1. Nov 13, 2007Aharon Levy - Oxford seminar1 Gluons in the proton and exclusive hard diffraction Aharon Levy Tel Aviv University and DESY Introduction soft, hard interactions gluons data on exclusive vector meson electroproduction sizes of gluon cloud sizes of photon configurations effective Pomeron trajectory comparison to theory

2. Nov 13, 2007Aharon Levy - Oxford seminar2

3. Nov 13, 2007Aharon Levy - Oxford seminar3 LHeC

4. Nov 13, 2007Aharon Levy - Oxford seminar4 Deep Inelastic kinematics Spin [20 fb -1 /point]

5. Nov 13, 2007Aharon Levy - Oxford seminar5 McAllister, HofstadterEe=188 MeVr min =0.4 fm Bloom et al. 10 GeV 0.05 fm CERN, FNAL fixed target 500 GeV 0.007 fm HERA 50 TeV 0.0007 fm HERA Kinematics E e =27.5 GeV E P =920 GeV s=(k+P) 2 = (320 GeV) 2 e p r b Transverse distance scale: Impact parameter : where t is the square of the 4-momentum transferred to the proton

6. Nov 13, 2007Aharon Levy - Oxford seminar6 Proton  momentum frame Partons frozen during time of interaction. Virtual photon samples the quark distribution. Assume that partons form incoherent beam. The parton density distributions are meant to be universal quantities.

7. Nov 13, 2007Aharon Levy - Oxford seminar7 Proton rest-frame e p r b Photon fluctuates into,, ….. states, which interact with the proton. r large  interaction soft, r small  interaction hard. soft and hard – studied by W (or x~1/W 2 ) dependence of the cross section.

8. Nov 13, 2007Aharon Levy - Oxford seminar8 high energy behavior  tot  s 0.08 Donnachie and Lanshoff – universal behavior of total hadron-hadron cross section : soft

9. Nov 13, 2007Aharon Levy - Oxford seminar9 Regge trajectories IP - Pomeron

10. Nov 13, 2007Aharon Levy - Oxford seminar10 hard The rise of F 2 with decreasing x is strongly dependent on Q 2. DIS: at small x

11. Nov 13, 2007Aharon Levy - Oxford seminar11 Below Q 2  0.5 GeV 2, see same energy dependence as observed in hadron- hadron interactions. Start to resolve the partons.   s 0.08 soft  hard

12. Nov 13, 2007Aharon Levy - Oxford seminar12 QCD based fits can follow the data accurately, yield parton densities. BUT: many free parameters (18-30) (only know how parton densities evolve) form of parameterisation fixed by hand (not given by theory) F 2  parton densities.  * ‘sees’ partons. parton density increases with decreasing x.

13. Nov 13, 2007Aharon Levy - Oxford seminar13 From Pumplin, DIS05 There are signs that DGLAP (Q 2 evolution) may be in trouble at small x (negative gluons, high  2 for fits). Need better data to test whether our parton densities are reasonable. The structure function F L will provide an important test. all is not well … Can also get information on gluon density from exclusive hard processes.

14. Nov 13, 2007Aharon Levy - Oxford seminar14 arXiv:0711.1721 Date: Mon, 12 Nov 2007 07:49:56 GMT (288kb) Title: Status of Deeply Inelastic Parton Distributions Authors: Johannes Bl\"umlein From EDS07

15. Nov 13, 2007Aharon Levy - Oxford seminar15 Exclusive VM electroproduction (V 0 =   DVCS)

16. Nov 13, 2007Aharon Levy - Oxford seminar16 soft to hard transition IP ‘soft’ ‘hard’ gg Expect  to increase from soft (~0.2, from ‘soft Pomeron’ value) to hard (~0.8, from xg(x,Q 2 ) 2 ) Expect b to decrease from soft (~10 GeV -2 ) to hard (~4-5 GeV -2 )

17. Nov 13, 2007Aharon Levy - Oxford seminar17 ingredients Use QED for photon wave function. Study properties of V-meson wf and the gluon density in the proton.

18. Nov 13, 2007Aharon Levy - Oxford seminar18 Mass distributions

19. Nov 13, 2007Aharon Levy - Oxford seminar19 Photoproduction process becomes hard as scale (mass) becomes larger.

20. Nov 13, 2007Aharon Levy - Oxford seminar20  (W) – ρ 0 Fix mass – change Q 2

21. Nov 13, 2007Aharon Levy - Oxford seminar21  (W) - , J/ , 

22. Nov 13, 2007Aharon Levy - Oxford seminar22  (Q 2 +M 2 ) - VM

23. Nov 13, 2007Aharon Levy - Oxford seminar23  (Q 2 ) Fit to whole Q 2 range gives bad  2 /df (~70) VMncomments ρ2.44±0.09Q 2 >10 GeV 2  2.75±0.13 ±0.07 Q 2 >10 GeV 2 J/  2.486±0.080 ±0.068 All Q 2  1.54±0.09 ±0.04 Q 2 >3 GeV 2 101 Q 2 (GeV 2 )

24. Nov 13, 2007Aharon Levy - Oxford seminar24 b(Q 2 ) – ρ 0,  Fit:

25. Nov 13, 2007Aharon Levy - Oxford seminar25 b(Q 2 +M 2 ) - VM ‘hard’ gg

26. Nov 13, 2007Aharon Levy - Oxford seminar26 Frankfurt - Strikman Kornelija Passek-Kumaricki - EDS07 DVCS

27. Nov 13, 2007Aharon Levy - Oxford seminar27 Information on  L and  T Use  0 decay angular distribution to get r 04 00 density matrix element  - ratio of longitudinal- to transverse- photon fluxes ( = 0.996) using SCHC

28. Nov 13, 2007Aharon Levy - Oxford seminar28 R=  L /  T (Q 2 ) When r 00 04 close to 1, error on R large and asymmetric  advantageous to use r 00 04 rather than R.

