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Photon Structure from HERA

Photon Structure from HERA. Steve Maxfield University of Liverpool on behalf of the H1 and ZEUS Collaborations. Photon Structure from ep collisions. Proton beam as source of partons probing photon structure. Dijet production in photoproduction and low Q2 DIS supplies:

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Photon Structure from HERA

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  1. Photon Structure from HERA Steve Maxfield University of Liverpool on behalf of the H1 and ZEUS Collaborations Stephen Maxfield DIS 2002, Krakow April/May 2002

  2. Photon Structure from ep collisions Proton beam as source of partons probing photon structure • Dijet production in photoproduction and low Q2 DIS supplies: • Constraints on g pdf’s • Test of perturbative QCD (NLO) • Complements eg measurements (direct sensitivity to gluon) …because we have resolved photon processes: pQCD (NLO) QED Constrained elsewhere • Parton from proton probes photon with resolution determined by Et of jets Stephen Maxfield DIS 2002, Krakow April/May 2002

  3. Photon Structure: Outline • Summarise status of photon structure measurements at HERA • Real photon: • Photoproduction of high Et dijets. Measurements from H1 and ZEUSsee talks by A. Lupi(ZEUS) and G.Frising(H1) • Virtual Photons: • Dijet cross-sections in Low Q2 DIS events. Measurements from H1 and ZEUS. see talks by K.Sedlak (H1) , M. Lightwood(ZEUS) • Not covered: - single-inclusive jet cross-section measurement see talk by J. Turnau-other important things I’ve forgotten For High Et jet production see talk byC.Foudas (ZEUS+H1) Stephen Maxfield DIS 2002, Krakow April/May 2002

  4. Real Photon Structure: Photoproduction Measurements of dijet cross-sections in photoproduction (ZEUS) Eur. Phys. J. C (2002) • Comparison with NLO calculations • (Frixione and Ridolfi) • Hadronisation corrections estimated using HERWIG 6.1 / PYTHIA 6.1 • Parton densities: Photon GRV-HO and AFG-HO • Proton CTEQ5M1 • Integrated Luminosity 38.6 pb-1 (1996/97) • Q2 < 1 GeV2 • 134 GeV < Wgp < 277 GeV •  2 jets (kt algorithm) • -1 < h < 2.4; Etjet1 > 14 GeV Etjet2 > 11 GeV …similarly from H1  Stephen Maxfield DIS 2002, Krakow April/May 2002

  5. Real Photon Structure Measurements of dijet cross-sections in photoproduction (H1) DESY 01-225 submitted to Eur. Phys. J. C • Comparison with NLO calculations • (Frixione and Ridolfi) • Hadronisation corrections estimated using HERWIG 5.9 / PYTHIA 5.7 • Parton densities: Photon GRV-HO and AFG-HO • Proton CTEQ5M • Integrated Luminosity 34.9 pb-1 (1995-1997) • Q2 < 1 GeV2 • 0.1 < y < 0.9 •  2 jets (kt algorithm) • -0.5 < h < 2.5; Etjet1 > 25 GeV Etjet2 > 15 GeV Note phase space and models slightly different from that of ZEUS Stephen Maxfield DIS 2002, Krakow April/May 2002

  6. Real Photon Structure: NLO Matrix elements • Test the NLO matrix elements of the hard scattering process by looking at: • distribution of scattering angle in centre-of-mass frame, cosq*=tanh(Dh/2) • Minimum Mjj required to ensure distribution not biased by Et cuts Different propagators  different cosq*distributions Distinguish using: direct resolved Stephen Maxfield DIS 2002, Krakow April/May 2002

  7. Testing the NLO Matrix element Mjj > 42 GeV • NLO prediction agrees with data for xgobs> 0.75 • underestimates resolved-like processes by (10-15)% • Reasonable description within errors† for resolved events with GRV-HO pdf • Shapes consistent with difference in dominant propagators †data 5-10% theory 5-10% Stephen Maxfield DIS 2002, Krakow April/May 2002

  8. H1: GRV-HO • Mjj cut needed to reveal dynamics • NLO can describe data well for low xg • Slight overestimates for high xg • Expected steeper behaviour in resolved processes  dynamics understood What about the pdf’s? renormalisation/factorisation scale uncertainty Stephen Maxfield DIS 2002, Krakow April/May 2002

  9. Real Photon Structure contd. • Sum over two highest Et jets. • In leading order, estimates xg • NLO calculations use pdf’s for real photons obtained from fits to eg scattering data • How well do these describe the data? • test of universality of pdf’s. • Can the ep data constrain the pdf’s? • Measure distribution of: Stephen Maxfield DIS 2002, Krakow April/May 2002

  10. Real Photon Structure contd. • NLO predictions give a ‘reasonable’ description of the data. • Neither GRV-HO nor AFG-HO pdf’s can describe the data everywhere. • Cannot distinguish between pdf’s. • Existing pdf’s inadequate? Stephen Maxfield DIS 2002, Krakow April/May 2002

  11. H1results • NLO predictions give a good description of the data. • GRV-HO or AFG-HO pdf’s can describe the data everywhere. • Cannot distinguish between pdf’s • Existing pdf’s adequate to describe the data Stephen Maxfield DIS 2002, Krakow April/May 2002

  12. Etjet2 Sensitivity of NLO calculations to jet Et cuts • NLO calculations IR sensitive unless min Et cuts on jets are asymmetric.  NLO prediction depends on min(Etjet2) cut. How strongly? • Significant dependence seen.Choice of Etjet2 cutchanges prediction • Note that HERWIG (LO + LL parton showers) gives an excellent description of this behaviour!  Resummed or NNLO calculations needed Stephen Maxfield DIS 2002, Krakow April/May 2002

