN-EP/2003-0235th May 2003 Measurement of Isolated PromptPhoton Production in Photon-PhotonCollisions at  √  s ee  = 183 − 209 GeV The OPAL Collaboration Abstract For the first time at LEP the production of prompt photons is studied in the collisions of quasi-real photons using the OPAL data taken at e + e − centre-of-mass energies between 183 GeV and209 GeV. The total inclusive production cross-section for isolated prompt photons in the kine-matic range of transverse momentum  p γ  T  >  3 . 0 GeV and pseudorapidity  | η γ  | <  1 is determinedto be  σ tot  = 0 . 32 ± 0 . 04 (stat) ± 0 . 04 (sys) pb. Differential cross-sections are compared to thepredictions of a next-to-leading-order (NLO) calculation. (To be submitted to Eur. Phys. J. C)  The OPAL Collaboration G.Abbiendi 2 , C.Ainsley 5 , P.F.˚Akesson 3 , G.Alexander 22 , J.Allison 16 , P.Amaral 9 ,G.Anagnostou 1 , K.J.Anderson 9 , S.Arcelli 2 , S.Asai 23 , D.Axen 27 , G.Azuelos 18 ,a , I.Bailey 26 ,E.Barberio 8 ,p , R.J.Barlow 16 , R.J.Batley 5 , P.Bechtle 25 , T.Behnke 25 , K.W.Bell 20 , P.J.Bell 1 ,G.Bella 22 , A.Bellerive 6 , G.Benelli 4 , S.Bethke 32 , O.Biebel 31 , O.Boeriu 10 , P.Bock 11 ,M.Boutemeur 31 , S.Braibant 8 , L.Brigliadori 2 , R.M.Brown 20 , K.Buesser 25 , H.J.Burckhart 8 ,S.Campana 4 , R.K.Carnegie 6 , B.Caron 28 , A.A.Carter 13 , J.R.Carter 5 , C.Y.Chang 17 ,D.G.Charlton 1 , A.Csilling 29 , M.Cuffiani 2 , S.Dado 21 , A.De Roeck 8 , E.A.De Wolf  8 ,s ,K.Desch 25 , B.Dienes 30 , M.Donkers 6 , J.Dubbert 31 , E.Duchovni 24 , G.Duckeck 31 ,I.P.Duerdoth 16 , E.Etzion 22 , F.Fabbri 2 , L.Feld 10 , P.Ferrari 8 , F.Fiedler 31 , I.Fleck 10 , M.Ford 5 ,A.Frey 8 , A.F¨urtjes 8 , P.Gagnon 12 , J.W.Gary 4 , G.Gaycken 25 , C.Geich-Gimbel 3 ,G.Giacomelli 2 , P.Giacomelli 2 , M.Giunta 4 , J.Goldberg 21 , E.Gross 24 , J.Grunhaus 22 ,M.Gruw´e 8 , P.O.G¨unther 3 , A.Gupta 9 , C.Hajdu 29 , M.Hamann 25 , G.G.Hanson 4 , K.Harder 25 ,A.Harel 21 , M.Harin-Dirac 4 , M.Hauschild 8 , C.M.Hawkes 1 , R.Hawkings 8 , R.J.Hemingway 6 ,C.Hensel 25 , G.Herten 10 , R.D.Heuer 25 , J.C.Hill 5 , K.Hoffman 9 , D.Horv´ath 29 ,c ,P.Igo-Kemenes 11 , K.Ishii 23 , H.Jeremie 18 , P.Jovanovic 1 , T.R.Junk 6 , N.Kanaya 26 ,J.Kanzaki 23 ,u , G.Karapetian 18 , D.Karlen 26 , K.Kawagoe 23 , T.Kawamoto 23 , R.K.Keeler 26 ,R.G.Kellogg 17 , B.W.Kennedy 20 , D.H.Kim 19 , K.Klein 11 ,t , A.Klier 24 , S.Kluth 32 ,T.Kobayashi 23 , M.Kobel 3 , S.Komamiya 23 , L.Kormos 26 , T.Kr¨amer 25 , P.Krieger 6 ,l , J.vonKrogh 11 , K.Kruger 8 , T.Kuhl 25 , M.Kupper 24 , G.D.Lafferty 16 , H.Landsman 21 , D.Lanske 14 ,J.G.Layter 4 , A.Leins 31 , D.Lellouch 24 , J.Letts o , L.Levinson 24 , J.Lillich 10 , S.L.Lloyd 13 ,F.K.Loebinger 16 , J.Lu 27 ,w , J.Ludwig 10 , A.Macpherson 28 ,i , W.Mader 3 , S.Marcellini 2 ,A.J.Martin 13 , G.Masetti 2 , T.Mashimo 23 , P.M¨attig m , W.J.McDonald 28 , J.McKenna 27 ,T.J.McMahon 1 , R.A.McPherson 26 , F.Meijers 8 , W.Menges 25 , F.S.Merritt 9 , H.Mes 6 ,a ,A.Michelini 2 , S.Mihara 23 , G.Mikenberg 24 , D.J.Miller 15 , S.Moed 21 , W.Mohr 10 , T.Mori 23 ,A.Mutter 10 , K.Nagai 13 , I.Nakamura 23 ,V   , H.Nanjo 23 , H.A.Neal 33 , R.Nisius 32 , S.W.O’Neale 1 ,A.Oh 8 , A.Okpara 11 , M.J.Oreglia 9 , S.Orito 23 , ∗ , C.Pahl 32 , G.P´asztor 4 ,g , J.R.Pater 16 ,G.N.Patrick 20 , J.E.Pilcher 9 , J.Pinfold 28 , D.E.Plane 8 , B.Poli 2 , J.Polok 8 , O.Pooth 14 ,M.Przybycie´n 8 ,n , A.Quadt 3 , K.Rabbertz 8 ,r , C.Rembser 8 , P.Renkel 24 , J.M.Roney 26 ,S.Rosati 3 , Y.Rozen 21 , K.Runge 10 , K.Sachs 6 , T.Saeki 23 , E.K.G.Sarkisyan 8 ,j , A.D.Schaile 31 ,O.Schaile 31 , P.Scharff-Hansen 8 , J.Schieck 32 , T.Sch¨orner-Sadenius 8 , M.Schr¨oder 8 ,M.Schumacher 3 , C.Schwick 8 , W.G.Scott 20 , R.Seuster 14 ,f  , T.G.Shears 8 ,h , B.C.Shen 4 ,P.Sherwood 15 , G.Siroli 2 , A.Skuja 17 , A.M.Smith 8 , R.Sobie 26 , S.S¨oldner-Rembold 16 ,d ,F.Spano 9 , A.Stahl 3 , K.Stephens 16 , D.Strom 19 , R.Str¨ohmer 31 , S.Tarem 21 , M.Tasevsky 8 ,R.J.Taylor 15 , R.Teuscher 9 , M.A.Thomson 5 , E.Torrence 19 , D.Toya 23 , P.Tran 4 , I.Trigger 8 ,Z.Tr´ocs´anyi 30 ,e , E.Tsur 22 , M.F.Turner-Watson 1 , I.Ueda 23 , B.Ujv´ari 30 ,e , C.F.Vollmer 31 ,P.Vannerem 10 , R.V´ertesi 30 , M.Verzocchi 17 , H.Voss 8 ,q , J.Vossebeld 8 ,h , D.Waller 6 , C.P.Ward 5 ,D.R.Ward 5 , P.M.Watkins 1 , A.T.Watson 1 , N.K.Watson 1 , P.S.Wells 8 , T.Wengler 8 ,N.Wermes 3 , D.Wetterling 11 G.W.Wilson 16 ,k , J.A.Wilson 1 , G.Wolf  24 , T.R.Wyatt 16 ,S.Yamashita 23 , D.Zer-Zion 4 , L.Zivkovic 241 School of Physics and Astronomy, University of Birmingham, Birmingham B15 2TT, UK 2 Dipartimento di Fisica dell’ Universit`a di Bologna and INFN, I-40126 Bologna, Italy 3 Physikalisches Institut, Universit¨at Bonn, D-53115 Bonn, Germany1  4 Department of Physics, University of California, Riverside CA 92521, USA 5 Cavendish Laboratory, Cambridge CB3 0HE, UK 6 Ottawa-Carleton Institute for Physics, Department of Physics, Carleton University, Ottawa,Ontario K1S 5B6, Canada 8 CERN, European Organisation for Nuclear Research, CH-1211 Geneva 23, Switzerland 9 Enrico Fermi Institute and Department of Physics, University of Chicago, Chicago IL 60637,USA 10 Fakult¨at f¨ur Physik, Albert-Ludwigs-Universit¨at Freiburg, D-79104 Freiburg, Germany 11 Physikalisches Institut, Universit¨at Heidelberg, D-69120 Heidelberg, Germany 12 Indiana University, Department of Physics, Bloomington IN 47405, USA 13 Queen Mary and Westfield College, University of London, London E1 4NS, UK 14 Technische Hochschule Aachen, III Physikalisches Institut, Sommerfeldstrasse 26-28, D-52056Aachen, Germany 15 University College London, London WC1E 6BT, UK 16 Department of Physics, Schuster Laboratory, The University, Manchester M13 9PL, UK 17 Department of Physics, University of Maryland, College Park, MD 20742, USA 18 Laboratoire de Physique Nucl´eaire, Universit´e de Montr´eal, Montr´eal, Qu´ebec H3C 3J7,Canada 19 University of Oregon, Department of Physics, Eugene OR 97403, USA 20 CLRC Rutherford Appleton Laboratory, Chilton, Didcot, Oxfordshire OX11 0QX, UK 21 Department of Physics, Technion-Israel Institute of Technology, Haifa 32000, Israel 22 Department of Physics and Astronomy, Tel Aviv University, Tel Aviv 69978, Israel 23 International Centre for Elementary Particle Physics and Department of Physics, Universityof Tokyo, Tokyo 113-0033, and Kobe University, Kobe 657-8501, Japan 24 Particle Physics Department, Weizmann Institute of Science, Rehovot 76100, Israel 25 Universit¨at Hamburg/DESY, Institut f¨ur Experimentalphysik, Notkestrasse 85, D-22607Ham-burg, Germany 26 University of Victoria, Department of Physics, P O Box 3055, Victoria BC V8W 3P6, Canada 27 University of British Columbia, Department of Physics, Vancouver BC V6T 1Z1, Canada 28 University of Alberta, Department of Physics, Edmonton AB T6G 2J1, Canada 29 Research Institute for Particle and Nuclear Physics, H-1525 Budapest, P O Box 49, Hungary 30 Institute of Nuclear Research, H-4001 Debrecen, P O Box 51, Hungary 31 Ludwig-Maximilians-Universit¨at M¨unchen, Sektion Physik, Am