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

2. Luca Stanco - Padova Heavy Quarkonium, Fermilab 2 Introduction - I photoproduction (Q 2 < 1 GeV 2 ) scattered e not seen in the main detector electroproduction (2 < Q 2 < 100 GeV 2 ) scattered e detected in calorimeter J/  detected through:  +  -, e + e - decay modes Kinematic variables:

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

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

5. Luca Stanco - Padova Heavy Quarkonium, Fermilab 5 Production Mechanisms - II Resolved photon processes (gg fusion): Color Singlet Model Non-Relativistic QCD suppressed with increasing Q 2 CSM CS+CO

6. Luca Stanco - Padova Heavy Quarkonium, Fermilab 6 Production Mechanisms – III (background) 1..Diffraction (subracted in ZEUS:  15%, not subtracted in H1:  2%) 2.Decay of diffractively or inelastically produced  ’ mesons:  ’  J/  not subtracted in data ! 3.Decay of  c mesons:  ’  J/   (low z) not subtracted in data ! 4.Decay of B mesons: B  J/  X (low z, high p t,  ) not subtracted in data ! Suppressed by cuts z<0.9, p * t,  (  p t,  in  p), additional activity in the detector ELASTIC diffraction: z = 1 Proton dissociation: z  1 ELASTIC diffraction: z = 1 Proton dissociation: z  1

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

8. Luca Stanco - Padova Heavy Quarkonium, Fermilab 8 Photoproduction: DATA and CSM NLO H1: 96-00,  +  - ZEUS: 96-97,  +  - (scaled to H1 phase space) 1.Good agreement between H1 and ZEUS 2.full NLO of the direct  -g fusion (M.Krämer) 3.theory in agreement with data both in shape and normalization, within the large theoretical uncertainties

9. Luca Stanco - Padova Heavy Quarkonium, Fermilab 9 Photoproduction: DATA and NRQCD LO The measurements explore the low z region: z=0.05 (H1), z=0.1 (ZEUS) 1.Large uncertainties in calculation due to LDMEs from CDF 2.Large values of LDME are here excluded 3..NRQCD LO calculation including direct and resolved photon (M.Krämer and M.Cacciari) provide a good description of whole z range with small LDMEs

10. Luca Stanco - Padova Heavy Quarkonium, Fermilab 10 Photoproduction: DATA and NRQCD LO Cut on low p T to exclude low z (and to increase hard scale) 1.NRQCD LO calculation resumming soft contributions at high z (M.Beneke, G.A.Schuler and S.Wolf)  : energy loss of J/  due to soft gluon radiation Resummation reduces discrepancies at high z

11. Luca Stanco - Padova Heavy Quarkonium, Fermilab 11  ’ Photoproduction: DATA and CSM 1.Underlying production mechanisms for J/  and  ’ are the same ! 2.Integrated on DATA: 0.33±0.10 +0.01 -0.02 3..LO-CSM prediction : 24% 0.55 < z < 0.9 50 < W < 180

12. Luca Stanco - Padova Heavy Quarkonium, Fermilab 12 Electroproduction: Q 2 and p * T,  dependence 1.Comparisons with CS and NRQCD LO contributions (B.A.Kniehl and L.Zwirner) 2.. CS contribution too low by  2.7 and too step in (p * T,  ) 2  missing higher orders ? 3..CS+CO too high at low Q 2 and (p * T,  ) 2 (  2) CS+CO desription improves at high Q 2 and (p * T,  ) 2 (smaller theoretical errors) H1: 97-00, e + e - and  +  -

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

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

15. Luca Stanco - Padova Heavy Quarkonium, Fermilab 15 H1: 96-00,  +  - ZEUS: 96-00,  +  - Polarization Measurements: photoproduction 1.variable more promising to disentangle 2.more data needed to decide on production mechanism(s)

16. Luca Stanco - Padova 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  ’ in line with expectations (but still large exper. errors) Electroproduction: CS (LO) alone too low, wrong (p * T,  ) 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)