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Excited Hadrons

Volker D. BurkertJefferson Lab

Excited baryons• Classification• NΔ transition form factors• Low mass N* excitations, Roper• A near-threshold resonance S11(1535) • Search for new baryon states• Coupled channels analysis

Exotic hybrid mesons• What are they, what do we know?• Near term plans• Plans for JLab Upgrade

• Summary

Workshop on Physics of Nucleons and Nuclei, October 16-17, 2006

Contributions: T. Barnes, A. Dzierba, C. Meyer, A. Szczepaniak

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Why excited baryons are important

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

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

Study of the excitation spectrum is necessary to understand the ground state and explore origin quark confinement.

LQCD calculation of gluon flux distribution in a 3-quark system.

gluon self coupling

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HO-Model Principal Energy Levels

SU(6)xO(3) Classification of BaryonsQ

uark

orb

ital a

ngul

ar m

omen

tum

3-Quark Shell Model

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Electromagnetic Excitation of N*’s

The experimental N* Program has two major components:

1) Transition form factors of known resonances to study their internal structure and confining potential

2) Spectroscopy of excited baryon states, search for new states. Both parts of the program are being pursuit in various decay channels, e.g. Nπ, pη, pπ+π-, KΛ, KΣ, pω, pρ0 using cross sections and polarization observables.

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Reach of Current Accelerators

JLAB

Spring-8

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S11(1535)

P11(1440)P33(1232)

SU(6)xO(3) Classification of BaryonsQ

uark

orb

ital a

ngul

ar m

omen

tum

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p(e,e’)X

Examples of Exclusive Processes in N* Studies

Hadronic mass

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Electromagnetic Excitation of N*’s

v N

p

p

e

e’

γv

N N’

N*,△

A3/2, A1/2, S1/2

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

Measure the electromagnetic excitations of low-lying baryon states (<2 GeV) and their transition form factors over the range Q2 = 0.1 – 7 GeV2 and measure the electro- and photo-production of final states with one and two pseudo-scalar mesons.

DOE Milestone 2012

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Resonance Analysis Tools

• Nucleon resonances are broad and overlapping, careful analyses of angular distributions for differential cross sections and polarization observables are needed.

• Amplitude & multipole analysis (GWU, SAID)

• Phenomenological analysis procedures have been developed, e.g. unitary isobar models (UIM), dispersion relations (DR), that separate non-resonant and resonant amplitudes in single channels.

• Dynamical coupled channel approaches for single and double pion analysis are being developed within the Exited Baryon Analysis Center (EBAC) effort. They are most important in the extraction of transition form factors for higher mass baryon states.

• Event-based partial wave analyses with maximum-likelihood fit, developed in the search for new mesons states are now being utilized for baryon resonance studies. They fully utilize correlations in the final state (CMU). (Comments by Curtis Meyer).

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The γNΔ(1232) Quadrupole Transition

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

pQCDlimit

pQCDlimit

Shape at low Q2

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REM remains small and negative at -2% to -4% from 0 ≤ Q2 ≤ 6 GeV2.

No trend towards sign change or asymptotic behavior. REM → +100%, RSM→constant. Higher energies needed.

Dynamical models need to include pion contributions to explain magnitude and Q2 dependence.

Quenched LQCD agrees with E/M, deviates at low Q2 in S/M. Effect of pion cloud?

NΔ Multipole Ratios REM, RSM

6.01.0 2.0 3.0 4.0 5.0

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The Roper resonance N1/2+(1440)P11

RQM: P11(1440) = [56,0+]r

P11(1440) = Q3GP11(1440) = (Q3)r(QQ)

The Roper resonance is not a gluonic excitation Q3G.

At large distances meson couplings may be important.

At short distances the Roper is best described as a radial excitation of the nucleon.

Photocoupling amplitudes carry information on the the internal structure of the state.

First observation of a sign change for any nucleon resonance.

