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Isolated Prompt Photon Production in Photon-Photon Collisions at s ee =183-209 GeV

Abstract 376. Isolated Prompt Photon Production in Photon-Photon Collisions at s ee =183-209 GeV. T. Sch ö rner-Sadenius on behalf of the OPAL Collaboration ICHEP 2002, Amsterdam 23-31 July 2002.

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Isolated Prompt Photon Production in Photon-Photon Collisions at s ee =183-209 GeV

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  1. Abstract 376 Isolated Prompt Photon Production in Photon-Photon Collisions at see=183-209 GeV T. Schörner-Sadenius on behalf of the OPAL Collaboration ICHEP 2002, Amsterdam 23-31 July 2002 ‘Prompt’  not coming from particle decays/fragmentation, but emitted from the hard underlying scattering.

  2. Prompt Photons in +X direct  • Allow to study photon’s hadronic structure and test QCD predictions: • Prompt  appear in resolved processes (in LO; in NLO also in direct events). • Small NLO corrections / scale dependency. • ‘Cleaner’ than dijets (hadrons, D*) less uncertainty from hadronisation (but prompt  have smaller Xsection ~O(EM)). • Might further be used for • studies of the underlying parton picture or • background studies in LHC Higgs searches. prompt  resolved  Schörner-Sadenius: Prompt Photons in 

  3. Theoretical Predictions Single-resolved • Fixed-order NLO QCD calculations (Fontannaz et al). • PYTHIA 6.130 for signal +X • Distinct samples for single/double resolved for three energy points. • Sas-1D (LO)  PDF. • Background with fake / real : • PHOJET 1.10 and HERWIG 5.9 for  background without prompt . • Also Vermaseren 1.0, PYTHIA 5.7, KORALZ, grc4f. Double-resolved Schörner-Sadenius: Prompt Photons in 

  4. LEP and the OPAL Detector Luminosity used: 639 pb-1 Schörner-Sadenius: Prompt Photons in 

  5. Event/ Selection (see=183-209 GeV) • Require 3 tracks. Reject events with scattered leptons above =33 mrad. • Reject e+e- annihilations: • 5 GeV < Wvis < 0.3see • Ecal < 0.25see. • Select isolated photons: • EM clusters with || < 1 and pT > 3 GeV. • Photon and hadronic final state balanced in . Schörner-Sadenius: Prompt Photons in 

  6. Photon Selection ctd’ iETi·(-Ri)  0.2·ET(1-cos)/(1-cosR) • Isolation criterion (<R): • Background: antineutrons, /0 decays  shape analysis: • Fit: a = 0.85  0.08 (stat) b = 0.11  0.08 (stat) fmax = Emax/E c = (iEiRi)/E g(f, ) = ag + bg + (1-a-b)·(cgn + (1-c)g) Schörner-Sadenius: Prompt Photons in 

  7. Single/Double Resolved Ratio rs/d (E  pz) + (Ejet  pzjet) • For the  +jet sample: using a cone jet finder with R=1, ET > 3 GeV. • Fit xmin distribution to data  rs/d = 0.59 ± 0.12 • For the complete data set use also xT=2pT/Wvis rs/d = 0.41 ± 0.08, • Average  rs/d~0.5. x = hadrons,(E  pz) Schörner-Sadenius: Prompt Photons in 

  8. The Total Cross-Section For isolated prompt photons (pT > 3 GeV, || < 1): tot = r·Nprompt/singleL + (1-r)·Nprompt/doubleL • Nprompt: 92.8, background subtracted (FSR, , , n). • rs/d: single / double resolved ratio. • : Taken from PYTHIA MC (0.51 / 0.61). • Tristan: (pT > 2GeV) = 1.72  0.67 pb at see = 58 GeV tot = 0.26 ± 0.04 (stat.) ± 0.03 (syst.) pb Schörner-Sadenius: Prompt Photons in 

  9. Comparisons with PYTHIA d/dpT d/d||: Flat, as expected! • Shape of distributions well described, but normalisation off (1.85; in case of TOPAZ ~3, MC also low for ZEUS) Schörner-Sadenius: Prompt Photons in 

  10. Comparisons with NLO Calculations • Shape and normalisation correct (Fontannaz et al)! d/d|| d/dpT Schörner-Sadenius: Prompt Photons in 

  11. Summary • Prompt photons provide means to test QCD and to determine hadronic structure of photon.  = 0.26 ± 0.04 (stat.) ± 0.03 (syst.) pb • PYTHIA badly normalized; good agreement of differential cross-sections with NLO QCD calculations. • Looking forward to final result: • Test of different photon PDFs (GRV, LAC1). • Investigate including FSR in signal  better NLO description? Schörner-Sadenius: Prompt Photons in 

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