Coulombwall 1, D-85748Garching, Germany 32 Max-Planck-Institute f¨ur Physik, F¨ohringer Ring 6, D-80805 M¨unchen, Germany 33 Yale University, Department of Physics, New Haven, CT 06520, USA a and at TRIUMF, Vancouver, Canada V6T 2A3 c and Institute of Nuclear Research, Debrecen, Hungary d and Heisenberg Fellow e and Department of Experimental Physics, Lajos Kossuth University, Debrecen, Hungary f  and MPI M¨unchen g and Research Institute for Particle and Nuclear Physics, Budapest, Hungary h now at University of Liverpool, Dept of Physics, Liverpool L69 3BX, U.K. i and CERN, EP Div, 1211 Geneva 23  j and Manchester University k now at University of Kansas, Dept of Physics and Astronomy, Lawrence, KS 66045, U.S.A.2  l now at University of Toronto, Dept of Physics, Toronto, Canada m current address Bergische Universit¨at, Wuppertal, Germany n now at University of Mining and Metallurgy, Cracow, Poland o now at University of California, San Diego, U.S.A.  p now at Physics Dept Southern Methodist University, Dallas, TX 75275, U.S.A. q now at IPHE Universit´e de Lausanne, CH-1015 Lausanne, Switzerland r now at IEKP Universit¨at Karlsruhe, Germany s now at Universitaire Instelling Antwerpen, Physics Department, B-2610 Antwerpen, Belgium t now at RWTH Aachen, Germany u and High Energy Accelerator Research Organisation (KEK), Tsukuba, Ibaraki, Japan v now at University of Pennsylvania, Philadelphia, Pennsylvania, USA w now at TRIUMF, Vancouver, Canada ∗ Deceased 1 Introduction We present the first LEP measurement of the inclusive production of isolated prompt photonsin photon-photon collisions,  γγ   → γ   +X, where X denotes the hadronic final state produced inaddition to the photon. The interacting photons are emitted by the beam electrons 1 . Electronsscattered at small angles into the beam pipe escape detection and, in this analysis, events withone or two detected scattered electrons are vetoed (“anti-tagging”). The interacting photonsthus carry a small four-momentum squared,  Q 2 , i.e. they are quasi-real.In leading order (LO), only processes where one (“single-resolved”) or both (“double-resolved”) of the incoming photons fluctuate into a hadronic state contribute to the productioncross-section for prompt photons. In these processes, a quark or a gluon from the hadronic stateparticipates in the hard interaction,  γ  q  →  γ  q (Fig. 1), qq  →  γ  g and gq  →  γ  q (Fig. 2). Pro-cesses with final state radiation (“FSR”) are a higher-order contribution to the direct process(Fig. 3).The hadronic structure of the photon has previously been studied by OPAL in the in-teractions of two quasi-real photons producing jets [1], hadrons [2] or D ∗ mesons [3] at hightransverse momentum. The inclusive production cross-section for isolated prompt photons isexpected to be about two orders of magnitude smaller than for di-jet production in a similarkinematic region of transverse energy  E  T  >  3 GeV and pseudorapidity  | η | <  1 [1]. Hadronisa-tion uncertainties, however, are expected to be much smaller than in the case of jet productionat similar transverse momenta, allowing a complementary study of the hadronic structure of photon interactions [4, 5]. The photoproduction of prompt photons has previously been studiedby NA14 [6] and by ZEUS [7].Approximately 649 pb − 1 of e + e − collision data taken by the OPAL experiment at centre-of-mass energies  √  s ee  from 183 GeV to 209 GeV are used in this analysis. Since the expectedproduction cross-section is small and the increase of the cross-section from the lowest to thehighest centre-of-mass energy is expected to be less than the statistical uncertainty of the 1 Positrons are also referred to as electrons. 