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S11(1535): A near-threshold resonances

S11(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 very slow falloff of the A1/2(S11) form factor with Q2 suggests a Q3 system rather than a meson-baryon (QQQ-QQ) molecule (no form factor calculations exist for the molecular case).

S11(1535)

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Cross section for Nπ and η-cusp γp->π0p, Θ*=180o, mostly resonant

0.6 0.7 0.8 0.9Eγ(GeV)

γp->π+n, Θ*=180o, mostly non-resonant.

0.6 0.7 0.8 0.9Eγ(GeV)

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SU(6)xO(3) Classification of Baryons

Missing states

Qua

rk o

rbita

l ang

ular

mom

entu

m

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Search for undiscovered baryon states

|Q3>

|Q2Q>

Symmetric CQM |Q3> predicts many more states than are observed in elastic πN → πN scattering analysis.

Discovery of any new state could have significant impact on our understanding of the relevant degrees of freedom in baryonic matter. Example: An additional P13 below 1900MeV would effectively rule out the |Q2Q> model.

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 become important.

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K+

New N* states in K production?

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• PWA of data on p → K+, K+, K0+

CLAS K+ K+

New N* states in K/K production?

Partial wave analyses yield tantalizing hints of several new states. However solutions are not unique due to insufficient polarization data.

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New N* candidate at 1710 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+

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Search for Exited Baryon States

Experiment reactions beam pol. target pol. recoil status===================================================================================G1/G10 γp→Nπ, pη, pππ, KΛ/Σ - - Λ,Σ complete

G8 γp→p(ρ,φ,ω) linear - - complete

-----------------------------------------------------------------------------------------------------

G9-FROST γp→Nπ, pη, pππ, KΛ lin./circ. long./trans. Λ,Σ 2007

G13 γD→KΛ, KΣ circ./lin. unpol. Λ,Σ 2006/2008

G14-HD γ(HD)→KΛ, KΣ, Nπ lin./circ. long./trans. Λ,Σ 2009/2010

CLAS

This program will, for the first time, provide complete amplitude information on the KΛ final state (more than 7 independent polarization measurements at each kinematics), and nearly complete information on the Nπ final states.

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γp→KY (K+Λ, K+Σ0, K0Σ+)

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γp→KY (K+Λ, K+Σ0, K0Σ+)

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γn→K0Λ

E=100MeVE=100MeV

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Coupled Channel Analysis (EBAC)

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Coupled Channel Analysis (EBAC)

Pion-nucleon and 2-pion-nucleon contributions to the non-resonant T matrix.

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Hybrid mesons

Flux Tube Model

– Provides a framework to understand gluonic excitations.

– The quarks in mesons are sources of color electric flux which is trapped in a flux tube connecting the quarks.

– Conventional mesons have the flux tube in the ground state. When the flux tube is excited hybrid mesons emerge. For static quarks the excitation level above the ground state is ~1 GeV.

– The excitation of the flux tube, when combined with the quarks, can lead to spin-parity quantum numbers that cannot be obtained in the quark model (JPC - exotics).

– The decay of hybrid mesons leads to complex final states.

1GeVqqGqq

JPC = 0+-, 1-+, 2+-

Comments by Ted Barnes, Alex Dzierba, Adam Szczepaniak

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LQCD supports the idea of flux tubes.

Flux distribution between static quarks.

Flux tubes lead to a linear confining potential.

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Exotic Hybrid Mesons Masses

With 3 light quarks the conventional and hybrid mesons form flavor nonets for each JPC.

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Evidence for Exotic Meson with JPC = 1-+

E852 - Evidence for the π1(1600) in π-p → η′π-

p

M(η’π-) (GeV)

1.5 2.0 2.5

M 1597 10 1045 MeV / c2

340 40 5050 MeV / c2

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Evidence for Exotic Mesons

E852 BNLJPC = 1-+

There is controversy about some of these channels.

• experimental issues.

• truncation of partial wave included in analysis.

• interpretation of line shape and phase motion.

