1. HEP2003 Photon Structure Albert De Roeck (CERN) 1 Measurements of the Photon Structure Function at LEP Albert De Roeck / CERN Representing the LEP Collaborations HEP2003 Europhysics Conference Aachen/Germany 17-23 July

2. HEP2003 Photon Structure Albert De Roeck (CERN) 2 New Photon Structure Function Developments  Progress on measurements of F 2  using the full LEP statistics (DELPHI/ALEPH)  Measurement of F 2 charm (OPAL)  First measurement of the electron structure function F 2 e (DELPHI)  New parton density parametrizations based on recent data and extraction of  s  Outlook & Future Pointlike part Hadron-like part qqqq + Total F 2  PQCD qqqq

3. HEP2003 Photon Structure Albert De Roeck (CERN) 3 Introduction: Kinematics Measurements at LEP: Single tag two-photon interactions (DIS) Tagged electron Target photon Reconstruction of x needs hadronic final state  s ee = 91  209 GeV

4. HEP2003 Photon Structure Albert De Roeck (CERN) 4 New Data from ALEPH (preliminary) LEP2 data from ALEPH based on 548.4 pb -1 Tikhonov unfolding procedure used Two ranges in Q 2 : 17.3 GeV 2 and 67.3 GeV 2 Comparison of F 2  for x  0.1

5. HEP2003 Photon Structure Albert De Roeck (CERN) 5 New Data from DELPHI LEP1 and LEP2 data from DELPHI based on 78/548 pb -1 No unfolding but fitting of different cross section components to data distributions LEP1 data

6. HEP2003 Photon Structure Albert De Roeck (CERN) 6 New Data from DELPHI LEP2 data Note: DELPHI chooses to present different F 2  values calculated/corrected with different hadronic models  difficult to compare with other LEP measurements

7. HEP2003 Photon Structure Albert De Roeck (CERN) 7 New Data from DELPHI measurements in different x intervals as a function of DELPHI points extracted using one model (TWOGAM) only. Large spread due to different models not shown

8. HEP2003 Photon Structure Albert De Roeck (CERN) 8 Data on F 2 charm from OPAL Pointlike part (PQCD) Hadron-like part depends on gluon in the photon D*

9. HEP2003 Photon Structure Albert De Roeck (CERN) 9 Data on F 2 charm from OPAL Limited by statistics (data other LEP experiments?) High-x: well described by PQCD calculation, essentially pointlike Low-x: somewhat higher than Predicted but large error Subtract NLO pointlike part at low x GRV-NLO/ Laenen et al.

10. HEP2003 Photon Structure Albert De Roeck (CERN) 10 Electron Structure Function  Does not need hadronic final state to calculate x  Kinematics better constrained  No corrections due virtuality of the target photon (P) needed  Photon flux spectrum obscures the sensitivity to the photon structure  Radiative corrections large at small x x-resolution x=0.1 z-resolution z= 0.01 x z

11. HEP2003 Photon Structure Albert De Roeck (CERN) 11 Electron Structure Function Measurements consistent with F 2  Note: Real structure functions have to be “given” virtuality before comparing with F 2 e

12. HEP2003 Photon Structure Albert De Roeck (CERN) 12 Electron Structure Function Measurements consistent with F 2  /reduced sensitivity

13. HEP2003 Photon Structure Albert De Roeck (CERN) 13 The World Data: F 2  versus x More than 50 measurements Kinematical coverage 10 -3  x  0.9 1.9   780 GeV 2 Used for new fits and the extraction of  s Thanks to R. Nisius, see http://www.mppmu.mpg.de/~nisius/welcomeaux/struc.html All except new DELPHI-LEP2 points

14. HEP2003 Photon Structure Albert De Roeck (CERN) 14 The World Data: F 2  versus Q 2 Fit: Significant increase of the slope with increasing x

15. HEP2003 Photon Structure Albert De Roeck (CERN) 15 Parametrizations and Fits  New radiatively generated (LO) quark and gluon densities in the photon on all F 2  data available by summer 2002 (208 data points)  NLO densities in progress… Cornet,Jankowski,Krawczyk & Lorca, hep-ph/0212160 ACOT HF scheme Fit  s from the F 2  data Use data with (x>0.45, Q 2 > 59 GeV 2 ) and fit  s Parametrizations Albino, Klasen & Soldner Rembold, hep-ph/0205069 NLO/MS Use all data, 5 parameter fit for (N, , ,  s,Q 0 2)

16. HEP2003 Photon Structure Albert De Roeck (CERN) 16 Outlook I Near Future What can be still expected from the LEP experiments? ALEPH: Possibly an analysis at larger Q 2 L3: Full analysis with all statistics OPAL: Low-x, Q 2 analysis for F 2  and F 2 e

17. HEP2003 Photon Structure Albert De Roeck (CERN) 17 Outlook II Far Future: Large extension of the kinematic range at a future linear collider e+e- option Photon collider option F 2  measurements with a quality similar to proton structure functions today !!

18. HEP2003 Photon Structure Albert De Roeck (CERN) 18 Summary The photon structure has now been investigated in a large range 10 -3  x  0.9 1.9   780 GeV 2 Precision has been improving over the years The charm content of the photon has been measured Will need data from the other 3 experiments The electron structure function has been measured, and serves as a valuable cross check of the photon structure function, but does not seem to add more discriminative power. New parametrizations of the parton densities in the photon become available.  s values have been extracted from fits to the photon structure data. Thanks to A. Finch, C. Mariotti, B. Muryn, R. Nisius, T. Szumlack, T. Wengler