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Measurement of the photon structure function F2γ(x,Q2) with the LUMI detector at L3 Gyongyi Baksay Florida Institute of Technology Melbourne, Florida, USA Advisor: Dr. Marcus Hohlmann FAS, 68th Annual Meeting Orlando, March 12-13, 2004 1 Topics of Discussion Introduction:CERN, L3, LUMI Theoretical considerations Data analysis and results Summary FAS, 68th Annual Meeting Orlando, March 12-13, 2004 2 Introduction Two-photon reactions dominant LEP, CERN, Switzerland, France (future LHC) highest centre-of-mass energy : 207 GeV (Giga-electron Volts) 3 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 FAS, 68th Annual Meeting Orlando, March 12-13, 2004 4 The photon QED: Photon mediator. Photon structureless: direct/bare photon Heisenberg uncertainty principle: ΔE t 1 Photon violates conservation of energy: f f f or f interacts => parton content resolved, photon reveals its structure. Photon extended object=> charged fermions+gluons Dual nature of photon: direct or resolved One possible description: Photon Structure Function FAS, 68th Annual Meeting Orlando, March 12-13, 2004 5 The different appearances of the photon Photon: QED-photon couples to fermions (quarks & leptons) Lepton pair production => process can be calculated in QED Quark pair production => QCD corrections 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. FAS, 68th Annual Meeting Orlando, March 12-13, 2004 6 e+e- e+e- * * e+e- + hadrons deep-inelastic scattering reaction tag >> 0 electron observed inside the detector antitag 0 other electron undetected “single-tag” HADRON CALORIMETER ELECTROMAGNETIC CALORIMETER etag LUMI LUMI - tag e- * BEAM PIPE e+ (*) LUMI eantitag + antitag 0 LUMI ELECTROMAGNETIC CALORIMETER HADRON CALORIMETER FAS, 68th Annual Meeting Orlando, March 12-13, 2004 7 Photon Structure Function F2(x,Q2) ~ probability that the probe photon with virtuality Q2 sees a parton (quark or gluon) with momentum fraction x inside the target quasi-real photon. dσ e(k)γ* (q)e tag (k ' )X (x, Q 2 ) dxdQ 2 2π 2 [(1 (1 y) 2 )F2γ (x, Q 2 ) y 2 FLγ (x, Q 2 )] 4 xQ y (p q)/(p K) 1 E tag E beam cos 2 (θ tag ), y 0 Single-tag variables: Q2 q2 2Etag Ebeam (1 costag ) x Q 2 Q 2 W 2 P 2 Q 2 2( p q) W 2 (q1 q2 ) 2 ( E * E ) 2 (q p) 2 q1 ( E * , q), q2 ( E , p) qi ( E * , p i* ), (i 1,2) i qi E * p i* 2 2 For single tagged events: P 0 2 2 i x q Q Q 0 2 1 2 1 2 Q Q2 W 2 q22 Q22 0 mass squared of the outgoing interactin g fermion : k 2 ( xq2 q1 ) 2 q12 2 xq1 q2 0 q12 Q2 x 2q1 q 2 2q1 q2 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. FAS, 68th Annual Meeting Orlando, March 12-13, 2004 8 Analysis Method 1) 2) 3) 4) Selection Split x and Q2 in several bins Unfolding energy of the target photon is not known Correction with MC (Pythia, Phojet, Twogam) Calculate measured cross section: unfolding Example: selection 1998 Q2 “well” measured N unfolded N background L acceptance trigger efficiency 5) F2(x,Q2) obtained using analytically calculated differential cross section (program Galuga) Correlations between the generated and measured Q2, x, W; MC: Phojet 9 FAS, 68th Annual Meeting Orlando, March 12-13, 2004 10 Evolution of F2 with x F2/ F2(x,Q2) vs x with the different contributions: quarks VDM, QCD, QPM gluons Preliminary results: F2(VDM) x 11 Expected LUMI-L3 results Q2 evolution of F2 add data points to the low x region! High statistics! Test of QCD and QED. FAS, 68th Annual Meeting Orlando, March 12-13, 2004 12 The Grand Daddy prominence Summary Photon is not just a simple structureless object. It’s more than that! It can fluctuate into other states (resolved photon, QCD corrections). The photon can be regarded as an object with an internal structure consisting of charged fermions and gluons. Photon structure function analyzed for e+e- e+e- * * e+e- + hadrons Results obtained at LEP/ L3 (using LUMI for tagging the scattered electron) provides the highest statistics ever obtained (highest c.m. energy). FAS, 68th Annual Meeting Orlando, March 12-13, 2004 13 Thank you! FAS, 68th Annual Meeting Orlando, March 12-13, 2004 14