(A. Dzierba et al., Phys.Rev.D73:072001,2006)

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Photons may be more suited to excite exotics

• In the flux tube model, using photon beams, the production rate of hybrid mesons is not suppressed compared to conventional mesons. N. Isgur, PRD (1999); A. Afanasev & A. Szczepaniak, PRD (2000); F. Close & J. Dudek (2004)

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A first search for exotic meson with

photons

a1

a2

2 Gluonic Meson?

1(1600)

0.8 1.2 1.6 2.0

102

Eve

nts

/ 20

MeV

45

35

25

15

5

Clarify evidence for exotic meson states, e.g. at 1600 MeV with high statistics. Prepare for full study with GlueX.

Events from previous CLAS experiment.

Expect 1-2 million 3-pion events,3 orders more than any previously published meson photoproduction results, allowing a partial wave analysis.

Experiment planned to run in 2007/2008.

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GlueX – Exotic meson program at 12GeV

To meet these goals GlueX will:

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GlueX – Mass reach

Other facilities:

PANDA (GSI/Darmstadt). cc and cc-hybrid production in pp annihilation.

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Summary• Transition form factors of the NΔ(1232) measured in large Q2 range.

– no sign of approaching asymptotic QCD limit, needs 12 GeV upgrade– pion dressing of vertex needed to describe form factors

• Roper P11 transition form factor determined for the first time. – zero-crossing of magnetic form factor – behaves like a Q3 radial excitation at short distances

• Tantalizing hints of new baryon states in KY and Nππ channels– require polarization data to resolve ambiguities in analysis

• Measurement of multiple polarization observables in Nπ, pη, and KY production needed to resolve ambiguities in baryon resonance analysis. EBAC essential to support the baryon resonance program with coupled channel calculations.

• Experiment to clarify status of some exotic meson candidates with photon beams in preparation.

• Full program with GlueX at the JLab 12GeV Upgrade will map out exotic hybrid meson mass spectrum with high precision.

Structure of Exotic MesonsStructure of Exotic Mesons …in 2-3 slides, 5 mins, concentrate on future …in 2-3 slides, 5 mins, concentrate on future developments.developments.

1. Definition(s) of exotic mesons1. Definition(s) of exotic mesons

2. Current theoretical expectations (< 2.5 2. Current theoretical expectations (< 2.5 GeV) GeV)

3. Future developments 3. Future developments

Ted BarnesPhysics Div. ORNL

Dept. of Physics and Astronomy, U.Tenn.

JLAB-RIA Workshop Wash. DC 16-17 Oct 2006

1. Exotic meson defn. (Wikipedia):

“…quantum numbers not possible for mesons in the quark model.” (hence .ne. qq )

FLAVOR EXOTICS: e.g. isospin=2, m: requires higher Fock states, e.g. q2q2. Controversial! (recall q4q)

SPIN-PARITY EXOTICS: J PC forbidden to qq, e.g. 1. Hybrid mesons “qqg”. Not controversial!

2. Current theoretical expectations for exotic mesons: (< 2.5 GeV)

SPIN-PARITY (J PC) exotics in this mass range are expected only from hybrids “qqg”.

All JPC can be formed from qqg.____________________________________________________________________________________________________________________________________________________________

The lightest hybrid multiplet is predicted to contain

JPC = 0, 1, 2, 1(bag model)all this and 0, 1, 2, 1 (flux tube model)9 flavor states (qq flavor nonets). Hence 36 new states (bag) or 72 new states (f.t.)

____________________________________________________________________________________________________________________________________________________________

At what mass? MH = 1.9 GeV (famous flux-tube estimate, Isgur and Paton),

ca. 1½ GeV (bag model), 2.0 GeV (LGT).____________________________________________________________________________________________________________________________________________________________

Decaying to what? Famous flux-tube prediction: H S+P modes, e.g. f1b

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This may be wrong. Simple S+S modes (’ should also be studied.

The best experimental exotic candidate, 1(1600), is seen in ’ !

