1. Recent Highlights of Physics on the Nucleon with CLAS Volker D. Burkert Jefferson Lab NSTAR 2007 September 5, 2007, Bonn, Germany

2. The CLAS Collaboration Idaho State University, Pocatello, Idaho INFN, Laboratori Nazionali di Frascati, Frascati, Italy INFN, Sezione di Genova, Genova, Italy Institut de Physique Nucléaire, Orsay, France ITEP, Moscow, Russia James Madison University, Harrisonburg, VA Kyungpook University, Daegu, South Korea University of Massachusetts, Amherst, MA Moscow State University, Moscow, Russia University of New Hampshire, Durham, NH Norfolk State University, Norfolk, VA Ohio University, Athens, OH Old Dominion University, Norfolk, VA Rensselaer Polytechnic Institute, Troy, NY Rice University, Houston, TX University of Richmond, Richmond, VA University of South Carolina, Columbia, SC Thomas Jefferson National Accelerator Facility, Newport News, VA Union College, Schenectady, NY Virginia Polytechnic Institute, Blacksburg, VA University of Virginia, Charlottesville, VA College of William and Mary, Williamsburg, VA Yerevan Institute of Physics, Yerevan, Armenia Brazil, Germany, Morocco and Ukraine, as well as other institutions in France and in the USA, have individuals or groups involved with CLAS, but with no formal collaboration at this stage. Arizona State University, Tempe, AZ University of California, Los Angeles, CA California State University, Dominguez Hills, CA Carnegie Mellon University, Pittsburgh, PA Catholic University of America CEA-Saclay, Gif-sur-Yvette, France Christopher Newport University, Newport News, VA University of Connecticut, Storrs, CT Edinburgh University, Edinburgh, UK Florida International University, Miami, FL Florida State University, Tallahassee, FL George Washington University, Washington, DC University of Glasgow, Glasgow, UK

3.  Introduction  Resonance transition form factors  Search for new baryon states (non-exotic)  Nucleon spin structure in the transition region  Generalized Parton Distributions  Conclusions Outline

4. Hadron Structure with e.m. Probes? resolution of probe low high N π Allows to address central question: What are the relevant degrees-of-freedom at varying distance scale? q e.m. probe LQCD/DSE quark mass (GeV)

5. D 13 (1520) S 11 (1535) P 33 (1232) SU(6)xO(3) Classification of Baryons Missing States? P 11 (1440) e e’ γvγv N N’N’ N*,△* A 1/2, A 3/2, S 1/2 π, η, ππ N

6. γ*NΔ - Transition Form Factors – G M Meson contributions play significant role even at fairly high Q 2. bare vertex dressed vertex pion cloud *

7.  Precise multipole ratios:  R EM,  R SM < 0.5.- 2%  R EM remains small and negative at -2% to -3.5% from 0 ≤ Q 2 ≤ 6 GeV 2. No trend towards asymptotic behavior. Helicity conservation - R EM → +100 (%).  R SM negative and increase in magnitude. Helicity conservation – R SM → constant  Dynamical models allow description of multipole ratios in large Q 2 range.  R EM < 0 favors oblate shape of  and prolate shape of the proton at large distances. NΔ Multipole Ratios R EM, R SM in 2007

8. Transition to the 2 nd Resonance Region CLAS P 11 (1440) Poorly understood in nrCQMs. Other models: - Light front kinematics (relativity) - Hybrid baryon with gluonic excitation |q 3 G> - Quark core with large meson cloud |q 3 m> - Nucleon-sigma molecule |Nσ> - Dynamically generated resonance S 11 (1535) Hard form factor (slow fall off with Q 2 ) Not a quark resonance, but KΣ dynamical system? D 13 (1520) Change of helicity structure with increasing Q 2 from λ=3/2 dominance to λ=1/2 dominance, predicted in nrCQMs, pQCD. Measure Q 2 dependence of Transition F.F.

9. P 11 (1440) CQM Comparison @ low Q 2 CLAS  Non-relativistic CQ Models do not reproduce sign of A 1/2 at Q 2 =0, and show no zero-crossing.  Relativistic CQ Models (LC) give correct sign and show zero-crossing but miss strength at Q 2 =0. LC CQM nrCQM → go to higher Q 2 to reduce effects of meson contributions.

