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Excited Baryon Program

- in part based on N* Workshop, Nov. 6-7, 2006, JLab -

Volker D. BurkertJefferson Lab

Town Meeting on QCD and Hadrons, Rutgers University, January 12-14, 2007

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Why excited baryons are important

Baryons (nucleons) make up most of the mass of the visible universe.

The 3-quark system are at the foundation of the development of the quark model.

Understanding the existence of the lowest excited Δ++ baryon required introduction of a new quantum number (later called ‘color’) by O. Greenberg.

Baryon represent the simplest system where the non-abelian character of QCD is manifest.

Study of the excited baryon states is necessary to fully understand the ground state nucleon and to explore quark confinement.

Lattice QCD calculation of gluon flux distribution in a system of 3 heavy quarks.

gluon self coupling

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SU(6)SF x O(3) Classification of BaryonsQ

uark

orb

ital a

ngul

ar m

omen

tum

Harmonic Oscillator-Potential - Principal Energy Levels

S11(1535)

P33(1232) P11(1440)

F15(1680)

D13(1520)Predicted states

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Why study hadron structure with e.m. probes?

resolution of probe

low

high

N

π • γN may excite states not seen in πN.• What are the appropriate degrees-of-freedom describing hadron structure at varying distance?

e.m. probe

q

LQCD - P.O. BowmanDSE - C. Roberts

dressed quark (glue, qq)

bare quark

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Electromagnetic Excitation of N*’s

The experimental N* Program has two major components:

1) Transition form factors of known states to probe their internal structure and confining mechanism

2) Search for undiscovered states. Both parts of the program are being pursued in various decay channels with CLAS, e.g. Nπ, pη, pπ+π-, KΛ, KΣ, pω, pρ0 using cross sections and polarization observables.

v Np

p

e

e’

γv

N N’

N*,△

A3/2, A1/2, S1/2

Ml+/-, El+/-, Sl+/-

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Inclusive Electron Scattering

Need to measure exclusive processes in full phase space to separate resonances from each other and from non-resonant contributions.

ep→eXCLAS

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The γ*NΔ(1232) Quadrupole Transition

pQCDlimit

pQCDlimit

Shape at low Q2

Non-zero values at higher Q2 reveal intrinsic quadrupole charge distribution.

SU(6): E1+=S1+=0

~ -0.03 -0.1

γ*NΔ Multipole Ratios REM, RSM before JLab

Sign @ Q2 > 0 ?

Q2 dependence?

REM= -2 to -4% at 0 ≤ Q2 ≤ 6 GeV2.

RSM < 0, increasing in magnitude.

REM < 0 favors oblate shape of Δ(1232).

Pion contributions needed to explain shape, magnitude.

γ*NΔ Multipole Ratios REM, RSM with JLab

No trend towards asymptotic behavior REM→+100%.

γ*pΔ+ - Magnetic Transition Form Factor G*M

Quark corecontribution

e

e *

0

e

e *

Pion contribution predicted to drop more rapidly with Q2 than the quark core. Probe core at sufficiently high Q2.

Pion cloudcontribution

Large pion contribution needed to explain NΔ transition.

Connection with elastic form factors and GPDs => Paul Stoler, Friday session

T.-S. H. Lee N. Sato

Lattice QCD results for P11(1440), S11(1535)

F. Lee, N*2004

Mπ2 (GeV2)

Both states are considered as possible nucleon-meson molecular states: P11(1440) = |Nσ >, S11(1535) = |YK>.

Masses of both states are well reproduced in quenched LQCD with valence quarks.

For a (Q3Q2) system one expects a faster drop of the transition form factors with Q2.

