Recent Highlights of Physics on the Nucleon with CLAS

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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
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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
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Outline

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

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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
p
low
N
high
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SU(6)xO(3) Classification of Baryons
P11(1440)
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?N? - Transition Form Factors GM

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N? Multipole Ratios REM, RSM in 2007

• Precise multipole ratios
• dREM, dRSM lt 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 lt 0 favors oblate shape of D and prolate
  shape of the proton at large distances.

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Transition to the 2nd Resonance Region
CLAS
Poorly understood in nrCQMs. Other models -
Light front kinematics (relativity) - Hybrid
baryon with gluonic excitation q3Ggt - Quark
core with large meson cloud q3mgt -
Nucleon-sigma molecule Nsgt - Dynamically
generated resonance
P11(1440)
S11(1535)
Hard form factor (slow fall off with Q2) Not a
quark resonance, but KS dynamical system?
Change of helicity structure with increasing Q2
from ?3/2 dominance to ?1/2 dominance,
predicted in nrCQMs, pQCD.
D13(1520)
Measure Q2 dependence of Transition F.F.
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P11(1440) CQM Comparison _at_ low Q2
CLAS
nrCQM
nrCQM
LC CQM
LC CQM

• Non-relativistic CQ Models do not reproduce sign
  of A1/2 at Q20, and show
• no zero-crossing.
• Relativistic CQ Models (LC) give correct sign
  and show zero-crossing but
• miss strength at Q20.

? go to higher Q2 to reduce effects of meson
contributions.
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P11(1440) Transition FF _at_ high Q2
CLAS

• Analysis with
• Unitary Isobar Model (UIM)
• Fixed-t Dispersion Relations (DR)

Np, ppp-
np
DR
UIM
pp0
np
pp0
Include gt 35,000 data points in fits.
PDG
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S11(1535) in p? and Np
CLAS
preliminary
PDG (2006) S11?pN (35-55) ? ?N
(45-60)
A1/2 from p? and Np are consistent
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Transition ?pD13(1520)
CLAS
A3/2
Previous pp0 based data
preliminary
preliminary
A1/2
Q2, GeV2
Q2, GeV2
nrQM
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Helicity Asymmetry for ?pD13
CLAS
CQMs and pQCD Ahel ? 1 at Q2?8
Helicity structure of transition changes rapidly
with Q2 from helicity 3/2 (Ahel -1) to helicity
1/2 (Ahel 1) dominance!
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New Results in ?p?pp0
CLAS
FA06 solution of SAID analysis

• A1/2 from Np analysis for
• S11(1535) now agrees with N? results as was found
  earlier in electro-production.
• Strong excitation of P13(1720) is consistent
  with earlier analysis of ppp- electro-couplings.

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Search for CQM predicted states.
CLAS

• To reduce ambiguities, the search for new excited
  states aims at complete or nearly complete
  measurements in ?p?pN, ?N, KY and ?n?pN, 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 ?p ? ?N, ?p, ppN with
  linearly polarized beams, and with polarized beam
  and polarized targets.

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Photoproduction of K?, KS0
CLAS
P13
Fit Bonn-Gatchina group, Anisovich et al., 2007
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?pgtK? Polarization transfer
CLAS
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.
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Excited Cascades ?
CLAS

• Advantage over search for Ns and Ys is narrow
  widths of ?s
• Possible production mechanism through decay of
  excited hyperons requires large acceptance and
  high luminosity experiments

?(1320)
Possible production mechanism
?(1530)
Missing mass MM(KK) works for narrow states,
but higher energy and higher statistics are
needed.
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Search in ?p?gtp-KK?
CLAS
?(1530)
A ? 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 ?p state.
It would contradict quark models. Requires more
statistics and PWA.
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CLAS
Experiment Status Plans of Search for New N
States
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? d?K0?, p-p, (ps)
CLAS
E?1.1-1.3 GeV All polar angles
lt 0.1 of all data
Photons produced coherently from aligned diamond
crystals are linearly polarized.
Online beam asymmetry for ?n?p-p
Identify Ks ?pp- ??pp-

• Plots show a 5 GeV run with the coherent edge at
  1.9 GeV

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?p ?K?
?
?
?
Projected Accuracy of Data (4 of over 100 bins)
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?n ?K0?
?
?
?
Projected Accuracy of Data (4 of over 100 bins)
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Spin structure of the nucleon in the transition
regime
CLAS

• 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.

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World data on polarized structure function
g1(x,Q2)
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.
CLAS provides a large body of precise g1 data
that is being used to improve our knowledge of
twist-2 PDFs.
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Impact on PDFs
CLAS
The dashed lines include the CLAS data in the
analysis (LSS06). E. Leader, A. Sidorov, D.
Stamenov, Phys.Rev.D75074027,2007.
The CLAS data do not change the average values of
PDFs, but reduce their uncertainties
significantly,
At xB0.4, the relative uncertainty of x?G is
reduced by a factor 3.
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Proton Integral G1 ?g1(x,Q2)dx
CLAS
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)
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Bjorken Sum G1p-n(Q2)
CLAS
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
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Integrated Asymmetries in ep?epp0
CLAS
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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 ? 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.
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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?, t.
Fit ALU asinf/(1 bcosf)
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CLAS
Comparison with GPD model
t-dependence of leading twist term a (sinF).

• VGG parameterization reproduces t gt 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.

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Summary
CLAS

• 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 Lees 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)