PowerPoint Presentation - NSTAR 2007

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

 Introduction

 Resonance transition form factors

 Search for new baryon states (non-exotic)

 Nucleon spin structure in the transition region

 Generalized Parton Distributions

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

          high                               e.m. probe
SU(6)xO(3) Classification of Baryons



                                                                              π, η, ππ
                                    e            γv       N*,△*

                                             N                           N’

                                                      A1/2, A3/2, S1/2
  γ*NΔ - Transition Form Factors – G*M




Meson contributions play
significant role even at
fairly high Q2.
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
CLAS Transition to the 2nd Resonance Region
 P11(1440)   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

 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
D13(1520)    dominance to λ=1/2 dominance, predicted in nrCQMs, pQCD.

                     Measure Q2 dependence of Transition F.F.
CLAS         P11(1440) CQM Comparison @ low Q2


                     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.
CLAS            P11(1440) Transition FF @ high Q2
 Analysis with
                                             Nπ, pπ+π-        DR     UIM nπ+
 1) Unitary Isobar Model (UIM)
 2) Fixed-t Dispersion Relations (DR)       nπ+ pπ0            pπ0

                                        Include > 35,000 data points in fits.

                                                              Talk in Session 5

                                                              Talk by V. Mokeev
CLAS            S11(1535) in pη and Nπ
                                             New CLAS results
            CLAS 2007                          nπ+              pπ0
            CLAS 2002
            previous results                   nπ+ pπ0          pη


                                         PDG 2006

                                     PDG (2006): S11→πN (35-55)%
A1/2 from pη and Nπ are consistent
                                                    → ηN (45-60)%
CLAS     Transition γ*pD13(1520)

                    Previous pπ0
                    based data


         Q2, GeV2                              Q2, GeV2

         -                         Nπ, pπ+π-   nπ+
                                   nπ+ pπ0     pπ0
CLAS Helicity Asymmetry for γ*pD13

               A21/2 – A23/2
    Ahel =
               A21/2 + A23/2

 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)
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

                          Strong excitation of
                         P13(1720) is consistent
                         with earlier analysis of
                         pπ+π- electro-

                                      Talk by
                                      W. Briscoe
CLAS      Search for CQM predicted states.

   – 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
                                       Talk by M. Bellis
CLAS   Photoproduction of K+Λ, K+Σ0
                   P11          P13          K exchange


         Fit: Bonn-Gatchina group, Anisovich et al., 2007
CLAS             γp—>K+Λ Polarization transfer
 w/o P13(1900)                with P13(1900)
                                                       Quark-Diquark Model
                                                       (E. Santopinto, 2005)

                                                      Includes *** / **** states

        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.
                 Talk by R. Schumacher         Talk by A. Sarantsev
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
    Possible production mechanism


Missing mass MM(K+K+) works for narrow
states, but higher energy and higher statistics
are needed.
CLAS   Search in γp―>π-K+K+Ξ*


            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.
                   Experiment Status & Plans of Search for New N* States
Reaction          Diff   Lin.   Circ.   Long.    Trans.   Recoil   Group     Publication/Status/Schedule
                  crs    beam   beam    Target   Targe
γp→pπ0             x                                                 G1      arXiv:0705.0816
γp→nπ+             x                                                 G1      analysis
γp→pη              x                                               G1, G10   PRL89, 222002, 2002
γp→pη’             x                                               G1, G10   PRL96, 062001, 2006
γp→K+Λ, K+Σ        x              x                         x      G1, G10   PRC69 042201, 2004; PRC73,
                                                                             035202, 2006; PRC75 035205, 2007
γp→K0*Σ+           x                                                 G1      PRC75 042201, 2007
γp→pπ-π+           x              x                                  G1      PRL95 162003, 2006, analysis
γp→pω, pρ0, nρ+    x      x                                          G8      2005 / Analysis
γn→ K0Λ, K0Σ,      x      x       x                         x        G13     2007 / Analysis
K+Σ-, K-Σ+
γp→pπ0, nπ+, pη           x       x       x        x                 G9-     2007/2009
γp→K+Λ, K+Σ               x       x       x        x         x       G9-     2007/2009
γp→pπ-π+                  x       x       x        x                 G9-     2007/2009
γn→ K0Λ, K0Σ,             x       x       x        x         x     G14-HD    2010
K+Σ-, K-Σ+
γn→pπ-,nπ+π-              x       x       x        x               G14-HD    2010
CLAS                                  γ d→K0Λ, π-p, (ps)

                                                                   Eγ=1.1-1.3 GeV
                                                                   All polar angles
                 Eγ=1.5 – 1.7

                                                                             < 0.1%
                                                                             of all data

Photons produced coherently from aligned           Online beam asymmetry
diamond crystals are linearly polarized.           for γn→π-p

      Ks               Identify:           Λ
                       Ks →π+π-
                                                           •   Plots show a 5 GeV run
                                                               with the coherent edge
                                                               at 1.9 GeV

       M(π+π-), GeV                        M(pπ-), GeV
            →→     +→
            γp   →K Λ
Projected Accuracy of Data (4 of over 100 bins)
               γn   →K Λ
               →→     0→

Projected Accuracy of Data (4 of over 100 bins)
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.
        World data on polarized structure
                function g1(x,Q2)
Spin structure function g1p have
been measured for the past 30

Accuracy and coverage is much
poorer than for spin-averaged
structure function F2p.

Consequently, the polarized parton
distribution functions have still
large uncertainties.

 CLAS provides a large
 body of precise g1 data that is
 being used to improve our
 knowledge of twist-2 PDFs.
CLAS            Impact on PDFs
                                The dashed lines include the CLAS
                                data in the analysis (LSS’06).
                                E. Leader, A. Sidorov, D. Stamenov,

   xΔs errors      xΔG errors                 The CLAS data do
                                              not change the
                                              average values of
                                              PDFs, but reduce
                                              their uncertainties

                                             At xB=0.4, the relative
                                             uncertainty of xΔG is
                                             reduced by a factor 3.
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
                         Agreement with PT at
                         the lowest points.
                         Low Q2 fit to data:
                        G1          2
                                        Q2  bQ4  cQ6  dQ8
                         Ji predicts b = 3.89
                         Fit: b = 3.810.31 (stat)
                                 +0.44 - 0.57 (syst)
CLAS   Bjorken Sum: Γ1p-n(Q2)

                      G1p  G1  A  Q 2 evolution
                              n g

                       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
CLAS Integrated Asymmetries in ep→epπ0
                Target                                      Beam-target

   Data will help improve analysis of 2nd and 3rd resonance regions at low Q2.
            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 ξ dependence.

  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.
CLAS DVCS/BH Beam Spin Asymmetry
 BSA mostly sensitive to GPD H      Large kinematics coverage

   Fully integrated asymmetry and
   one of 65 bins in Q2, x=ξ, t.          Fit: ALU = asinf/(1  bcosf)
CLAS                     Comparison with GPD model

t-dependence of leading
twist term a (sinΦ).

          VGG Model
(Vanderhaeghen, Guichon, Guidal)

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.
CLAS                    Summary
 • CLAS is making major contributions to many areas of hadron

 • 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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