1. Isolated Prompt Photon Production in Photon-Photon Collisions at  s ee =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. Abstract 376

2. ICHEP02 AmsterdamSchörner-Sadenius: Prompt Photons in  2 Prompt Photons in    +X  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. direct  resolved  prompt 

3. ICHEP02 AmsterdamSchörner-Sadenius: Prompt Photons in  3 Theoretical Predictions  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. Single-resolved Double-resolved

4. ICHEP02 AmsterdamSchörner-Sadenius: Prompt Photons in  4 LEP and the OPAL Detector Luminosity used: 639 pb -1

5. ICHEP02 AmsterdamSchörner-Sadenius: Prompt Photons in  5 Event/  Selection (  s ee =183-209 GeV)  Require  3 tracks. Reject events with scattered leptons above  =33 mrad.  Reject e+e- annihilations:  5 GeV < W vis < 0.3  s ee  E cal < 0.25  s ee.  Select isolated photons:  EM clusters with |  | 3 GeV.  Photon and hadronic final state balanced in .

6. ICHEP02 AmsterdamSchörner-Sadenius: Prompt Photons in  6 Photon Selection ctd’  Isolation criterion (  <R):  Background: antineutrons,  /  0 decays  shape analysis:  Fit: a = 0.85  0.08 (stat) b = 0.11  0.08 (stat) f max = E max /E   c = (  i E i R i  )/E  g(f,  ) = ag  + bg  + (1-a-b)·(cg n + (1-c)g  )  i E Ti ·  (  -R i  )  0.2·E T  (1-cos  )/(1-cosR)

7. ICHEP02 AmsterdamSchörner-Sadenius: Prompt Photons in  7 Single/Double Resolved Ratio r s/d  For the  +jet sample: using a cone jet finder with R=1, E T > 3 GeV.  Fit x  min distribution to data  r s/d = 0.59 ± 0.12  For the complete data set use also x T =2p T  /W vis  r s/d = 0.41 ± 0.08,  Average  r s/d ~0.5. x =x = (E   p z  ) + (E jet  p zjet )  hadrons,  (E  p z )

8. ICHEP02 AmsterdamSchörner-Sadenius: Prompt Photons in  8 The Total Cross-Section  tot = r·N prompt /  single L + (1-r)·N prompt /  double L  N prompt : 92.8, background subtracted (FSR, , , n).  r s/d :single / double resolved ratio.   : Taken from PYTHIA MC (0.51 / 0.61).  Tristan:  (p T > 2GeV) = 1.72  0.67 pb at  s ee = 58 GeV  tot = 0.26 ± 0.04 (stat.) ± 0.03 (syst.) pb For isolated prompt photons (p T > 3 GeV, |   | < 1):

9. ICHEP02 AmsterdamSchörner-Sadenius: Prompt Photons in  9 Comparisons with PYTHIA d  /dp T   Shape of distributions well described, but normalisation off (1.85; in case of TOPAZ ~3, MC also low for ZEUS) d  /d|   |: Flat, as expected!

10. ICHEP02 AmsterdamSchörner-Sadenius: Prompt Photons in  10 Comparisons with NLO Calculations  Shape and normalisation correct (Fontannaz et al)! d  /d|   | d  /dp T 

11. ICHEP02 AmsterdamSchörner-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?