3. Future developments:

_____________________________________________________________

Experiment -

JLAB:

High statistics investigation of meson spectroscopy using photoproduction(Gluex/HallD/Jlab).

Other facilities:

PANDA (GSI/Darmstadt). cc and cc-hybrid production in pp annihilation. “Super-LEAR” Start date ca. 2012. 350 collaborators.____________________________________________________________

Theory -

Improved LGT studies of hybrid spectroscopy. LGT H strong decays.

Understanding photoproduction of meson resonances (CEX vs diffractive).

M 1370 16 3050 MeV / c2

385 40 10565 MeV / c2

p p

p 0 n

M 1597 10 1045 MeV / c2

340 40 5050 MeV / c2 p p

BNL (E852) confirmed by Crystal Barrel

BNL (E852)

Adam Szczepaniak, University of Indiana

(other approaches use dispersion relations -

Roy eqs. - yield similar amplitudes)

What is the QCD nature of exotic signals? For reactions involving ground state pseudo-scalars there exists the possibility of effective theory formulation (chiral GB’s have derivative couplings and the U(1) GB couples via (heavy) glueball). What follows is a coupled channel analysis. For example to order p4 (schematically)

Meson spectrum up to √1.2 GeV (Gasser, et al., Pelaez et al., Oset et al.,

Lesniak et al.)Includes the ρ,K*; σ,κ, f0, a0

; f2

Effective Lagrangian projected onto the exotic channel (Marco,Bass) reproduces

the E852ηπ and η’π signals (AS et al.)

P-wave

P -waveP -wave’’2 coupled channels2 coupled channels

S, D -S, D -wavewave’’3 coupled channels3 coupled channels

_

...but can we say something about the what is the quark content ... Yes, study the large-Nc dependence of S-matrix poles

resonance pole moves

deep into the complex plane

resonance pole moves

towards the real axis

The Nc dependence of the E852 exotic is currently under study (J.R.Pelaez, AS)

like a quark model state

complicated !

Partial Wave Analysis at CMU

XNp

)( 0* pNp

Curtis A. Meyer, Carnegie Mellon University

In the CLAS experiment at Jefferson Lab, there significant photoproductiondata that can be used to provide information on baryon resonances.

Now:

Future:The GlueX experiment will study photoproduction of mesons to search forexotic-quantum-number hybrids. States that involve the confining gluonicfield in their quantum numbers.

Photoproduction of hadrons

A partial wave analysis combines information from the initial state with angular and energy distributions of the final state particles to reconstruct the quantum numbers of intermediate states.

))(( 2 bpp

s-channel process in CLAS

t-channel process in GlueX

Missing Baryons?Baryons are made from three quarks:

proton = uud neutron = ddu

Predict a spectrum of baryons of spin (J) and parity (P)

p,n Positive Parity States (Observed)

Negative Parity States (Observed)

Positive Parity States (About 50% Missing)

What are the effective degrees of freedom in a baryon?

If “quarks”, then we are missing states.If “quarks” and “small di-quarks”, then we might have all states.

Analysis ProcedureFor two-body final states, the analysis can be simplified. For more than two bodies, the correlations between particles provides significant. This information is most effectively retained by keeping the events themselves, and not trying to bin the data.

Using 11TB of CLAS data from a recent run period,simultaneously analyzing reactions:

p ! p p ! p 0

p ! p p ! K+

p ! p+-

700,000 Events 250,000 Events8,000,000 Events1,200,000 Events¼ 1 Events

These channels have not been extensively studiedand are supposed to couple to some missing baryons.

The Machinery

Very CPU intense process.

Can impose constraints at the event level between different final states E.g. and 0 have to couple to the same states

Get total and differential cross sections as a free by-product. Theyare projected out.

Get partial wave intensities and phases as a function of energy, which can then be fit to a resonance picture.

We have the ability to put some models directly into the fit andse how well they describe the data.

Preliminary Results

p

p

and 0 showing similar coupling to the same partial wave.

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Exotic Mesons