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Photon structure as revealed in ep collisions

Photon structure as revealed in ep collisions. Alice Valk árová Insti tute of Particle and Nuclear Physics Charles University of Prague. On behalf of the H1 and ZEUS collaborations. Deep inelastic scattering. Q 2 >>1 GeV 2 : probing proton structure

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Photon structure as revealed in ep collisions

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  1. Photon structure as revealed in ep collisions Alice Valkárová Institute of Particle and Nuclear Physics Charles University of Prague On behalf of the H1 and ZEUS collaborations PHOTON 2003, Frascati

  2. Deep inelastic scattering Q2>>1 GeV2: probing proton structure via pointlike virtual photons Photoproduction Q2 ~ 0: probing photon structure via dijet system resolved direct PHOTON 2003, Frascati

  3. Probes photon structure via dijet system Photoproduction events in H1 detector xγ - fraction of photon’s momentum in hard subprocess Direct photon event ET,η – transverse momentum and pseudorapidity of jets xγ = 1 2 hard jets Resolved photon event xγ < 1 2 hard jets +photon remnant PHOTON 2003, Frascati

  4. σepis convolution of partonic cross sections and pdf’s: cross section of the subprocess- calculable flux of photons pdf of the proton (experimentally constraint) pdf of the photon xp - fraction of proton’s momentum in hard subprocess Q2 – virtualityof γ* and y - inelasticity ET,η – transverse momentum and pseudorapidity of jets Θ*- angle of dijet system in 2-jet CMS μ ET is the hard scale At HERA the photon structure can be probed at higher scales than at LEP PHOTON 2003, Frascati

  5. Photon structure from HERA The measured of dijet cross section  the parton densities in the photon Two different approaches: • Extraction:extracting the pdf, treating the measured cross section, the known pdf of proton and QCD matrix element as input fundamental, parton level quantity • (but difficult and only LO procedure) • Universality test:checking if existing parametrizations of pdf are adequate for the description of the dijet cross section within NLOQCD  NLO approach (but hadronization corrections should be known (small) ) PHOTON 2003, Frascati

  6. High ET dijetsmotivation Soft physics supressed (hadronization corrections small) High ET jets provide hard scale for perturbative QCD calculations Test perturbative QCD at NLO Test parametrizations of proton pdfgluon in the proton,0.05<xp<0.6 Test parametrizations of photon pdfq/g in the photon, 0.1<xγ<1. gluon poorly constrained by F2γ measured in e+e- collisions at LEP, jets at HERA sensitive to gluons already at LO PHOTON 2003, Frascati

  7. Dijets in photoproduction: H1 and ZEUS H1 cuts: Q2<1 GeV2 0.1<y<0.9 Etmax>25 GeV, Etsec>15 GeV -0.5<η<2.5 Mjj>65 GeV kt long.inv. jet algorithm ZEUS cuts: Q2<1 GeV2 0.2<y<0.85 Etmax>14 GeV, Etsec>11 GeV -1.<η<2.4 0.1<ηmean<1.3 Mjj>39 GeV kt long.inv.jet algorithm PHOTON 2003, Frascati

  8. H1 collab.Eur.Phys.J C25 (2002),13 H1 lower xγ, higher xp Data described by NLO with GRV and AFG pdf’s of photon PHOTON 2003, Frascati

  9. GRV AFG ZEUS collab.,Eur.Phys.J C23 (2002),615 ZEUS NLOdescribes data not too bad overall, however neither GRV nor AFG pdf’s provide a perfect description everywhere PHOTON 2003, Frascati

  10. The dependence of the NLO cross section on the cuts on ET • Assymetric ET1/ET2 jetcuts • to avoid infrared sensitivity of NLO calculations ZEUS H1 • dependence on ET2 significantly • different for data and NLO • HERWIG describes dependence quite well ZEUS H1 Comparison of data &NLO depends on the cut value! Theoretical progress needed!! PHOTON 2003, Frascati

  11. Inclusive jets: H1 and ZEUS two samples: Q2< 1 GeV2; ET > 21 GeV, -1 <η <2.5 Q2< 10-2 GeV2; 5< ET< 21 GeV, -1< η<2.5 kt long. inv. jet algorithm H1 Q2 <1 GeV2; ET >13 GeV, -1 <η <2.5 kt long. inv. jet algorithm ZEUS PHOTON 2003, Frascati

  12. Inclusive jets in photoproduction-ZEUS ZEUS collab.,DESY-03-012 The measured cross section falls over five orders of magnitude NLO  a good description of the data within uncertainties PHOTON 2003, Frascati

  13. Inclusive jets in photoproduction-H1 H1 collab.,DESY-02-225 Analysis of “low” ET and “high” ET jet sample agree well in their domain of overlap The measured cross section falls by 6 orders of magnitude between ET=5 and 75 GeV NLO calculations describe ET dependence of dijet cross section within errors quite satisfactorily PHOTON 2003, Frascati

  14. Inclusive jets in photoproduction-H1 NLO QCD with GRV-HO gives the best description of the data AFG prediction lower by 10-15% NLO calculations describe the measured η distribution both in normalisation and shape within uncertainties PHOTON 2003, Frascati

  15. Conclusions • Inclusive cross sections in photoproduction described by NLO calculations very well. • Dijet cross sections described by NLO not so perfectly. • Theoretical uncertainties are dominating, • theoretical progress needed!! • The current precision does not allow one to discriminate between different photon parton distribution functions. PHOTON 2003, Frascati

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