3  measurement [4], all data are combined for the final result. The luminosity-weighted mean e + e − centre-of-mass energy is approximately 196.6 GeV. The measured differential and total cross-sections are compared to the leading order predictions of the Monte Carlo generator PYTHIA [8]and to a next-to-leading-order (NLO) calculation [9]. The measurement is restricted to isolatedprompt photons to suppress background from neutral particle decays into photons. 2 The OPAL detector A detailed description of the OPAL detector can be found in [10], and therefore only a brief account of the main features relevant to the present analysis will be given here.The central tracking system is located inside a solenoidal magnet which provides a uniformaxial magnetic field of 0.435 T. The magnet is surrounded by a lead-glass electromagneticcalorimeter (ECAL) and a hadronic sampling calorimeter (HCAL). The HCAL is surroundedby muon chambers. There are similar layers of detectors in the endcaps. The region aroundthe beam pipe on both sides of the detector is covered by the forward calorimeters and thesilicon-tungsten luminometers.Starting with the innermost components, the tracking system consists of a high precisionsilicon microvertex detector (SI), a precision vertex drift chamber (CV), a large volume jetchamber (CJ) with 159 layers of axial anode wires and a set of   z   chambers measuring the trackcoordinates along the beam direction 2 .The ECAL covers the complete azimuthal range for polar angles satisfying  | cos θ |  <  0 . 98.The barrel section, which covers the polar angle range  | cos θ |  <  0 . 82, consists of a cylindricalarray of 9440 lead-glass blocks with a depth of 24 . 6 radiation lengths. Each block subtendsan angular region of approximately 40 × 40 mrad 2 . Deposits of energy in adjacent blocks aregrouped together to form clusters. The intrinsic energy resolution of   σ E  /E   = 0 . 2% ⊕ 6 . 3% / √  E  is substantially degraded (by a factor ≃ 2) due to the presence of at least two radiation lengthsof material in front of the lead glass.The endcap sections consist of 1132 lead-glass blocks with a depth of more than 22 radiationlengths, covering the range of polar angles between 0 . 81  < | cos θ | <  0 . 98. The HCAL consistsof streamer tubes and thin multiwire chambers instrumenting the gaps in the iron yoke of themagnet, which provides the absorber material of 4 or more interaction lengths.Scintillators in the barrel and endcap regions provide time measurements for the largefraction of photons which convert in the material in front of the ECAL. They are also used toreject the background from cosmic ray interactions. The barrel time-of-flight (TOF) scintillatorbars are located outside the solenoid in front of the barrel ECAL and match its geometricalacceptance ( | cos θ | <  0 . 82).The forward calorimeters (FD) at each end of the OPAL detector consist of cylindrical lead- 2 In the OPAL coordinate system the  x  axis points towards the centre of the LEP ring, the  y  axis pointsupwards and the  z  axis points in the direction of the electron beam. The polar angle  θ  is defined with respectto the  z  axis. The azimuthal angle  φ  and the radius  r  denote the usual spherical coordinates. 4
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