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Recent Highlights of Physics on the Nucleon with CLAS

Recent Highlights of Physics on the Nucleon with CLAS. Volker D. Burkert Jefferson Lab. NSTAR 2007 September 5, 2007, Bonn, Germany. The CLAS Collaboration. Rensselaer Polytechnic Institute, Troy, NY Rice University, Houston, TX University of Richmond, Richmond, VA

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Recent Highlights of Physics on the Nucleon with CLAS

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  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 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 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

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

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

  5. D13(1520) S11(1535) Missing States? P33(1232) e’ π, η, ππ γv e N*,△* N N’ N A1/2, A3/2, S1/2 SU(6)xO(3) Classification of Baryons P11(1440)

  6. bare vertex dressed vertex pion cloud Meson contributions play significant role even at fairly high Q2. * γ*NΔ - Transition Form Factors – GM

  7. NΔ Multipole Ratios REM, RSM in 2007 • Precise multipole ratios: dREM, dRSM < 0.5.- 2% • REM remains small and negative at -2% to -3.5% from 0 ≤ Q2≤ 6 GeV2. No trend towards asymptotic behavior. Helicity conservation - REM→+100 (%). • RSM negative and increase in magnitude. Helicity conservation – RSM→ constant • Dynamical models allow description of multipole ratios in large Q2 range. • REM < 0 favors oblate shape of D and prolate shape of the proton at large distances.

  8. CLAS Transition to the 2nd Resonance Region Poorly understood in nrCQMs. Other models: - Light front kinematics (relativity) - Hybrid baryon with gluonic excitation |q3G> - Quark core with large meson cloud |q3m> - Nucleon-sigma molecule |Nσ> - Dynamically generated resonance P11(1440) S11(1535) Hard form factor (slow fall off with Q2) Not a quark resonance, but KΣ dynamical system? Change of helicity structure with increasing Q2 from λ=3/2 dominance to λ=1/2 dominance, predicted in nrCQMs, pQCD. D13(1520) Measure Q2 dependence of Transition F.F.

  9. CLAS P11(1440) CQM Comparison @ low Q2 nrCQM nrCQM LC CQM LC CQM • Non-relativistic CQ Models do not reproduce sign of A1/2 at Q2=0, and show no zero-crossing. • Relativistic CQ Models (LC) give correct sign and show zero-crossing but miss strength at Q2=0. → go to higher Q2 to reduce effects of meson contributions.

  10. Talk in Session 5 Talk by V. Mokeev CLAS P11(1440) Transition FF @ high Q2 • Analysis with • Unitary Isobar Model (UIM) • Fixed-t Dispersion Relations (DR) Nπ, pπ+π- nπ+ DR UIM nπ+ pπ0 pπ0 Include > 35,000 data points in fits. PDG

  11. New CLAS results CLAS 2007 pπ0 nπ+ CLAS 2002 previous results pη nπ+ pπ0 pη PDG 2006 CQM CLAS S11(1535) in pη and Nπ preliminary PDG (2006): S11→πN (35-55)% → ηN (45-60)% A1/2 from pη and Nπ are consistent

  12. PDG Nπ, pπ+π- nπ+ - nπ+ pπ0 pπ0 CLAS Transition γ*pD13(1520) A3/2 Previous pπ0 based data preliminary preliminary A1/2 Q2, GeV2 Q2, GeV2 nrQM:

  13. A21/2 – A23/2 Ahel = A21/2 + A23/2 Ahel D13(1520) CLAS Helicity Asymmetry for γ*pD13 CQMs and pQCD Ahel→ +1 at Q2→∞ Helicity structure of transition changes rapidly with Q2 from helicity 3/2 (Ahel= -1) to helicity 1/2 (Ahel= +1) dominance!

  14. Talk by W. Briscoe CLAS New Results in γp→pπ0 FA06 solution of SAID analysis • A1/2 from Nπ analysis for • S11(1535) now agrees with Nη results as was found earlier in electro-production. • Strong excitation of P13(1720) is consistent with earlier analysis of pπ+π- electro-couplings.

  15. Search for CQM predicted states. Talk by M. Bellis CLAS • 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, K0Y 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.

  16. P11 K exchange P13 CLAS Photoproduction of K+Λ, K+Σ0 P13 Fit: Bonn-Gatchina group, Anisovich et al., 2007

  17. Quark-Diquark Model (E. Santopinto, 2005) Includes *** / **** states Talk by R. Schumacher Talk by A. Sarantsev CLAS γp—>K+Λ Polarization transfer w/o P13(1900) with P13(1900) Coupled channel fit: Bonn-Gatchina group, Anisovich et al, 2007 Fit shows strong preference for second P13 state. Existence of this state would be evidence against the quark-diquark model.

  18. CLAS 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 Ξ(1320) Possible production mechanism Ξ(1530) Missing mass MM(K+K+) works for narrow states, but higher energy and higher statistics are needed.

  19. CLAS Search in γp―>π-K+K+Ξ* Ξ(1530) 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 Ξπstate. It would contradict quark models. Requires more statistics and PWA.

  20. CLAS Experiment Status & Plans of Search for New N* States

  21. Eγ=1.5 – 1.7 Ks Λ M(π+π-), GeV M(pπ-), GeV CLAS Eγ=1.1-1.3 GeV All polar angles γd→K0Λ, π-p, (ps) < 0.1% of all data Photons produced coherently from aligned diamond crystals are linearly polarized. Online beam asymmetry for γn→π-p Identify: Ks→π+π- Λ→pπ- • 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 →K0Λ → → → Projected Accuracy of Data (4 of over 100 bins)

  24. CLAS 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 g1(Q2, x) in the low and moderate Q2 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.

  25. Spin structure function g1p 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. World data on polarized structure function g1(x,Q2) CLAS provides a large body of precise g1 data that is being used to improve our knowledge of twist-2 PDFs.

  26. xΔG errors xΔs errors CLAS Impact on PDFs 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, At xB=0.4, the relative uncertainty of xΔG is reduced by a factor 3.

  27. CLAS Proton Integral G1 = g1(x,Q2)dx Shows expected trend toward DIS result at high Q2 At low Q2 we observe a negative slope as expected from GDH Sum Rule. Agreement with PT at the lowest points. Low Q2 fit to data: Ji predicts b = 3.89 Fit: b = 3.810.31 (stat) +0.44 - 0.57 (syst)

  28. CLAS Bjorken Sum: Γ1p-n(Q2) Agreement with PT up to Q2 = 0.25 GeV2. NNLO PQCD in reasonable agreement with the data  Higher twists are small even down to Q2 = 0.75 GeV2

  29. Target Beam-target Data will help improve analysis of 2nd and 3rd resonance regions at low Q2. CLAS Integrated Asymmetries in ep→epπ0

  30. Physical content of GPDs The nucleon matrix element of the fundamental Energy-Momentum Tensor contains 3 form factors. M2(t) : Mass distribution inside the nucleon in transverse space J(t) : Angular momentum distribution d1(t) : Forces and pressure distribution These form factors are related to GPDs through 2nd moments! Separate through ξdependence. 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.

  31. CLAS DVCS/BH Beam Spin Asymmetry Large kinematics coverage BSA mostly sensitive to GPD H Fully integrated asymmetry and one of 65 bins in Q2, x=ξ, t. Fit: ALU = asinf/(1 + bcosf)

  32. VGG Model (Vanderhaeghen, Guichon, Guidal) CLAS Comparison with GPD model t-dependence of leading twist terma (sinΦ). VGG parameterization reproduces –t > 0.5GeV2 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.

  33. CLAS 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)

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