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Inclusive jet photoproduction at HERA

B.Andrieu (LPNHE, Paris) On behalf of the collaboration. Outline:. u. Introduction & motivation QCD calculations and Monte Carlo Detector & experimental facts Results Summary. u. u. u. u. Inclusive jet photoproduction at HERA. Jet photoproduction (parton level).

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Inclusive jet photoproduction at HERA

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  1. B.Andrieu (LPNHE, Paris) On behalf of the collaboration Outline: u Introduction & motivation QCD calculations and Monte Carlo Detector & experimental facts Results Summary u u u u Inclusive jet photoproduction at HERA

  2. Jet photoproduction (parton level) g=direct (pointlike) + resolved (hadronlike (VDM) + gqq ) e’ mg e e’ e (xg) k k i j j (xp) (xp) l l p p mp mp photon flux in e PDF of parton i (j ) in g (p) { { (direct) factorisation scales in g (p) ; (resolved) renormalisation scale partonic cross section B.AndrieuInclusive jet photoproduction at HERA -

  3. Jet photoproduction (hadron level) LO QCD partons  jets of hadrons  detector signals LO hard process Soft processes Compare jets at the parton, hadron and detector level ] Jet algorithms must ensure • infrared and collinear safety • minimal sensitivity to non-perturbative processes ]inclusive k (D=1) algorithm(Ellis&Soper, PRD48 (1993) 3160) • dij = min (ETi,ETj ) Rij /D; ET weighted recombination scheme • Cone (R=1) algorithm used for comparison with previous data Higher order QCD processes B.AndrieuInclusive jet photoproduction at HERA -

  4. Motivation • High ET jets( non-perturbative effects and scale uncertainty reduced) ]Direct insight into parton dynamics ]Precise tests of perturbative QCD predictions ] Constrain photon and proton PDFs ] Search for new physics • Low ET jets( non-perturbative effects and scale uncertainty important) ]Test phenomenological models of underlying event + fragmentation • Inclusive vs dijet + More statistics, extended kinematical range  No direct reconstruction of xg,xp + No infrared sensitivity w.r.t. kinematical cuts as for dijet B.AndrieuInclusive jet photoproduction at HERA -

  5. QCD calculations and Monte Carlo { Multiple Interactions (PYTHIA) pTmia =1.2 GeV Dual Parton Model (PHOJET) Soft Underlying Event (HERWIG) 35 % resolved Underlying event: • Most precise QCD calculations up to NLO (parton level) • NLO QCD weighted parton Monte Carlo(Frixione, NPB507(1997) 295) • Photon & proton PDFs: GRV & CTEQ5M • Other choice : photon  AFG proton  MRST99, CTEQ5HJ (enhanced gluon at high xp ) • LO QCD Monte Carlo event generators to correct data and calculations to the hadron level: PYTHIA, PHOJET, HERWIG • LQCD = 200 MeV • Fragmentation: LUND String (PYTHIA, PHOJET) or Cluster (HERWIG) B.AndrieuInclusive jet photoproduction at HERA -

  6. H1 detector at HERA g p e p { Electron Tagger Photon Detector Luminosity dL/L= 1.5% Central Tracking dpT /pT = 0.6 % . pT e LAr Calorimeter dE /E = 50 % / (E) (hadrons & jets) = 300 GeV { 2% (highET) Systematic uncertainty 4% (lowET) B.AndrieuInclusive jet photoproduction at HERA -

  7. Experimental facts (I) • Inclusive cross section as a function of ETjet and hjet • count the number of jets in a given kinematical range • hjet measured in laboratory frame (hcms ~hjet – 2) • High ET jets (ETjet > 21 GeV) • L= 24 pb-1, untagged (e+undetected)data Q2 < 1 GeV2 , 95 < W gp < 285 GeV (0.1 < y < 0.9) • Low ET jets (5 GeV< ETjet < 21 GeV) • L= 0.5 pb-1, tagged (e+ detected)data Q2 < 0.01 GeV2 , 164 < W gp < 242 GeV (0.3 < y < 0.65) B.AndrieuInclusive jet photoproduction at HERA -

