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Photoproduction of jets and prom p t photons Jaroslav Cvach Ins t itute of Physics AS CR, Prague

Photoproduction of jets and prom p t photons Jaroslav Cvach Ins t itute of Physics AS CR, Prague. Inclusive jets Dijets Parton-parton dynamics Prompt photons Summar y. Parton level ( photo ) production. Resolved ( x g < 1). Direct ( x g ~ 1). Resolved/direct processes:

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Photoproduction of jets and prom p t photons Jaroslav Cvach Ins t itute of Physics AS CR, Prague

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  1. Photoproduction of jets and prompt photonsJaroslav CvachInstitute of Physics AS CR, Prague • Inclusive jets • Dijets • Parton-parton dynamics • Prompt photons • Summary

  2. Parton level(photo)production Resolved (xg < 1) Direct (xg ~ 1) Resolved/direct processes: • resolved photons are meaningful when Q2ET2 • distinction can only clearly be made in LO • use xγ to separate resolved and direct enhanced samples QCD calculations: • renormalisation and factorisation scales in parton cross sections and PDF fij lead to an uncertainty • different NLO calculation differ in their treatment of infrared and collinear divergences J. Cvach, Photoproduction of jets

  3. Motivation • High ET jets ]Precise tests of perturbative QCD predictions ]Constrain photon and proton PDFs ]Direct insight into parton-parton dynamics ]Search for new physics • Low ET jets(non-perturbative effects and scale uncertainty important) ]Test phenomenological models of underlying event + fragmentation ]Test limits of pQCD applicability ]Investigation of resolved photon processes • Inclusive vs dijet + More statistics, extended kinematic range  No direct reconstruction of xγ,xp + No infrared sensitivity w.r.t. kinematical cuts as for dijet • Prompt photons • Direct insight into the hard process not biased by fragmentation • Low cross section J. Cvach, Photoproduction of jets

  4. { Multiple Interactions (PYTHIA) pTmia =1.2 GeV Dual Parton Model (PHOJET) Soft Underlying Event (HERWIG) 35% resolved Underlying event: NLO QCD calculations and Monte Carlo • NLO weighted parton MC - Frixione, NP B507(1997) 295 (H1 - inclusive and dijets, ZEUS – dijets) • photon & proton PDFs: GRV & CTEQ5M • Other choice : photon: AFG, proton: MRST99, CTEQ5HJ (g↑ at ↑xp ) • Klasen, Kleinwort, Kramer, EPJ Direct C1 (1998) 1, (ZEUS – inclusive jets) • photon & proton PDFs: GRV & MRST99 • Fontannaz, Guillet, Heinrich, hep-ph/0105121 (H1-prompt photons) • photon: AFG, proton: MRST2 • LO QCD Monte Carlo event generators to correct data and calculations to hadron level: • Fragmentation: LUND String (PYTHIA, PHOJET) or cluster (HERWIG) J. Cvach, Photoproduction of jets

  5. hadronisation correction uncertainty renormalisation and factorisation scale uncertainty calorimeter energy scale uncertainty Inclusive jets:ET H1 • cross section falls by more than 6 orders of magnitude from ETjet = 5 to 75 GeV • LO QCD underestimates the cross section (less so at high ETjet) • NLO QCD reproduces the data well, but needs hadronisation corrections at low ET • different choices of photon and proton pdf's describe the data within errors (variations at the level of 5-10%) J. Cvach, Photoproduction of jets

  6. high ET low ET h distribution H1 trend different from calculation  ]inadequacy of photon PDFs?]higher-order terms needed? J. Cvach, Photoproduction of jets

  7. Inclusive jets:ET ZEUS • cross section falls by more than 5 orders of magnitude from ETjet = 17 to 95 GeV • NLO QCD gives excellent descrip-tion • corrections to hadron level applied to NLO < 2.5% • Data used: • To test the scaling hypothesis • For αS extraction J. Cvach, Photoproduction of jets

  8. fit of toformula: in each bin i constants Cjifrom NLO calculat-ions Fit of the E-scale dependence of measured αSjetto renormalization group equation Jet photoproduction:aSextraction ZEUS J. Cvach, Photoproduction of jets

  9. aSsummary ZEUS ⇦ this measurement • αS value consistent with the current world average0.1183+-0.0027 • Similar accuracy obtained from subjet multiplicity in DIS and • and NLO QCD fits to F2 • HERA is very competitive in determining αS J. Cvach, Photoproduction of jets

  10. Dijets: MJJ, ET,mean cross sections H1 • NLO QCD calculations describe the shape and normalization of Mjjcross section well • Similar statement from ZEUS measuments • In the dijet cross sections dσ /dET,meanthe scale uncertainties are reduced to ~ 5% for ET,mean > 30 GeV, hadr. corrections < 5%. • NLO describes also well dσ /dET,max cross section • Validity of pQCD description of parton -parton and γ-parton interactions in photoproduction J. Cvach, Photoproduction of jets