  13. Real Photon Structure Summary • NLO calculations give a reasonable description of the data • Taking into account systematic errors and differences in the phase space, the H1 and ZEUS results are not inconsistent. • Both experiments agree that data shows sensitivity to g pdfs’s (but do not allow discrimination between the pdf’s obtained from eg.) • …and that the data should be included in future global fits. • NNLO, Resummed calculations highly desirable to improve the constraints. Stephen Maxfield DIS 2002, Krakow April/May 2002

  14. Virtual Photon Structure + + • ‘Target’ photon with finite Q2 : Low Q2 DIS events • Introduces a second hard scale. Now have Et2, Q2 • For Q2 large enough, non-perturbative contributions to the g* structure (VDM) strongly suppressed (~1/Q4) w.r.t to calculable perturbative terms. • g*pdf’snot strictly required. • But useful as includes resummation of near collinear terms • Provides framework for a smooth transition from photoproduction to DIS Stephen Maxfield DIS 2002, Krakow April/May 2002

  15. Virtual Photon Structure • Triple differential cross-sections measured by H1 and ZEUS • Cross sections compared with LO QCD • + pdf’s for resolved photon including suppression of resolved component with increasing Q2 (SaS) Stephen Maxfield DIS 2002, Krakow April/May 2002

  16. ZEUS • HERWIG5.9 • proton pdf MRSA • photon pdf SaS1D • no s.u.e • Normalised to highest xg bin • Direct component sufficient where Q2 > Et2 • Resolved virtual photon component needed whereEt2 > Q2 • See expected evolution of resolved component Stephen Maxfield DIS 2002, Krakow April/May 2002

  17. ZEUS • In SaS 1D model: • Resolved component everywhere dominated by perturbative component Stephen Maxfield DIS 2002, Krakow April/May 2002

  18. Q2 suppression modified in charm enriched events… • 103.7 pb-1 (1996-2000) • Etjet1 > 7.5 GeV Etjet2 > 6.5 GeV • |hjet| < 2.4 • 0.2 < y < 0.65 • Tag charm with D*+→ K-p+p+s +c.c. • …and require D* to have • pT > 3 GeV |h| < 1.5 + Look at ratio of resolved to direct events: Q2 evolution modified • Data consistent with flat in Q2 • described by LO MC with SaS1D pdf’s for g*. Stephen Maxfield DIS 2002, Krakow April/May 2002

  19.  not all due to restricted D* phase space Interplay of three hard scales: Et2, Q2, mc2 Stephen Maxfield DIS 2002, Krakow April/May 2002

  20. H1 • HERWIG 5.9 • RAPGAP 2.8 • proton pdf GRV LO • photon pdf SaS1D • 10% s.u.e • Both LO predictions underestimate the cross section • normalisation uncertain What else? Stephen Maxfield DIS 2002, Krakow April/May 2002

  21. Longitudinally polarised photons g*L • Lepton beam acts as source of transverse and longitudinal photons: • Fluxes comparable for small y: • Gauge invariance ensures g*L contributions to cross-sections vanish as Q2→0 • Explore sensitivity to g*L by measuring y-dependence of cross-section Stephen Maxfield DIS 2002, Krakow April/May 2002

  22. PDF’s of g*L Notation: M2 = hard scale (e.g. Et2) P2 = photon virtuality • QED (vanishes for P2→0) • QCD improved PDF’s: • J. Chýla Phys. Lett. B488,289, hep-ph/0006232 • g*L contribution significant • exhibits hadron-like scaling behaviour. • cf g*T ~ ln M2/P2 •  g*Lsignificance increases as P2 → M2 Stephen Maxfield DIS 2002, Krakow April/May 2002

  23. g*L • Version of HERWIG5.9 modified to include g*L pdf • Inclusion ofg*Limproves description Stephen Maxfield DIS 2002, Krakow April/May 2002

  24. y-dependence of cross-section • Shape of y-distribution not described by gT* pdf’s alone Stephen Maxfield DIS 2002, Krakow April/May 2002

  25. g*L • Description of y dependence also improved Stephen Maxfield DIS 2002, Krakow April/May 2002

  26. Direct Photon DGLAP Other possibilities:Unordered emissions: CASCADE Resolved Photon ~unordered emission CASCADE: Event generator based on CCFM H.Jung and G.P.Salam, Eur. Phys. J. C19 (2001), 351-360 Stephen Maxfield DIS 2002, Krakow April/May 2002

  27. CASCADE •  • CASCADE also gives better description of the data in general But… Q2 dependence not well-reproduced Stephen Maxfield DIS 2002, Krakow April/May 2002

  28. CASCADE xgdependence similar Stephen Maxfield DIS 2002, Krakow April/May 2002

  29. Next Steps: • Would really like to see NLO predictions compared with the data. • H1 and ZEUS both working on this. • Expect that the ‘share’ of the cross-section carried by g pdf’s will be reduced. • Extraction of gL? Stephen Maxfield DIS 2002, Krakow April/May 2002

  30. Conclusions • Dijet cross-section measurements in photoproduction of sufficient precision to constrain pdf’s of real photons. • Constraints would be tighter if NNLO/resummed calculations were available. • Virtual photon structure shows sensitivity to longitudinal polarisation of photon. • Potential to measure pdf of gL* • Comparison of data with NLO predictions including virtual photon structure underway Stephen Maxfield DIS 2002, Krakow April/May 2002

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