1. 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     

2. DIS03, St-Petersburg, 25 Apr. 2003 B.Andrieu Inclusive jet photoproduction at HERA -1 Jet photoproduction (parton level) pp j (xp)(xp) k l e e e’ p p pp 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) { {

3. DIS03, St-Petersburg, 25 Apr. 2003 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

4. DIS03, St-Petersburg, 25 Apr. 2003 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

5. DIS03, St-Petersburg, 25 Apr. 2003 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 {

6. DIS03, St-Petersburg, 25 Apr. 2003 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 {

7. DIS03, St-Petersburg, 25 Apr. 2003 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)

8. DIS03, St-Petersburg, 25 Apr. 2003 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

9. DIS03, St-Petersburg, 25 Apr. 2003 B.Andrieu Inclusive jet photoproduction at HERA -8 Experimental facts (III) Systematic uncertainties: LAr hadronic energy scale  10-20 % (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 %

10. DIS03, St-Petersburg, 25 Apr. 2003 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

11. DIS03, St-Petersburg, 25 Apr. 2003 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

12. DIS03, St-Petersburg, 25 Apr. 2003 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  0.07 +-0.2 (syst.)

13. DIS03, St-Petersburg, 25 Apr. 2003 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

14. DIS03, St-Petersburg, 25 Apr. 2003 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

15. DIS03, St-Petersburg, 25 Apr. 2003 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?

16. DIS03, St-Petersburg, 25 Apr. 2003 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

17. DIS03, St-Petersburg, 25 Apr. 2003 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 -