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. 25 March 2003Joachim Tandler, DIS 03, St. Petersburg2 Clean experimental signature! HERA: Elastic VM Production M VM VM mass,0.8 – 3.1 GeV Q2Q2 = -q 2 = (k-k‘) 2 Virtuality of exchanged  * 0 – 100 GeV 2 W= (q+p) 2  *p centre of mass energy 20 – 290 GeV x= Q 2 /(p·q) = 1/(1+(W/Q) 2 ) x Bjørken variable10 -4 – 0.9 t= (P-P‘) 2 4-momentum transfer squared at p vertex -20 – 0 GeV 2 VM =  J/  M

3. 25 March 2003Joachim Tandler, DIS 03, St. Petersburg3 Models for VM Production  *  VM oscillation (before interaction) Soft pomeron exchange  t    ‘ t, „trajectory“ (    ‘ = 0.25 GeV -2 ) d  dt  e b0t (W/W 0 ) 4 (  t  1) Vector Dominance  Regge soft regime Predictions:  Slow rise  W)  W   0.22 Shrinkage: b(W) = b 0 + 4  ‘ ln(W/W 0 )

4. 25 March 2003Joachim Tandler, DIS 03, St. Petersburg4 Models for VM Prodution Perturbative QCD In proton rest frame  *  qq oscillation qq scatters off the proton VM is formed (after interaction) qq lifetime   1/(xM p ): small x  long lifetime  L (x,Q 2 ) =  d 2 rdz |  L (r,z,Q 2 )| 2 ·  qq 2 (x,r)  L :  * L  qq wave function  qq : qq-proton cross-section z = E q /E  * r: transverse separation of qq r = [z(1-z)Q 2 +m q 2 ]  ½ is small, if Q 2 large or quarks heavy: VM=cc or bb qq _ _ _ _ _ _ _ _ _ _ _

5. 25 March 2003Joachim Tandler, DIS 03, St. Petersburg5 Predictions:  Fast rise  W)  [x  0.2 ] 2, (x  Q 2 /W 2 )  W   0.8  Universal t dependence d  /dt  e bt b = b 2g  4 GeV -2 and  ‘  0  Naively:  J  =  9 : 1 : 2 : 8 Two gluon exchange (LO) in colour singlet state  L   s 2 (Q eff 2 )/Q 6 |xG(x,Q eff 2 )| 2 Models for VM Production Q eff 2  7 GeV 2 hard regime pQCD Questions: Do these models describe HERA data? At which scale, Q eff 2, has xG to be evaluated? I.e. which combination of Q 2, M VM and t? Ryskin: Q eff 2 = ¼ (Q 2 + M VM 2 + t) M

6. 25 March 2003Joachim Tandler, DIS 03, St. Petersburg6 Q 2 as scale  electroproduction Q2Q2 soft hard Fit  W)  W  soft hard No saturation yet

7. 25 March 2003Joachim Tandler, DIS 03, St. Petersburg7 M VM as scale Vector Meson Photoproduction M VM soft hard optical theorem

8. 25 March 2003Joachim Tandler, DIS 03, St. Petersburg8 J/  Photoproduction Fast rise with W:  = 0.69 ± 0.02 Described by pQCD with M J/  as hard scale Sensitivity to the gluon density  L  |xG(x,Q 2 )| 2 J/  Photoproduction

9. 25 March 2003Joachim Tandler, DIS 03, St. Petersburg9 Two scales: M VM and Q 2 J/  Electroproduction   is flat in Q 2 Q2Q2 No change of W dependence with Q 2 Fit  W)  W 

10. 25 March 2003Joachim Tandler, DIS 03, St. Petersburg10 Two scales: M VM and Q 2 Compatible with photoproduction? New! W dependence in t- bins Fit with d  dt  (W/W 0 ) 4 (  t  1) Extract the pomeron trajectory J/  Electroproduction

11. 25 March 2003Joachim Tandler, DIS 03, St. Petersburg11 Two scales: M VM and Q 2 Pomeron trajectory in J/  DIS  Yes, it is compatible with photoproduction  Hard process in photoproduction and DIS New! J/  Electroproduction

12. 25 March 2003Joachim Tandler, DIS 03, St. Petersburg12 J/  Electroproduction Q 2 sets the size r of the colour dipole Cross section well described in shape and normalisation (MRT) by pQCD models   qq (r) is well modelled different n for  L and  T as expected , J/  Electroproduction J/   Fit  Q 2 )  (Q 2 +M J/  2 ) -n LL TT

13. 25 March 2003Joachim Tandler, DIS 03, St. Petersburg13 Size of the proton?  J/  and  at high Q2 are point-like J/    p-diss d  /dt  e bt, b is related to the transverse sizes of the scattering objects „b = b 2g + b VM “ pQCD: universal b slope b = b 2g  4 GeV -2 (gluon could of the proton) , J/  Electroproduction

14. 25 March 2003Joachim Tandler, DIS 03, St. Petersburg14 VMs at High t Expectations: Soft: d  /dt  e bt Hard: power-like (BFKL) (naïve: t 4 )  p  VM  Y, VM =  J/   Indeed power-like behavior

15. 25 March 2003Joachim Tandler, DIS 03, St. Petersburg15 Q 2 +M 2 universal scale? Naïve SU(4) approach Use scale Q 2 + M 2  Fails for the J/   VM wave function effects?  J/  Q 2 +M 2

16. 25 March 2003Joachim Tandler, DIS 03, St. Petersburg16 Ratio  V /  tot W independence for  cannot be explained by pQCD or Regge  still unknown soft physics? J/   pQCD:  V   S /Q 6 |xG(x,Q 2 )| 2  V /  tot  W 2 /b Regge:  V  W 4 (  0  1)  V /  tot  W 2 (  0  1) /b Q 2 +M 2 4 GeV 2

17. 25 March 2003Joachim Tandler, DIS 03, St. Petersburg17 Summary New Pomeron trajectory in high-Q 2 J/  confirms J/  to be hard alredy in photoproduction Various kinematic regions  Validity range of pQCD in terms of M VM, Q 2, 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  production at high Q 2 still soft?