  11. xγ < 0.75 ET, η jet cross sections ZEUS • The NLO gives good description except for -1< ηjet1 < 0 • Good agreement with NLO for xγ >0.75 When both jets in 1< ηjet1,2 <2.4, NLO lies below data at high ETjet J. Cvach, Photoproduction of jets

  12. ZEUS GRV AFG Photon structure H1 • uncorrelated syst. errors shown by • hatched histogram • Calo energy scale uncertainty • Prediction agrees well with the data neither GRV nor AFG pdf’s provide a perfect description everywhere J. Cvach, Photoproduction of jets

  13. ZEUS H1 ZEUS H1 Sensitivity of NLO calculation on ETjet 25 < ETjet1 < 35 GeV • Differences between ZEUS and H1 appear to be due to the different cuts on ET2 • Comparison of data vs NLOdepends on the cut value! • ET2dependence significantlydifferent for data and NLO • HERWIG describes dependence very well • For xγ > 0.8converges to the data as ET2cut is decreased; the cross section is less sensitive to the ET2 cut value Theoretical work on improving the dijet calculations is needed! J. Cvach, Photoproduction of jets

  14. Dynamics of resolved and direct processes • Scatering angle θ*of 2→2 parton scattering coincides with scattering angle in dijet CMS • Small angle jet angular distribution given by the spin of the exchanged particle • Resolved (gluon dominant) • Direct (quark exchange) Dijet θ* distribution should be steeper for resolved processes (xγ < 0.75) compared to direct ones (xγ > 0.75) as |cos θ*| →1 J. Cvach, Photoproduction of jets

  15. ZEUS cos θ* • To study cos θ*without bias at small angles→ cut on low mass dijet mass Mjjis applied (this cut eliminates bias from low ETjet cut) • Cut on ηmean=(η1jet + η2jet)/2 ensures the PS uniformity in cosθ* • Dijet angular resolution is steeper for the resolved sample • NLO describes the shape and normalization of data both for direct and resolved parts (using GRV-HO for the photon PDF) • H1 analysis brings the same conclusion J. Cvach, Photoproduction of jets

  16. Integrated jet shape in θ-φ plane of radius r Jet shapes • The dominant mechanism for high ET production in γp goes via • (resolved) • (direct) • Majority of quark jets in γ(e) direction ηjet< 0 • Increasing fraction of gluon jets with increasing ηjet • Tagging quark and gluon jets can disentangle the underlying hard process • gluon – “thick” jet: Ψ(r = 0.3) < 0.6 and nsbj≥ 6 • quark - “thin” jet: Ψ(r = 0.3) < 0.8 and nsbj<4 for ycut = 0.0005 in ktcluster algorithm J. Cvach, Photoproduction of jets

  17. Inclusive cross sections for jets with • Thick jets peak in forward direction • PYTHIA normalized to the total no. of events • Good description of thin jets Inclusive sample ZEUS • Thick jets reasonably well described by PYTHIA (MI), HERWIG fails • Thick jets exhibit a softer spectrum than thin jets J. Cvach, Photoproduction of jets

  18. ZEUS • PYTHIA gives the best description of data • The difference between 2 samples: • Thick - thick:resolved qg → qg • Thin - thin: direct Dijet sample • Cross sections normalized at |cosθ*|=0.1 • Asymmetry of |cosθ*thick| is due to different dominant diagrams: • gexchange→ +1, • q exchange→ -1 Jet shapes give a powerful handle on dynamics

  19. ET Q2 xγ Dijets at low Q2 • H1 new analysis of 58 pb-1 data with e+inSpacal → 2 < Q2< 80 GeV2 • Large 0.1 < y < 0.85 range to study contribution from longitudinal photon • Comparison with NLO in environment with two hard scales: Q2and jet ET when neither of scales is large • For ET2≫ Q2 virtual photon behaves as both direct and resolved similar to photoproduction • No collinear divergencies in NLO for virtual photon • PDF for virtual photon is perturbati-vely calculable, multiple parton interactions small Tripple differential dijet cross section of xγ in bins (Q2, ET ) compared to the NLO calculation by Jetvip J. Cvach, Photoproduction of jets

  20. Stability of Jetvip calculations • Jetvip uses phase space slicing method to integrate the NLO QCD equations • Detailed investigations of cross section dependence on parameter yc • Small dependence for the direct processes – calculation are stable • Sizable dependence for the resolved processes • As there are not any other NLO calculations for resolved we used yc = 3.10-3 (value which ~maximizes cross section and is recommended by the author (B. Pötter) J. Cvach, Photoproduction of jets

  21. Prompt photons • High ETphoton in the final state • Signature: well isolated compact elmg. shower and track veto • Difficult: background from π0, η0 • Prompt photonsproduced both in direct and resolved processes • Tests of pQCD calculations: • NLO matrix element • PDFs of the photon and the proton J. Cvach, Photoproduction of jets