  8. Hadronisation corrections fragmentation (after parton showers) underlying event (after fragmentation) reverse order  consistent results (1+dhadr.) = (1+dfrag.).(1+du.e.) dfrag.< 0 andwhen ET or h du.e.> 0 and when ET  or h dhadr.~ 30 (10) % for ET <10 (>20) GeV Cone: dhadr.~ 40 (20) % for ET <15 (>15) GeV Data corrections bin migrations ] Important due to steeply falling ET spectrum selection efficiencies Exclude regions of large migrations high ET ] h<0 (photon region) low ET ] h>1.5 (proton region) Experimental facts (II) HO QCDpartons jets of hadrons detector signals Hadron level cross sections obtained using Monte Carlo B.AndrieuInclusive jet photoproduction at HERA -

  9. Experimental facts (III) • Systematic uncertainties: • LArhadronic energy scale  10-20 % (10 %) for low (high) ET • Correction for detector effects < 10 % (8 %)for low (high) ET (statistical 1/2 difference between Monte Carlo) • Luminosity 1.5 % • All other uncertainties (SPACAL energy scale, fraction of hadronic energy flow carried by tracks, background subtraction, trigger efficiency) 1 % • Theoretical uncertainties: • Hadronisation correction uncertainty  30 % (10 %) for low (high) ET (statistical  1/2 difference between Monte Carlo) • Renormalisation & factorisation scale (x2, /2)uncertainty < 10 % B.AndrieuInclusive jet photoproduction at HERA -

  10. ET& hdistribution (high ET) • LO too low at lowET and high h • Agreement with NLO very good, even w/o hadronisation corrections • All predictions using different PDFs agree with the data B.AndrieuInclusive jet photoproduction at HERA -

  11. ET distribution (W gpbins, high ET) ET&h fixed: <xg,xp>1/W gp • LO prediction • low ET & high W gp ] too low • NLO prediction • high W gp ] very good agreement • lowW gp ] reasonable agreement ] promising region to constrain gluon at high xp B.AndrieuInclusive jet photoproduction at HERA -

  12. ET distribution: full range • LO prediction fails to reproduce shape • NLO prediction ]good agreement over 6 orders of magnitude! ] hadronisation corrections needed • Fit Range: 5 < ET< 35 GeV n = 7.5 0.3 (stat) +0.1–0.5 (syst.) ] compatible with similar fit on charged particle cross section (EPJ C10 (1999) 363) n = 7.03  0.07 +-0.2 (syst.) B.AndrieuInclusive jet photoproduction at HERA -

  13. h distribution (ETbins,high ET) W gp &h fixed: <xg,xp>ET • Good agreement with NLO even w/o hadronisation corrections • Precision of data equivalent to (or even better than) scale uncertainty ] challenge for theory to reduce uncertainty B.AndrieuInclusive jet photoproduction at HERA -

  14. h distribution (ET & W gpbins,high ET) h fixed: <xg,xp>ET / W gp • Good agreement with NLO even w/o hadronisation corrections • Cross section maximum shifted towards lower h values for higher W gp (Lorentz boost) and lower ET • All PDFs consistent with data • Precision of data equivalent to (or better than) scale uncertainty ] could be used to better constrain PDFs fits B.AndrieuInclusive jet photoproduction at HERA -

  15. h distribution (ETbins,low ET) • 12 < ET < 21 GeV ] good agreement ] both NLO and hadronisation corrections needed • 5 < ET < 12 GeV ] data indicative of a trend different from calculation ] challenge for Monte Carlo to accurately estimate hadronisation corrections? ] inadequacy of photon PDFs? ] higher-order terms needed? B.AndrieuInclusive jet photoproduction at HERA -

  16. - Comparison with pp Scaled cross section (independent of energy up to scaling violations) • xT< 0.2 ] shape similar for gp and pp ] resolved photon ~ hadron • xT> 0.2 ]gp harder than pp spectrum • enhanced quark density in the resolved photon w.r.t. a hadron • dominance of direct ] point-like photon - - ] Confirmation of the dual nature of the photon B.AndrieuInclusive jet photoproduction at HERA -

  17. Summary • New measurement of inclusive jet photoproduction cross section (L x 80 compared with previous one) using the kalgorithm • Kinematical range extended to ET =75 GeV (~ xT = 0.5) • Experimental uncertainties already competitive with (scale) uncertainties • Good agreement over 6 orders of magnitude in ET distribution • NLO and hadronisation corrections needed, especially at low ET • Nodiscrimination of PDFs, but data helpful in global PDFs fits and future measurement promising for the gluon at high xp • Determinationof hadronisation corrections challenging for theory and phenomenology • Comparisonof scaled cross section with pp data confirms the dual nature of the photon with a transition around xT = 0.2 - B.AndrieuInclusive jet photoproduction at HERA -

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