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Charmonium Production at HERA

This international workshop focused on charmonium production mechanisms and data analysis at FERMILAB in 2003. Highlights include production mechanisms, proton dissociation, data analysis, and polarization measurements. The workshop provided insights into heavy quarkonium production processes.

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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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