Legendre Moments

W(GeV) W(GeV) W(GeV)

σT +εσL for γ*p→π+nQ2=3GeV2

~const. ~cosΘ ~ (a + bcos2Θ)

The Roper P11, S11 and D13 states become dominant contributions at high Q2

Δ(1232)

D13(1520)

S11(1535)D13(1520)

no Roper

with Roper

Δ

no Roper

with Roper

CLAS

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|Q3G>

nr|Q3>

nr |Q3>

|Q3G> r|Q3>LC

r |Q3>LC

Exclude gluonic excitation Q3G. At short distances consistent with Q3- radial excitation. At large distances meson couplings may be important.

preliminary

preliminary

Nature of the Roper N(1440)P11 ?

zero crossing

LC Models: S. Capstick & B. Keister; S. Simula; I. Aznauryan

CLAS

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Photocoupling amplitudes N(1535)S11

N(1535) in the CQM is a L3Q = 1, P=-1 state. It has also been described as a bound (KΣ) molecule with a large coupling to pη.

The slow falloff of the A1/2 amplitude seen in pη and Nπ suggests a small Q3 system rather than a large KΣ molecule.

Nπ

pη

CLAS

What is the nature of the N(1535) ?

preliminary

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Photocoupling amplitudes N(1520)D13

A1/2 is dominant amplitude at high Q2 as expected from asymptotic helicity conservation.

Q2(GeV2) Q2(GeV2)

CLAS

A1/2 amplitudes P11, S11, D13, (F15) appear to behave similarly at high Q2.

preliminary

preliminary

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Test helicity conservation

Q3A1/2 Q5A3/2S11

P11

F15

D13

No scaling seen for helicity non-conserving amplitude A3/2

F15

D13

Helicity conserving amplitude appears to approach scaling, but needs to be confirmed at higher Q2.

→ Expect approach to flat behavior for Q3A1/2, Q5A3/2 at high Q2

CLAS

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SU(6)xO(3) Classification of Baryons

Predicted states

Qua

rk o

rbita

l ang

ular

mom

entu

m

Summary of recent N* and Δ* findings

Does not support several N* and * reported by PDG2006:

*** (1600)P33, N(1700)D13, N(1710)P11, (1920)P33

** N(1900)P13, (1900)S31, N(1990)F17, (2000)F35,

N(2080)D13, N(2200)D15, (2300)H39, (2750)I313

* (1750)P31, (1940)D33, N(2090)S11, N(2100)P11,

(2150)S31, (2200)G37, (2350)D35, (2390)F37

R. Arndt, W. Briscoe, I. Strakovsky, R. WorkmanAnalysis of elastic πN→πN (2006)

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Discover new baryon states

|Q3>

|Q2Q>

SU(6) symmetric quark model |Q3> predicts many states that have not been seen in elastic πN scattering analysis.

Discovery of new states could have significant impact on our understanding of the relevant degrees of freedom in baryonic matter.

The diquark-quark model |Q2Q> has frozen degrees of freedom → fewer states. It accommodates all observed **** states.

Search for new states in different final states, e.g. Nππ, KΛ, KΣ, pω, pη’. Analyses are more complex and channel couplings are likely important.

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Predicted SU(6) x O(3) States

SU(6) x O(3)

Partial wave L2J,2I

Mass (MeV)

Decays

[N1/2+]4 P11 1880 Δπ, ∑K

[N1/2+]5 P11 1975 Δπ, Nω, Nρ

[N3/2+]2 P13 1870 Nπ, ∑K, Δπ

[N3/2+]3 P13 1910 Δπ, Nω, Nρ

[N1/2-]3 S11 1945 Nρ, Δπ, KΛ*

[N3/2-]3 D13 1960 Δπ, ΛK, Nρ

Examples of states predicted in the symmetric quark model with masses near 1900 MeV. ( S. Capstick, W. Roberts )

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K+

New N* states in KY production?

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• PWA of data on p→ K+, K+, K0+

K+Σ K+Λ

J. McNabb et al, PRC69 (2004)

New N* states in KΛ/KΣ production?

• Analyses find needs for various new candidate states. • Solutions based on unpolarized cross sections alone have ambiguities; demonstrates the need for polarization measurements.

