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

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

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

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

5. CAM2003, Merida, Mexico October 24-26, 2003 Subdetectors of LUMI 1.A calorimeter made of 304 Bismuth Germanium Oxide (BGO) crystals, arranged in 16 sectors of 19 crystals (of varying size) each. 2.A silicon detector placed in front of each monitor measures the polar angle  and the azimuthal angle  with very good accuracy.

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

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

8. CAM2003, Merida, Mexico October 24-26, 2003 Photon Structure Function F 2  (x,Q 2 ) ~ probability F 2  (x,Q 2 ) ~ probability that the probe photon with virtuality Q 2 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. Why do we measure the photon structure function of photon 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

9. CAM2003, Merida, Mexico October 24-26, 2003 Goal: measure the cross section for the single-tagged  *  process to extract the photon structure function F 2  (x, Q 2 ) P  0 For single tagged events: P  0 Direct process (QPM)VDM Single Resolved Main Processes contributing to the e + e -  e + e -  *   e + e - + hadrons cross section Single-tag variables:, k ’ X (hadrons)

10. CAM2003, Merida, Mexico October 24-26, 2003 Analysis Method 1)Selection 2)Split x and Q 2 in several bins 3)Unfolding (energy of the target photon is not known  Correction with MC 4)Calculate measured cross section: 5)Compare measured cross section to analytical calculations (program Galuga) unfolding Q 2 “well” measured

11. CAM2003, Merida, Mexico October 24-26, 2003 Evolution of F 2  with x F 2  (x,Q 2 ) vs x with the different contributions:VDM, QCD, QPM after unfolding with PHOJET F2/F2/ x F 2  (VDM) quarks gluons

12. CAM2003, Merida, Mexico October 24-26, 2003 Q 2 evolution of F 2  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. Expected LUMI-L3 results add data points to the low x region! Test of QCD Present measurements: data collected in 1998, 1999, 2000 at center-of-mass energies between

13. CAM2003, Merida, Mexico October 24-26, 2003 Selections, high enough statistics Binning for (x, Q 2 ), comparable to other experiments; assure high enough statistics for each bin Unfolding for x Calculate measured differential cross section Extract F 2  versus x Work done What improves compared to previous measurements? Work to be done Work to be done Determine F 2  versus Q 2  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 Q 2 (6-43 GeV 2) with a low W, means low x=> test of QCD

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