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Charmonium Production at HERA

International Workshop on Heavy Quarkonium FERMILAB , September 20-22, 2003. Charmonium Production at HERA. Luca Stanco – INFN Padova. H1 and ZEUS collaborations. Outline: Introduction J/ y Production Mechanisms Inelastic J/ y and y ’ photoproduction Inelastic J/ y electroproduction

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Charmonium Production at HERA

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  1. International Workshop on Heavy Quarkonium FERMILAB, September 20-22, 2003 Charmonium Production at HERA Luca Stanco – INFN Padova H1 and ZEUS collaborations • Outline: • Introduction • J/y Production Mechanisms • InelasticJ/y and y’photoproduction • InelasticJ/yelectroproduction • Polarization Measurements • Conclusions and Outlook Heavy Quarkonium, Fermilab

  2. Introduction - I Kinematic variables: • photoproduction (Q2 < 1 GeV2) • scattered e not seen in the main detector • electroproduction (2 < Q2 < 100 GeV2) • scattered e detected in calorimeter • J/y detected through: • m+m- , e+e- decay modes Heavy Quarkonium, Fermilab

  3. Introduction - II charmonium production ? Color Singlet Model (CSM) (orders of magnitude too low) Non-Relativistic QCD (NRQCD) Heavy Quarkonium, Fermilab

  4. Production Mechanisms - I Direct photon gluon fusion: CSM CS+CO Color Singlet Model Non-Relativistic QCD cc also in colour octet state Additional free parameters Long distance matrix elements (LDMEs) LDMEs not calculablefrom experiments cc must have J/y quantum numbers Only one free parameter given by Heavy Quarkonium, Fermilab

  5. Production Mechanisms - II Resolved photon processes (gg fusion): suppressed with increasing Q2 CSM CS+CO Color Singlet Model Non-Relativistic QCD Heavy Quarkonium, Fermilab

  6. Proton dissociation: z  1 Proton dissociation: z  1 Proton dissociation: z  1 ELASTIC diffraction: z = 1 ELASTIC diffraction: z = 1 Production Mechanisms – III (background) • .Diffraction(subracted in ZEUS:  15%, not subtracted in H1:  2%) • Decay of diffractively or inelastically produced y’ mesons: y’  J/y ppnot subtracted in data ! • Decay of cc mesons: c’  J/yg (low z) not subtracted in data ! • Decay of B mesons: B  J/y X (low z, high pt,y) not subtracted in data ! Suppressed by cuts z<0.9, p*t,y (pt,y in g-p), additional activity in the detector Heavy Quarkonium, Fermilab

  7. Production Mechanisms and DATA J/y variables z > 0.4 50 < W < 180 From MC: 44% of diffractive proton-dissociation, (17% for z < 0.9) 27% 17% Heavy Quarkonium, Fermilab

  8. Photoproduction: DATA and CSM NLO • Good agreement between H1 and ZEUS • full NLO of the direct g-g fusion (M.Krämer) • theory in agreement with data both in shape and normalization, within the large theoretical uncertainties H1: 96-00, m+m- ZEUS: 96-97, m+m- (scaled to H1 phase space) Heavy Quarkonium, Fermilab

  9. Photoproduction: DATA and NRQCD LO The measurements explore the low z region: z=0.05 (H1), z=0.1 (ZEUS) • Large uncertainties in calculation due to LDMEs from CDF • Large values of LDME are here excluded • .NRQCD LO calculation including direct and resolved photon (M.Krämer and M.Cacciari) provide a good description of whole z range with small LDMEs Heavy Quarkonium, Fermilab

  10. Photoproduction: DATA and NRQCD LO Cut on low pT to exclude low z (and to increase hard scale) • NRQCD LO calculation resumming soft contributions at high z (M.Beneke, G.A.Schuler and S.Wolf) • L: energy loss of J/y due to soft gluon radiation Resummation reduces discrepancies at high z Heavy Quarkonium, Fermilab

  11. y’Photoproduction: DATA and CSM 0.55 < z < 0.9 50 < W < 180 • Underlying production mechanisms for J/y and y’ are the same ! • Integrated on DATA: 0.33±0.10+0.01-0.02 • .LO-CSM prediction : 24% Heavy Quarkonium, Fermilab

  12. Electroproduction: Q2 and p*T,y dependence • Comparisons with CS and NRQCD LO contributions (B.A.Kniehl and L.Zwirner) • . CS contribution too low by  2.7 and too step in (p*T,y)2 missing higher orders ? • .CS+CO too high at low Q2 and (p*T,y)2(  2) CS+CO desription improves at high Q2 and (p*T,y)2 (smaller theoretical errors) H1: 97-00, e+e- and m+m- Heavy Quarkonium, Fermilab

  13. Electroproduction: z and rapidity dependence ZEUS: 99-00, m+m- (not easyly scalable to H1 phase space) z: missing resummation of soft terms for CS+CO comparisons CS below data but shape consistent CS+CO above the data Heavy Quarkonium, Fermilab

  14. Polarization Measurements Polarization of J/y provides information on production processes independent of normalization uncertainties Polarization is measured in decay angular distributions in J/y rest frame system: q*: angle m+ to z’ axis (opposite direction to that of the proton) f*: angle m+ to plane determined by incoming photon and proton l n l=+1: transverse polarization, l=-1: longitudinal polarization Heavy Quarkonium, Fermilab

  15. Polarization Measurements: photoproduction l H1: 96-00, m+m- ZEUS: 96-00, m+m- l • variable n more promising todisentangle • more data needed to decide on production mechanism(s) n n Heavy Quarkonium, Fermilab

  16. Conclusions and Outlook • Photoproduction: • medium z: good agreement with CSM NLO calculations • low z: resolved photon contributions improve agreement • NRQCD with small LDMEs gives reasonable description • y’ in line with expectations (but still large exper. errors) • Electroproduction: • CS (LO) alone too low, wrong (p*T,y)2 dependence • NRQCD with small LDMEs gives reasonable description • DATA: • major improvements in data statistics needed for • conclusions from polarization measurements •  only possible with HERA II data • Theory: • NLO calculation in NRQCD is needed ! • (both for photo- and electro-production) Heavy Quarkonium, Fermilab

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