Recent Highlights of Physics on Recent Highlights of Physics on the Nucleon with CLASthe Nucleon with CLAS

Volker D. Burkert

Jefferson Lab

NSTAR 2007

September 5, 2007, Bonn, Germany

The CLAS Collaboration

Idaho State University, Pocatello, IdahoINFN, Laboratori Nazionali di Frascati, Frascati, Italy

INFN, Sezione di Genova, Genova, ItalyInstitut de Physique Nucléaire, Orsay, France

ITEP, Moscow, RussiaJames Madison University, Harrisonburg, VAKyungpook University, Daegu, South KoreaUniversity of Massachusetts, Amherst, MA Moscow State University, Moscow, RussiaUniversity of New Hampshire, Durham, NH

Norfolk State University, Norfolk, VAOhio University, Athens, OH

Old Dominion University, Norfolk, VA

Rensselaer Polytechnic Institute, Troy, NYRice University, Houston, TX

University of Richmond, Richmond, VAUniversity of South Carolina, Columbia, SC

Thomas Jefferson National Accelerator Facility, Newport News, VAUnion College, Schenectady, NY

Virginia Polytechnic Institute, Blacksburg, VAUniversity of Virginia, Charlottesville, VA

College of William and Mary, Williamsburg, VAYerevan 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, AZUniversity of California, Los Angeles, CACalifornia State University, Dominguez Hills, CACarnegie Mellon University, Pittsburgh, PACatholic University of AmericaCEA-Saclay, Gif-sur-Yvette, FranceChristopher Newport University, Newport News, VAUniversity of Connecticut, Storrs, CTEdinburgh University, Edinburgh, UKFlorida International University, Miami, FLFlorida State University, Tallahassee, FLGeorge Washington University, Washington, DCUniversity of Glasgow, Glasgow, UK

Introduction

Resonance transition form factors

Search for new baryon states (non-exotic) Nucleon spin structure in the transition region

Generalized Parton Distributions

Conclusions

Outline

Hadron Structure with e.m. Probes?

resolution of probe

low

high

N

πAllows to address central question: What are the relevant degrees-of-freedom at varying distance scale?

q

e.m. probe

LQCD/DSEqu

ark

mas

s (G

eV)

D13(1520)S11(1535)

P33(1232)

SU(6)xO(3) Classification of Baryons

Missing States?

P11(1440)

e

e’

γv

N N’

N*,△*

A1/2, A3/2, S1/2

π, η, ππ

N

γ*NΔ - Transition Form Factors – GM

Meson contributions play significant role even at fairly high Q2.

bare vertex

dressed vertex

pion cloud

*

Precise multipole ratios: REM, RSM < 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 and prolate shape of the proton at large distances.

NΔ Multipole Ratios REM, RSM in 2007

Transition to the 2nd Resonance RegionCLAS

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?

D13(1520)Change of helicity structure with increasing Q2 from λ=3/2 dominance to λ=1/2 dominance, predicted in nrCQMs, pQCD.

Measure Q2 dependence of Transition F.F.

P11(1440) CQM Comparison @ low

Q2 CLAS

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.

LC CQM

LC CQM

nrCQMnrCQM

→ go to higher Q2 to reduce effects of meson contributions.

P11(1440) Transition FF @ high Q2CLAS

pπ0nπ+

Nπ, pπ+π- nπ+

pπ0

DR UIMAnalysis 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

S11(1535) in pη and Nπ

pπ0nπ+

pπ0nπ+

pη

New CLAS results

CLAS

preliminary

pη

CLAS 2007CLAS 2002previous results

CQM

A1/2 from pη and Nπ are consistent

PDG 2006

PDG (2006): S11→πN (35-55)% → ηN (45-60)%

Transition γ*pD13(1520)

A1/2

A3/2

Q2, GeV2 Q2, GeV2

CLAS

Previous pπ0

based data

pπ0nπ+

Nπ, pπ+π- nπ+

pπ0

preliminary

preliminary

PDG

-nrQM:

Helicity Asymmetry for γ*pD13

CQMs and pQCD

Ahel → +1 at Q2→∞

Ahel =A2

1/2 – A23/2

A21/2 + A2

3/2

D13(1520)A

hel

CLAS

Helicity structure of transition changes rapidly with Q2 from helicity 3/2 (Ahel= -1) to helicity 1/2 (Ahel=

+1) dominance!

New Results in γp→pπ0CLASFA06 solutionof 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.

Talk by W. Briscoe

– 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.

Search for CQM predicted states.CLAS

Talk by M. Bellis

Photoproduction of K+Λ, K+Σ0

Fit: Bonn-Gatchina group, Anisovich et al., 2007

CLAS

P13

P13P11 K exchange

γp—>K+Λ Polarization transferw/o P13(1900) with P13(1900)

CLAS

Fit shows strong preference for second P13 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. Schumacher Talk by A. Sarantsev

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

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)

Reaction Diffcrs

Lin. beam

Circ. beam

Long.Target

Trans. Target

Recoil

Group Publication/Status/Schedule

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, K+Σ-, K-Σ+

x x x x G13 2007 / Analysis

γp→pπ0, nπ+, pη

x x x x G9-FROST

2007/2009

γp→K+Λ, K+Σ x x x x x G9-FROST

2007/2009

γp→pπ-π+ x x x x G9-FROST

2007/2009

γn→ K0Λ, K0Σ, K+Σ-, K-Σ+

x x x x x G14-HD 2010

γn→pπ-,nπ+π- x x x x G14-HD 2010

Experiment Status & Plans of Search for New N* StatesCLAS

CLAS γd→K0Λ, π-p, (ps)

Eγ=1.5 – 1.7

Online beam asymmetry for γn→π-p

Photons produced coherently from aligned diamond crystals are linearly polarized.

Identify: Ks →π+π- Λ→pπ-

Eγ=1.1-1.3 GeVAll polar angles

< 0.1% of all data

Ks

M(π+π-), GeV

Λ

M(pπ-), GeV

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

γp →K+ΛProjected Accuracy of Data (4 of over 100 bins)

→→ →

γn →K0Λ Projected Accuracy of Data (4 of over 100 bins)

→→ →

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.

CLAS

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.

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.

Impact on PDFs

At xB=0.4, the relative uncertainty of xΔG is reduced by a factor 3.

CLAS

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 errorsxΔs errors

Proton Integral 1 = 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:

1 2

8M 2Q2 bQ4 cQ6 dQ8

Ji predicts b = 3.89

Fit: b = 3.810.31 (stat) +0.44 - 0.57 (syst)

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

evolution Q2611Anp g

Bjorken Sum: Γ1p-n(Q2)CLAS

Integrated Asymmetries in ep→epπ0CLAS

Beam-targetTarget

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

Physical content of GPDs

M2(t) : Mass distribution inside the nucleon in transverse space J(t) : Angular momentum distribution d1(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 2nd 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.

Fit: ALU = sincosFully integrated asymmetry and one of 65 bins in Q2, x=ξ, t.

DVCS/BH Beam Spin Asymmetry

Large kinematics coverage

CLASBSA mostly sensitive to GPD H

t-dependence of leadingtwist term a (sinΦ).

Comparison with GPD model

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.

VGG Model (Vanderhaeghen, Guichon, Guidal)

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)

CLAS