10. P 11 (1440) Transition FF @ high Q 2 CLAS pπ0pπ0 nπ+nπ+ Nπ, pπ + π - nπ+nπ+ pπ0pπ0 DR UIM Analysis with 1)Unitary Isobar Model (UIM) 2)Fixed-t Dispersion Relations (DR) Talk in Session 5 PDG Include > 35,000 data points in fits. Talk by V. Mokeev

11. S 11 (1535) in pη and Nπ pπ0pπ0 nπ+nπ+ pπ0pπ0 nπ+nπ+ pηpη New CLAS results CLAS preliminary pηpη CLAS 2007 CLAS 2002 previous results CQM A 1/2 from pη and Nπ are consistent PDG 2006 PDG (2006): S 11 → πN (35-55)% → ηN (45-60)%

12. Transition γ*pD 13 (1520) A 1/2 A 3/2 Q 2, GeV 2 CLAS Previous pπ 0 based data pπ0pπ0 nπ+nπ+ Nπ, pπ + π - nπ+nπ+ pπ0pπ0 preliminary PDG - nrQM:

13. Helicity Asymmetry for γ*pD 13 CQMs and pQCD A hel → +1 at Q 2 →∞ A hel = A 2 1/2 – A 2 3/2 A 2 1/2 + A 2 3/2 D 13 (1520) A hel CLAS Helicity structure of transition changes rapidly with Q 2 from helicity 3/2 (A hel = -1) to helicity 1/2 (A hel = +1) dominance!

14. New Results in γp → pπ 0 CLAS FA06 solution of SAID analysis  A 1/2 from Nπ analysis for S 11 (1535) now agrees with Nη results as was found earlier in electro-production.  Strong excitation of P 13 (1720) is consistent with earlier analysis of pπ + π - electro- couplings. Talk by W. Briscoe

15. –To reduce ambiguities, the search for new excited states aims at “complete” or nearly complete measurements in γp → πN, ηN, K + Y and γn → πN, K 0 Y and using combinations of beam, target, and recoil polarizations: differential cross sections with unpolarized, circularly polarized, and linearly polarized photon beams, recoil polarizations for hyperons, longitudinally or transversely polarized proton and neutron (deuteron) targets. –Other reactions include γp → ρN, ωp, ππN with linearly polarized beams, and with polarized beam and polarized targets. Search for CQM predicted states. CLAS Talk by M. Bellis

16. Photoproduction of K + Λ, K + Σ 0 Fit: Bonn-Gatchina group, Anisovich et al., 2007 CLAS P13 P 11 K exchange

17. γp—>K + Λ Polarization transfer w/o P 13 (1900) with P 13 (1900) CLAS Fit shows strong preference for second P 13 state. Existence of this state would be evidence against the quark-diquark model. Includes *** / **** states (E. Santopinto, 2005) Quark-Diquark Model Coupled channel fit: Bonn-Gatchina group, Anisovich et al, 2007 Talk by R. SchumacherTalk by A. Sarantsev

18. Excited Cascades Ξ* Advantage over search for N*’s and Y*’s is narrow widths of Ξ’s Possible production mechanism through decay of excited hyperons – requires large acceptance and high luminosity experiments CLAS Ξ(1320) Ξ(1530) Missing mass MM(K + K + ) works for narrow states, but higher energy and higher statistics are needed. Possible production mechanism

19. Search in γp―>π - K + K + Ξ* CLAS A Ξ state at 1.62GeV and 50 MeV width could be the 1* candidate in PDG. Such a state would be consistent with a dynamically generated Ξπ state. It would contradict quark models. Requires more statistics and PWA. Ξ(1530)

20. ReactionDiff crs Lin. beam Circ. beam Long. Target Trans. Targe t RecoilGroupPublication/Status/Schedule γp→pπ 0 xG1arXiv:0705.0816 γp→nπ + xG1analysis γp→pη xG1, G10PRL89, 222002, 2002 γp→pη ’ xG1, G10PRL96, 062001, 2006 γp→K + Λ, K + Σ xxxG1, G10PRC69 042201, 2004; PRC73, 035202, 2006; PRC75 035205, 2007 γp→K 0* Σ + x G1PRC75 042201, 2007 γp→p π - π + xxG1PRL95 162003, 2006, analysis γp→pω, pρ 0, nρ + xxG82005 / Analysis γn → K 0 Λ, K 0 Σ, K + Σ-, K - Σ + xxxxG132007 / Analysis γp→p π 0, nπ +, pη xxxxG9- FROST 2007/2009 γp→ K+Λ, K+Σ xxxx xG9- FROST 2007/2009 γp→p π - π + xxxx G9- FROST 2007/2009 γn → K 0 Λ, K 0 Σ, K + Σ -, K - Σ + xxxx xG14-HD2010 γn →p π -,nπ + π - xx xx G14-HD2010 Experiment Status & Plans of Search for New N* States CLAS