29. Nov 13, 2007Aharon Levy - Oxford seminar29 Photon configuration - sizes small k T large k T large config. small config.  T : large size small size strong color forces color screening large cross section small cross section  *:  * T,  * L  * T – both sizes,  * L – small size Light VM: transverse size of ~ size of proton Heavy VM: size small  cross section much smaller (color transparency) but due to small size (scale given by mass of VM) ‘see’ gluons in the proton   ~ (xg) 2  large 

30. Nov 13, 2007Aharon Levy - Oxford seminar30  L /  tot (W)   L and  T same W dependence  L  in small configuration  T  in small and large configurations small configuration  steep W dep large configuration  slow W dep  large configuration is suppressed

31. Nov 13, 2007Aharon Levy - Oxford seminar31  L /  tot (t)  size of  * L   * T  large configuration suppressed

32. Nov 13, 2007Aharon Levy - Oxford seminar32  (W) - DVCS Final state  is real   T using SCHC  initial  * is  * T but W dep of  steep  large  * T configurations suppressed

33. Nov 13, 2007Aharon Levy - Oxford seminar33 Effective Pomeron trajectory ρ 0 photoproduction Get effective Pomeron trajectory from d  /dt(W) at fixed t Regge:

34. Nov 13, 2007Aharon Levy - Oxford seminar34 Effective Pomeron trajectory ρ 0 electroproduction

35. Nov 13, 2007Aharon Levy - Oxford seminar35 Comparison to theory All theories use dipole picture Use QED for photon wave function Use models for VM wave function – usually take a Gaussian shape Use gluon density in the proton Some use saturation model, others take sum of nonperturbative + pQCD calculation, and some just start at higher Q 2 Most work in configuration space, MRT works in momentum space. Configuration space – puts emphasis on VM wave function. Momentum space – on the gluon distribution. W dependence – information on the gluon Q 2 and R – properties of the wave function

36. Nov 13, 2007Aharon Levy - Oxford seminar36 ρ 0 data - Comparison to theory Martin-Ryskin-Teubner (MRT) – work in momentum space, use parton-hadron duality, put emphasis on gluon density determination. Phys. Rev. D 62, 014022 (2000). Forshaw-Sandapen-Shaw (FSS) – improved understanding of VM wf. Try Gaussian and DGKP (2- dim Gaussian with light-cone variables). Phys. Rev. D 69, 094013 (2004). Kowalski-Motyka-Watt (KMW) – add impact parameter dependence, Q 2 evolution – DGLAP. Phys. Rev. D 74, 074016 (2006). Dosch-Ferreira (DF) – focusing on the dipole cross section using Wilson loops. Use soft+hard Pomeron for an effective evolution. Eur. Phys. J. C 51, 83 (2007).

37. Nov 13, 2007Aharon Levy - Oxford seminar37 Q2Q2 KMW – good for Q 2 >2GeV 2 miss Q 2 =0 DF – miss most Q 2 FSS – Gauss better than DGKP

38. Nov 13, 2007Aharon Levy - Oxford seminar38 Q2Q2 Data seem to prefer MRST99 and CTEQ6.5M

39. Nov 13, 2007Aharon Levy - Oxford seminar39 W dependence KMW - close FSS: Sat-Gauss – right W-dep. wrong norm. MRT: CTEQ6.5M – slightly better in W-dep.

40. Nov 13, 2007Aharon Levy - Oxford seminar40  L /  tot (Q 2 )

41. Nov 13, 2007Aharon Levy - Oxford seminar41  L /  tot (W) All models have mild W dependence. None describes all kinematic regions.

42. Nov 13, 2007Aharon Levy - Oxford seminar42 Summary and conclusions HERA data shows transition from soft to hard interactions. The cross section is rising with W and its logarithmic derivative in W, , increases with Q 2. The exponential slope of the t distribution decreases with Q 2 and levels off at about b = 5 GeV -2. Transverse size of gluon density (0.6 fm) inside the charge radius of the proton (0.8 fm). The ratio of cross sections induced by longitudinally and transversely polarised virtual photons increases with Q 2, but is independent of W and t. The large configurations of the transversely polarised photon are suppressed. The effective Pomeron trajectory has a larger intercept and smaller slope than those extracted from soft interactions. All these features are compatible with expectations of perturbative QCD. None of the models which have been compared to the measurements are able to reproduce all the features of the data. Precision measurements of exclusive vector meson electroproduction can help determine the gluon density in the proton.