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Measurements of the Photon Structure Function at LEP

Measurements of the Photon Structure Function at LEP. Albert De Roeck / CERN Representing the LEP Collaborations HEP2003 Europhysics Conference Aachen/Germany 17-23 July. New Photon Structure Function Developments. Total F 2 . q q. q q. PQCD. +. Pointlike part. Hadron-like part.

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Measurements of the Photon Structure Function at LEP

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  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 Photon Structure Albert De Roeck (CERN)

  2. New Photon Structure Function Developments Total F2 q q q q PQCD + Pointlike part Hadron-like part • Progress on measurements of F2using the full LEP statistics (DELPHI/ALEPH) •  Measurement of F2charm (OPAL) •  First measurement of the electron structure function F2e (DELPHI) •  New parton densityparametrizations based on recent data • and extraction of s •  Outlook & Future Photon Structure Albert De Roeck (CERN)

  3. Introduction: Kinematics Measurements at LEP: Single tag two-photon interactions (DIS) Tagged electron Reconstruction of x needs hadronic final state see = 91  209 GeV Target photon 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 Q2: 17.3 GeV2 and 67.3 GeV2 Comparison of F2 for x  0.1 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 Photon Structure Albert De Roeck (CERN)

  6. New Data from DELPHI LEP2 data Note: DELPHI chooses to present different F2 values calculated/corrected with different hadronic models difficult to compare with other LEP measurements Photon Structure Albert De Roeck (CERN)

  7. New Data from DELPHI <F2 > measurements in different x intervals as a function of <Q2> DELPHI points extracted using one model (TWOGAM) only. Large spread due to different models not shown Photon Structure Albert De Roeck (CERN)

  8. Data on F2charm from OPAL D* Hadron-like part depends on gluon in the photon Pointlike part (PQCD) Photon Structure Albert De Roeck (CERN)

  9. Data on F2charm 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. 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 z x-resolution x=0.1 z-resolution z= 0.01 Photon Structure Albert De Roeck (CERN)

  11. Electron Structure Function Measurements consistent with F2 Note: Real structure functions have to be “given” virtuality before comparing with F2e Photon Structure Albert De Roeck (CERN)

  12. Electron Structure Function Measurements consistent with F2/reduced sensitivity Photon Structure Albert De Roeck (CERN)

  13. The World Data: F2 versus x More than 50 measurements Kinematical coverage 10-3 x  0.9 1.9  <Q2>  780 GeV2 Used for new fits and the extraction of s All except new DELPHI-LEP2 points Thanks to R. Nisius, see http://www.mppmu.mpg.de/~nisius/welcomeaux/struc.html Photon Structure Albert De Roeck (CERN)

  14. The World Data: F2 versus Q2 Significant increase of the slope with increasing x Fit: Photon Structure Albert De Roeck (CERN)

  15. Parametrizations and Fits Cornet,Jankowski,Krawczyk & Lorca, hep-ph/0212160 Parametrizations  New radiatively generated (LO) quark and gluon densities in the photon on all F2 data available by summer 2002 (208 data points)  NLO densities in progress… ACOT HF scheme Fit sfrom the F2 data Albino, Klasen & Soldner Rembold, hep-ph/0205069 Use data with (x>0.45, Q2 > 59 GeV2) and fit s NLO/MS Use all data, 5 parameter fit for (N, ,,s,Q02) 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 Q2 L3: Full analysis with all statistics OPAL: Low-x, Q2 analysis for F2 and F2e 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 F2measurements with a quality similar to proton structure functions today !! 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  <Q2>  780 GeV2 • 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 Photon Structure Albert De Roeck (CERN)

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