21.  γ  d → K 0 Λ, π - p, (p s ) Eγ=1.5 – 1.7 Online beam asymmetry for γn → π - p Photons produced coherently from aligned diamond crystals are linearly polarized. Identify: K s → π + π - Λ → pπ - Eγ=1.1-1.3 GeV All polar angles < 0.1% of all data KsKs M(π + π - ), GeV Λ M(pπ - ), GeV Plots show a 5 GeV run with the coherent edge at 1.9 GeV

22. γp →K + Λ Projected Accuracy of Data (4 of over 100 bins) →→→

23. γn → K 0 Λ Projected Accuracy of Data (4 of over 100 bins) →→→

24. Spin structure of the nucleon in the transition regime For the first time information on multi-parton interactions (higher twist) was obtained by precise measurements of g 1 (Q 2, x) in the low and moderate Q 2 regime. By isolating higher twist from the leading twist-2 term, CLAS data can be used to provide precise constraints on the twist-2 quark and gluon spin distribution functions. CLAS

25. World data on polarized structure function g 1 (x,Q 2 ) CLAS provides a large body of precise g 1 data that is being used to improve our knowledge of twist-2 PDFs. Spin structure function g 1p have been measured for the past 30 years. Accuracy and coverage is much poorer than for spin-averaged structure function F2p. Consequently, the polarized parton distribution functions have still large uncertainties.

26. Impact on PDFs At x B =0.4, the relative uncertainty of xΔG is reduced by a factor 3. C LAS The dashed lines include the CLAS data in the analysis (LSS’06). E. Leader, A. Sidorov, D. Stamenov, Phys.Rev.D75:074027,2007. The CLAS data do not change the average values of PDFs, but reduce their uncertainties significantly, xΔG errors xΔs errors

27. Proton Integral  1 =  g 1 (x,Q 2 )dx Shows expected trend toward DIS result at high Q 2 At low Q 2 we observe a negative slope as expected from GDH Sum Rule. Agreement with  PT at the lowest points. Low Q 2 fit to data: Ji predicts b = 3.89 Fit: b = 3.81  0.31 (stat) +0.44 - 0.57 (syst) CLAS

28. Agreement with  PT up to Q 2 = 0.25 GeV 2. NNLO PQCD in reasonable agreement with the data  Higher twists are small even down to Q 2 = 0.75 GeV 2 Bjorken Sum: Γ 1 p-n (Q 2 ) CLAS

29. Integrated Asymmetries in ep → epπ 0 CLAS Beam-target Target Data will help improve analysis of 2 nd and 3 rd resonance regions at low Q 2.

30. Physical content of GPDs M 2 (t) : Mass distribution inside the nucleon in transverse space J(t) : Angular momentum distribution d 1 (t) : Forces and pressure distribution The nucleon matrix element of the fundamental Energy-Momentum Tensor contains 3 form factors. These form factors are related to GPDs through 2 nd moments! If we can determine these form factors through the GPDs, we explore the spatial distribution of quark angular momentum, the quark mass distribution, and the distribution of pressure and forces on the quarks in the nucleon. Separate through ξ dependence.

31. Fit: A LU =  sin  cos  Fully integrated asymmetry and one of 65 bins in Q 2, x=ξ, t. DVCS/BH Beam Spin Asymmetry Large kinematics coverage CLAS BSA mostly sensitive to GPD H

32. t-dependence of leading twist term a (sinΦ). Comparison with GPD model VGG parameterization reproduces –t > 0.5GeV 2 behavior, but over- estimates asymmetry at small t. The latter could indicate that VGG misses some important contributions to the DVCS cross section that enters in the denominator. VGG Model (Vanderhaeghen, Guichon, Guidal) CLAS

33. Summary CLAS is making major contributions to many areas of hadron physics Major focus is N* physics –the search for new baryon state and determination of properties –resonance transition form factors –theory support from EBAC (see: Harry Lee’s talk) Spin structure of the nucleon Deeply exclusive processes and GPDs Properties of hadrons and quarks in nuclei, and using the nucleus as a laboratory (not discussed) CLAS