Inclusive jet photoproduction at HERA B.Andrieu (LPNHE, Paris) On behalf of the collaboration Outline: Introduction & motivation QCD calculations and Monte Carlo Detector & experimental facts Results Summary
DIS03, St-Petersburg, 25 Apr B.Andrieu Inclusive jet photoproduction at HERA -1 Jet photoproduction (parton level) pp j (xp)(xp) k l e e e’ p p pp j (xp)(xp) i (x)(x) k l = direct (pointlike) + resolved (hadronlike (VDM) + qq ) PDF of parton i (j ) in (p) photon flux in e factorisation scales in (p) ; renormalisation scale partonic cross section (direct) (resolved) { {
DIS03, St-Petersburg, 25 Apr B.Andrieu Inclusive jet photoproduction at HERA -2 Jet photoproduction (hadron level) Higher order QCD processes LO hard processSoft processes LO QCD partons jets of hadrons detector signals 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) d ij = min (E T i,E T j ) R ij /D ; E T weighted recombination scheme Cone (R=1) algorithm used for comparison with previous data
DIS03, St-Petersburg, 25 Apr B.Andrieu Inclusive jet photoproduction at HERA -3 Motivation High E T 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 E T 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 x ,x p + No infrared sensitivity w.r.t. kinematical cuts as for dijet
DIS03, St-Petersburg, 25 Apr B.Andrieu Inclusive jet photoproduction at HERA -4 QCD calculations and Monte Carlo 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 x p ) LO QCD Monte Carlo event generators to correct data and calculations to the hadron level: PYTHIA, PHOJET, HERWIG QCD = 200 MeV Fragmentation: LUND String (PYTHIA, PHOJET) or Cluster (HERWIG) Underlying event: Multiple Interactions (PYTHIA) p T mia =1.2 GeV Dual Parton Model (PHOJET) Soft Underlying Event (HERWIG) 35 % resolved {
DIS03, St-Petersburg, 25 Apr B.Andrieu Inclusive jet photoproduction at HERA -5 H1 detector at HERA e p Central Tracking dp T /p T = 0.6 %. p T LAr Calorimeter dE /E = 50 % / (E) (hadrons & jets) { 2% (high E T ) 4% (low E T ) Systematic uncertainty Electron Tagger Photon Detector ep d L / L = 1.5% = 300 GeV Luminosity {
DIS03, St-Petersburg, 25 Apr B.Andrieu Inclusive jet photoproduction at HERA -6 Experimental facts (I) Inclusive cross section as a function of E T jet and jet count the number of jets in a given kinematical range jet measured in laboratory frame ( cms ~ jet – 2) High E T jets (E T jet > 21 GeV) L = 24 pb -1, untagged (e + undetected) data Q 2 < 1 GeV 2, 95 < W p < 285 GeV (0.1 < y < 0.9) Low E T jets (5 GeV< E T jet < 21 GeV) L = 0.5 pb -1, tagged (e + detected) data Q 2 < 0.01 GeV 2, 164 < W p < 242 GeV (0.3 < y < 0.65)
DIS03, St-Petersburg, 25 Apr B.Andrieu Inclusive jet photoproduction at HERA -7 Experimental facts (II) Hadronisation corrections fragmentation (after parton showers) underlying event (after fragmentation) reverse order consistent results (1+ hadr. ) = (1+ frag. ). (1+ u.e. ) frag. < 0 and when E T or u.e. > 0 and when E T or hadr. ~ 30 (10) % for E T 20) GeV Cone: hadr. ~ 40 (20) % for E T 15) GeV Data corrections bin migrations Important due to steeply falling E T spectrum selection efficiencies Exclude regions of large migrations high E T <0 (photon region) low E T >1.5 (proton region) Hadron level cross sections obtained using Monte Carlo HO QCD partonsjets of hadronsdetector signals
DIS03, St-Petersburg, 25 Apr B.Andrieu Inclusive jet photoproduction at HERA -8 Experimental facts (III) Systematic uncertainties: LAr hadronic energy scale % (10 %) for low (high) E T Correction for detector effects < 10 % (8 %) for low (high) E T (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) E T (statistical 1/2 difference between Monte Carlo) Renormalisation & factorisation scale (x2, /2) uncertainty < 10 %
DIS03, St-Petersburg, 25 Apr B.Andrieu Inclusive jet photoproduction at HERA -9 E T & distribution (high E T ) LO too low at low E T and high Agreement with NLO very good, even w/o hadronisation corrections All predictions using different PDFs agree with the data
DIS03, St-Petersburg, 25 Apr B.Andrieu Inclusive jet photoproduction at HERA -10 E T distribution (W p bins, high E T ) E T & fixed: 1/W p LO prediction low E T & high W p too low NLO prediction high W p very good agreement lowW p reasonable agreement promising region to constrain gluon at high x p
DIS03, St-Petersburg, 25 Apr B.Andrieu Inclusive jet photoproduction at HERA -11 E T distribution: full range LO prediction fails to reproduce shape NLO prediction good agreement over 6 orders of magnitude! hadronisation corrections needed Fit Range: 5 < E T < 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 (syst.)
DIS03, St-Petersburg, 25 Apr B.Andrieu Inclusive jet photoproduction at HERA -12 distribution ( E T bins, high E T ) 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 W p & fixed: E T
DIS03, St-Petersburg, 25 Apr B.Andrieu Inclusive jet photoproduction at HERA -13 distribution ( E T & W p bins, high E T ) Good agreement with NLO even w/o hadronisation corrections Cross section maximum shifted towards lower values for higher W p (Lorentz boost) and lower E T All PDFs consistent with data Precision of data equivalent to (or better than) scale uncertainty could be used to better constrain PDFs fits fixed: E T / W p
DIS03, St-Petersburg, 25 Apr B.Andrieu Inclusive jet photoproduction at HERA -14 distribution ( E T bins, low E T ) 12 < E T < 21 GeV good agreement both NLO and hadronisation corrections needed 5 < E T < 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?
DIS03, St-Petersburg, 25 Apr B.Andrieu Inclusive jet photoproduction at HERA -15 x T < 0.2 shape similar for p and pp resolved photon ~ hadron x T > 0.2 p harder than pp spectrum enhanced quark density in the resolved photon w.r.t. a hadron dominance of direct point-like photon Comparison with pp - Scaled cross section (independent of energy up to scaling violations) - - Confirmation of the dual nature of the photon
DIS03, St-Petersburg, 25 Apr B.Andrieu Inclusive jet photoproduction at HERA -16 Summary New measurement of inclusive jet photoproduction cross section ( L x 80 compared with previous one) using the k algorithm Kinematical range extended to E T =75 GeV (~ x T = 0.5) Experimental uncertainties already competitive with (scale) uncertainties Good agreement over 6 orders of magnitude in E T distribution NLO and hadronisation corrections needed, especially at low E T No discrimination of PDFs, but data helpful in global PDFs fits and future measurement promising for the gluon at high x p Determination of hadronisation corrections challenging for theory and phenomenology Comparison of scaled cross section with pp data confirms the dual nature of the photon with a transition around x T = 0.2 -