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Results on Diffractive Vector Meson Production in ZEUS

Results on Diffractive Vector Meson Production in ZEUS. Joachim Tandler Bonn University DIS 03 St. Petersburg, 23-27 March 2003. Motivation Experimental results: different VMs, different kinematic ranges Summary. on behalf of. HERA: Elastic VM Production. Clean experimental signature!.

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Results on Diffractive Vector Meson Production in ZEUS

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  1. Results on Diffractive Vector Meson Production in ZEUS Joachim Tandler Bonn University DIS 03 St. Petersburg, 23-27 March 2003 • Motivation • Experimental results: different VMs, different kinematic ranges • Summary on behalf of

  2. HERA: Elastic VM Production Clean experimental signature! M VM = r, w, f, J/Y Joachim Tandler, DIS 03, St. Petersburg

  3. Models for VM Production Vector Dominance  Regge g*  VM oscillation (before interaction) Soft pomeron exchange a(t) = a0 + a‘ t, „trajectory“ (a0 = 1.08, a‘ = 0.25 GeV-2) ds/dt  eb0t (W/W0)4 (a(t) - 1) soft regime Predictions:  Slow rise s(W)  Wd, d  0.22 Shrinkage: b(W) = b0 + 4a‘ ln(W/W0) Joachim Tandler, DIS 03, St. Petersburg

  4. Perturbative QCD Models for VM Prodution • In proton rest frame • g* qqoscillation • qqscatters off the proton • VM is formed (after interaction) _ _ _ qq _ qqlifetime t 1/(xMp): small x  long lifetime sL (x,Q2) = d2rdz |YL(r,z,Q2)|2 · sqq2(x,r) YL: g*Lqqwave function sqq: qq-proton cross-section z= Eq/Eg* _ _ _ • r: transverse separation ofqq • r = [z(1-z)Q2 +mq2]½ is small, if Q2 large or quarks heavy: VM=cc or bb _ _ _ _ Joachim Tandler, DIS 03, St. Petersburg

  5. Models for VM Production pQCD M Two gluon exchange (LO) in colour singlet state sLas2(Qeff2)/Q6 |xG(x,Qeff2)|2 hard regime Qeff2 7 GeV2 • Questions: • Do these models describe HERA data? • At which scale, Qeff2, has xG to be evaluated? I.e. which combination of Q2, MVM and t?Ryskin: Qeff2 = ¼ (Q2 + MVM2 + t) Predictions:  Fast rise s(W)  [x 0.2]2, (x  Q2/W2)Wd, d  0.8  Universal t dependence ds/dt  ebt b = b2g  4 GeV-2 and a‘  0 Naively: r : w : f : J/y =9 : 1 : 2 : 8 Joachim Tandler, DIS 03, St. Petersburg

  6. Q2 as scale Fit s(W)  Wd r electroproduction No saturation yet Q2 soft hard soft hard Joachim Tandler, DIS 03, St. Petersburg

  7. MVM as scale optical theorem Vector Meson Photoproduction soft MVM hard Joachim Tandler, DIS 03, St. Petersburg

  8. J/y Photoproduction • Fast rise with W: d = 0.69 ± 0.02 • Described by pQCD with MJ/y as hard scale Sensitivity to the gluon density sL |xG(x,Q2)|2 J/y Photoproduction Joachim Tandler, DIS 03, St. Petersburg

  9. Two scales: MVM and Q2 Fit s(W)  Wd J/yElectroproduction Q2 No change of W dependence with Q2  d is flat in Q2 Joachim Tandler, DIS 03, St. Petersburg

  10. Two scales: MVM and Q2 New! • W dependence in t-bins • Fit with ds/dt  (W/W0)4 (a(t) - 1) • Extract thepomeron trajectory J/yElectroproduction Compatible with photoproduction? Joachim Tandler, DIS 03, St. Petersburg

  11. Two scales: MVM and Q2 New! J/yElectroproduction • Pomeron trajectory in J/y DIS •  Yes, it is compatible with photoproduction •  Hard process in photoproduction and DIS Joachim Tandler, DIS 03, St. Petersburg

  12. J/yElectroproduction Fit s(Q2)  (Q2+MJ/y2)-n J/y • Q2 sets the size r of the colour dipole • Cross section well described in shape and normalisation (MRT) by pQCD models  sqq(r) is well modelled • different n for sL andsTas expected r sL sT r, J/yElectroproduction Joachim Tandler, DIS 03, St. Petersburg

  13. Size of the proton? ds/dt  ebt, b is related to the transverse sizes of the scattering objects „b = b2g + bVM“ pQCD: universal b slope b = b2g  4 GeV-2 (gluon could of the proton) r r p-diss  J/y and r at high Q2 are point-like J/y r, J/yElectroproduction Joachim Tandler, DIS 03, St. Petersburg

  14. VMs at High t Expectations: • Soft: ds/dt  ebt • Hard: power-like (BFKL) (naïve: t4)  Indeed power-like behavior g p  VMY,VM = r,f,J/y Joachim Tandler, DIS 03, St. Petersburg

  15. Q2+M2universal scale? r, f, J/y • Naïve SU(4) approach • Use scale Q2 + M2 • Fails for the J/y • VM wave function effects? Q2+M2 Joachim Tandler, DIS 03, St. Petersburg

  16. Ratio sV/stot r J/y Q2+M2 4 GeV2 pQCD: sVaS/Q6 |xG(x,Q2)|2 sV/stot W2l/b • W independence for r cannot be explained by pQCD or Regge  still unknown soft physics? Regge: sVW4 (a(0) - 1) sV/stot W2 (a(0) - 1) /b Joachim Tandler, DIS 03, St. Petersburg

  17. Summary New Pomeron trajectory in high-Q2 J/yconfirms J/y to be hard alredy in photoproduction Various kinematic regions Validity range of pQCD in terms of MVM, Q2, t Generally, pQCD and the colour dipole model are able to describe the hard regime Still not clear, what is finally the scale to use Is elastic r production at high Q2 still soft? Joachim Tandler, DIS 03, St. Petersburg

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