A. Sarantsev et al., C. Bennhold, et al.,

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N* candidate at 1720 MeV in pπ+π- ?

electroproduction

W(GeV)

no 3/2+ (1720)full

M. Ripani et al, Phys.Rev.Lett. 91, 2003

photoproduction

W(GeV)

Background

Resonances

Interference

full calculation

no 3/2+

CLAS

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Search for New Baryon States

reactions beam pol. target pol. recoil status_____________________________________________________________

γp→Nπ,pη,pππ,KΛ/Σ - - Λ,Σ complete

γp→p(ρ,φ,ω) linear - - complete---------------------------------------------------------------------------------------------

γp→Nπ, pη, pππ, KΛ lin./circ. long./trans. Λ,Σ 2007

γD→KΛ, KΣ circ./lin. unpol. Λ,Σ 2006/2009

γ(HD)→KΛ,KΣ,Nπ lin./circ. long./trans. Λ,Σ 2009/2010

This program will, for the first time, provide complete amplitude information on the KΛ final state, and nearly complete information on the Nπ final states.

CLAS

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Instrumentation for Excited Baryon Search

Frozen Spin Target

FROST

CLASPhoton Tagger

LInearly polarized photon beam

P(H)

P(D)

0

20

40

60

(%)

days

BNL - Fall’06

Polarized HD - Target

Considered to be used at CLAS.

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

→→ →

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

→→ →

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Need for Theory Support

• For small resonance cross sections, channel couplings due to unitary constraints can lead to strong distortions of amplitudes.

• Requires coupled-channel computation that includes all major channels.

• The Excited Baryon Analysis Center (EBAC) was established in 2006 at JLab to provide theoretical support for the excited baryons experimental program.

CLAS12

1m

JLab Upgrade to 12 GeV

Luminosity > 1035cm-2s-1

• General Parton Distributions• Transverse parton distributions• Longitudinal Spin Structure • N* Transition Form Factors• Heavy Baryon Spectroscopy • Hadron Formation in Nuclei

Solenoid, ToF,Central Tracker

Forward Tracker,Calorimeter, Particle ID

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NΔ Transition - Future Program

Transition towards asymptotic behavior?

+100 ??

Projections for A1/2 @ 12 GeVCLAS12

Full transition to quark core behavior ?

CLAS published

CLAS preliminary

CLAS12 projected

DVCS - a new tool in N* physics

ep eN*

t, ξ dependence of N* transition - map out Transition-GPDs

GPDs

p N*

ee

hard processBjorken regime

x+ x-

Decouple γ virtuality from momentum transfer to the nucleon

Nucleon dynamics at the parton level

N*’s

Mn(GeV)

CLAS (preliminary)

ep→e+n

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Strangeness = -2 Ξ Baryons Advantage - Narrow widths, easier to separate from background.Disadvantage – No s-channel production, low cross sections. Flavor SU(3) predicts same number of Ξ’s as N*’s and Δ*’s. Only 3 Ξ’s have established JP.γp -> K+K+X-

Needs higher energy for spectroscopy -> 2007/2008.JLab @ 12 GeV is a good place for cascade spectroscopy.

γp -> K+K+Ξ0π-

Ξ(1320) Ξ(1530)

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Conclusions• Exclusive electroproduction of mesons has become a precise tool

to map out the intrinsic structure of established baryon states.

• With large acceptance detectors in use, and the development of highly polarized electron/photon beams and polarized targets the search for new baryon states has advanced to a much higher level of sensitivity.

• Planned precision measurements with polarized beams, targets, and recoil polarization measurements with CLAS will provide the basis for unraveling the S=0 baryon spectrum in the critical mass region near 2 GeV.

• Making full use of the precise data produced by the new equipment requires sound theoretical methods in the search for complex resonance structure, and in understanding the physics at the core of baryons. This effort is underway with the Excited Baryon Analysis Center at JLab and with continuing efforts in Lattice QCD.

• Jlab @ 12 GeV and CLAS12 allows extension of N* transition form factors to much higher Q2, and spectroscopy of heavy strange baryons.