  22. ZEUS Shower shape variables: ZEUS: longitudinal shape H1: radial shape Distribution of energy in shower: ZEUS: fmax fraction of energy of the most energetic cell H1: hot core fraction = energy in core / cluster energy Signal extracted by fit: H1: likelihood discriminators in (ET , η) Discriminating variables are well described by the fit H1 H1 J. Cvach, Photoproduction of jets

  23. Inclusive prompt photons • New H1 results presented as ep cross sections for 5 < ET < 10 GeV, -1 < η < 0.9 at 0.2<y<0.7 • Inclusive cross sections compared to the NLO QCD calculations of Fontannaz, Guillet, Heinrich • photon PDF: AFG proton PDF: MRST2 • NLO describes data quite well with tendency to overshoot data at large rapidities • PYTHIA (not shown) describes data well in shape but is low by 30% in normalization • ZEUS 1997 data above H1 data J. Cvach, Photoproduction of jets

  24. NLO describes well also cross sections in xγand xpvariables • Correction for MI calculated by PYTHIA → improvement of description at large ηγ, small xγ Prompt photon and jet cross sections H1 • New H1 results presented as ep cross sections for ETjet < 4.5 GeV, -1 < ηjet< 2.3 • Avoid symmetric ETcuts for NLO comparison! • NLO describes data within errors • Substantial and negative NLO corrections at ηjet< 0 J. Cvach, Photoproduction of jets

  25. Summary • HERA measures jets in large kinematic domain and large dynamic range, e.g. jet ET cross section spans over 6 orders of magnitude • Inclusive single jet cross section are in agreement with NLO QCD and can be used to extract very precise αSvalue • Dijet cross sections are accurately measured but comparison with NLO QCD is often difficult due to theoretical uncertainties in the treatment of infrared and collinear divergencies. Comparison of data with calculations is for experimentalists often more tedious than the data analysis itself. • At HERA II a ‘classical’ photoproduction of jets will step down in favour of jet production in diffraction and with jets heavy flavours Theoretical progress in this field is needed! J. Cvach, Photoproduction of jets

  26. Backup foils J. Cvach, Photoproduction of jets

  27. g H1 e p ZEUS { Electron Tagger Photon Detector Luminosity Untagged e:Q2 < 1 GeV2 dL/L= 2.25% dL/L= 1.5% Central Tracking dpT /pT = 0.6% . pT LAr Calorimeter dE /E = 50 % / (E) U/scint Calorimeter dE /E = 35 % / (E) Syst. uncertainty 1%, all ET Jets: inclusive k (D=1) algorithm, ET weight recomb. scheme { 2% (highET) Systematic uncertainty 4% (lowET) Photoproduction & jets at HERA Tagged e: Q2 < 0.01 GeV2 untagged:Q2 < 1 GeV2 dpT /pT = 0.6% . pT+ 0.7% J. Cvach, Photoproduction of jets

  28. Inclusive jets two samples: Q2< 1 GeV2; ET > 21 GeV, 95<Wγp<285 GeV, £=24 pb-1 Q2< 10-2 GeV2; 5< ET< 21 GeV, 164<Wγp<242 GeV, £=.5 pb-1 -1 <η <2.5, Ep= 820 GeV H1 Q2 <1 GeV2; ET >17 GeV, 142<Wγp<293 GeV, £=82 pb-1 -1 <η <2.5 , Ep= 920 GeV ZEUS J. Cvach, Photoproduction of jets

  29. Dijets in photoproduction H1 cuts: Q2 <1 GeV2 0.1<y<0.9 ETmax>25 GeV, ETsec>15 GeV -0.5<η<2.5 Mjj>65 GeV 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 • Probe partons in photon for 0.1<xγ<1., sensitive to quark and gluon densities • Probe partons in proton for 0.05<xp<0.6, sensitive to high xpgluon in proton • Sensitive to cos θ* form of matrix element at small θ* • Sensitive to final state partons via jet shapes • Test field for NLO calculations J. Cvach, Photoproduction of jets

  30. HERA data J. Cvach, Photoproduction of jets

  31. Prompt photons in DIS ZEUS • New ZEUS analysis and first HERA data from DIS: Q2> 35 GeV • Up quark contribution more dominant than in F2 • Large normalization factors applied to PYTHIA (∗2.4) and HERWIG (∗8.3) • Moderate description by NLO (Kramer, Spiesberger) J. Cvach, Photoproduction of jets

  32. Publications • ZEUS inclusive: Phys. Lett. B560 (2003) 7 • H1 inclusive: preprint DESY 02-225, hep-ex/0302034 • ZEUS jet shapes: ICHEP 2003, abstract 518 • ZEUS dijets: Eur.Phys.J C23 (2002),615-631 • H1 dijets: Eur.Phys.J C25 (2002),13 • ZEUS prompt photons: Phys. Lett. B472 (2000) 175 • H1 prompt photons: ICHEP 2003, abstract 093 J. Cvach, Photoproduction of jets

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