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Gyongyi Baksay Marcus Hohlmann (Advisor) Florida Institute of Technology Melbourne, Florida, USA

Measurement of the photon structure function F 2  with single tag events for the Q 2 range 6-43 GeV 2 in two-photon collisions at LEP2. Gyongyi Baksay Marcus Hohlmann (Advisor) Florida Institute of Technology Melbourne, Florida, USA. Topics of Discussion. Measurement of F 2 .

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Gyongyi Baksay Marcus Hohlmann (Advisor) Florida Institute of Technology Melbourne, Florida, USA

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  1. Measurement of the photon structure function F2 with single tag events for the Q2 range 6-43 GeV2 in two-photon collisions at LEP2 Gyongyi Baksay Marcus Hohlmann (Advisor) Florida Institute of Technology Melbourne, Florida, USA CAM2003, Merida, Mexico October 24-26, 2003

  2. Topics of Discussion • Measurement of F2 • Introduction LEP, L3, LUMI,  physics • Theoretical considerations • Data analysis and results • Future plans and conclusion CAM2003, Merida, Mexico October 24-26, 2003

  3. Introduction Two-photon reactions dominant LEP, CERN, Switzerland, France (future LHC) collides e+ e-, highest centre-of-mass energy : 208GeV CAM2003, Merida, Mexico October 24-26, 2003

  4. The L3 experiment MAIN SUBSYSTEMS:central tracker (SMD, TEC) electromagnetic (ECAL), hadronic (HCAL) calorimeters, and muon chambers. e+ e- Tagging:Luminosity Monitor (LUMI), Very Small Angle Tagger (VSAT), Active Lead Rings (ALR), Electromagnetic Calorimeter endcaps CAM2003, Merida, Mexico October 24-26, 2003

  5. Subdetectors of LUMI • A calorimeter made of 304 Bismuth Germanium Oxide (BGO) crystals, arranged in 16 sectors of 19 crystals (of varying size) each. • A silicon detector placed in front of each monitor measures the polar angle  and the azimuthal angle  with very good accuracy. CAM2003, Merida, Mexico October 24-26, 2003

  6. The different appearances of the photon, (QED and QCD) Photon: quarks Mediator of the electromagnetic force, couples to charged objects (QED) Lepton pair production => process can be calculated in QED Quark pair production => QCD corrections leptons (e, , ) Photon interactions receive several contributions: photon fluctuates into a hadronic state which subsequently interacts “bare photon” Does not reveal a structure The QED structure functions can only be used for the analysis of leptonic final states. For hadronic final states the leading order QED diagrams are not sufficient and QCD corrections are important. CAM2003, Merida, Mexico October 24-26, 2003

  7. Single tag events large scattering angle => electron observed inside the detector => “tag” one scattered electrons detected => “single-tag” event e+e- e+e- ** e+e- + hadrons deep-inelastic scattering reaction e+ e-  CAM2003, Merida, Mexico October 24-26, 2003

  8. Photon Structure Function Why do we measure the photon structure function of photon ? • understand complex behavior of interacting photons (test QED, QCD) • creation of matter out of two photons F2(x,Q2) ~ probabilitythat the probe photon with virtuality Q2 sees a parton (quark or gluon) with momentum fraction x inside the target quasi-real photon. The Bjorken variable x tells us what fraction of the photon four momentum was carried by the particle which participated to the interaction: the target photon itself or a parton (quark or gluon) inside the photon. CAM2003, Merida, Mexico October 24-26, 2003

  9. Goal: measure the cross section for the single-tagged * process to extract the photon structure function F2 (x, Q2) , k’ X (hadrons) Main Processes contributing to the e+e- e+e- * e+e- + hadrons cross section Single-tag variables: VDM Direct process (QPM) For single tagged events: P 0 Single Resolved CAM2003, Merida, Mexico October 24-26, 2003

  10. unfolding Analysis Method • Selection • Split x and Q2 in several bins • Unfolding (energy of the target photon is not known • Correction with MC 4) Calculate measured cross section: • Compare measured cross section to analytical calculations (program Galuga) Q2 “well” measured CAM2003, Merida, Mexico October 24-26, 2003

  11. Evolution of F2 with x F2(x,Q2) vs x with the different contributions:VDM, QCD, QPM F2/ quarks F2(VDM) gluons x after unfolding with PHOJET CAM2003, Merida, Mexico October 24-26, 2003

  12. Expected LUMI-L3 results add data points to the low x region! Test of QCD Q2 evolution of F2 • Comparison: • LEP (ALEPH, DELPHI, L3, OPAL) • TPC detector at PEP/SLAC, Stanford; • PLUTO, JADE, and TASSO detectors at • DESY, Hamburg, Germany; • TOPAZ, AMY detectors at KEK-TRISTAN, • Japan. Present measurements: data collected in 1998, 1999, 2000 at center-of-mass energies between CAM2003, Merida, Mexico October 24-26, 2003

  13. Work done • Selections, high enough statistics • Binning for (x, Q2), comparable to other experiments; assure high enough statistics for each bin • Unfolding for x • Calculate measured differential cross section • Extract F2 versus x Work to be doneDetermineF2 versus Q2 What improves compared to previous measurements? • Higher statistics, better precision • The physics advantage of this high energy reached at LEP2 comes from the increased center-of-mass energy between the virtual photon probe and the real photon target. This means that the photon structure can be measured at lower x values than was done before. • Better detector conditions=>LUMI Q2 (6-43 GeV2) with a low W, means low x=> test of QCD CAM2003, Merida, Mexico October 24-26, 2003

  14. CAM2003, Merida, Mexico October 24-26, 2003

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