1 Excited Hadrons Volker D. Burkert Jefferson Lab Excited baryons Classification NΔ transition form factors Low mass N* excitations, Roper A near-threshold resonance S 11 (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

2 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

3 HO-Model Principal Energy Levels SU(6)xO(3) Classification of Baryons Quark orbital angular momentum 3-Quark Shell Model

4 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, pρ 0 using cross sections and polarization observables.

5 Reach of Current Accelerators JLAB Spring-8

6 S 11 (1535) P 11 (1440) P 33 (1232) SU(6)xO(3) Classification of Baryons Quark orbital angular momentum

7 p(e,e’)X Examples of Exclusive Processes in N* Studies Hadronic mass

8 Electromagnetic Excitation of N*’s vv N  p   p  e e’ γvγv N N’N’ N*,△ A 3/2, A 1/2, S 1/2 M l+/-, E l+/-, S l+/-  Measure the electromagnetic excitations of low-lying baryon states (<2 GeV) and their transition form factors over the range Q 2 = 0.1 – 7 GeV 2 and measure the electro- and photo-production of final states with one and two pseudo-scalar mesons. DOE Milestone 2012

9 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).

10 The γNΔ(1232) Quadrupole Transition SU(6): E 1+ =S 1+ =0 pQCD limit pQCD limit Shape at low Q 2

11  R EM remains small and negative at -2% to -4% from 0 ≤ Q 2 ≤ 6 GeV 2.  No trend towards sign change or asymptotic behavior. R EM → +100%, R SM → constant. Higher energies needed.  Dynamical models need to include pion contributions to explain magnitude and Q 2 dependence.  Quenched LQCD agrees with E/M, deviates at low Q 2 in S/M. Effect of pion cloud? NΔ Multipole Ratios R EM, R SM

12 The Roper resonance N 1/2 +(1440)P 11 RQM: P 11 (1440) = [56,0+] r P 11 (1440) = Q 3 G P 11 (1440) = (Q 3 ) r (QQ)  The Roper resonance is not a gluonic excitation Q 3 G.  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.

13 S 11 (1535): A near-threshold resonances S 11 (1535) in the CQM is a L 3Q =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 A 1/2 (S 11 ) form factor with Q 2 suggests a Q 3 system rather than a meson-baryon (QQQ-QQ) molecule (no form factor calculations exist for the molecular case). S 11 (1535)

14 Cross section for Nπ and η-cusp γp->π 0 p, Θ*=180 o, mostly resonant Eγ(GeV) γp->π + n, Θ*=180 o, mostly non-resonant Eγ(GeV)

15 SU(6)xO(3) Classification of Baryons Missing states Quark orbital angular momentum

16 Search for undiscovered baryon states |Q3>|Q3> |Q2Q>|Q2Q>  Symmetric CQM |Q 3 > 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 P 13 below 1900MeV would effectively rule out the |Q 2 Q> model.  The diquark-quark model |Q 2 Q> 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.

17 K+K+ New N* states in K  production?

18 PWA of data on  p → K + , K + , K 0  + 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.

19 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 +

20 Search for Exited Baryon States Experimentreactionsbeam pol.target pol.recoil status =================================================================================== G1/G10γp → Nπ, pη, pππ, KΛ/Σ - -Λ,Σcomplete G8 γp → p(ρ,φ,ω) linear--complete G9-FROST γp → Nπ, pη, pππ, KΛ lin./circ.long./trans.Λ,Σ2007 G13 γD → KΛ, KΣcirc./lin.unpol.Λ,Σ 2006/2008 G14-HD γ(HD) → KΛ, KΣ, Nπlin./circ.long./trans.Λ,Σ 2009/2010 C LAS 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.

21 γp→KY (K + Λ, K + Σ 0, K 0 Σ + )

22 γp→KY (K + Λ, K + Σ 0, K 0 Σ + )

23 γn→K0Λγn→K0Λ  E=100MeV

24 Coupled Channel Analysis (EBAC)

25 Coupled Channel Analysis (EBAC)  Pion-nucleon and 2- pion-nucleon contributions to the non-resonant T matrix.

26 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 (J PC - exotics). –The decay of hybrid mesons leads to complex final states. 1GeV qqG qq J PC = 0 +-, 1 -+, 2 +- Comments by Ted Barnes, Alex Dzierba, Adam Szczepaniak

27 LQCD supports the idea of flux tubes. Flux distribution between static quarks. Flux tubes lead to a linear confining potential.

28 Exotic Hybrid Mesons Masses With 3 light quarks the conventional and hybrid mesons form flavor nonets for each J PC.

29 Evidence for Exotic Meson with J PC = 1 -+ E852 - Evidence for the π 1 (1600) in π - p → η′ π - p M(η’π - ) (GeV)

30 Evidence for Exotic Mesons E852 BNL J PC = 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)

31 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)

32 A first search for exotic meson with photons a1a1 a2a2 p2p2 Gluonic Meson? p 1 (1600) Events/ 20 MeV  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.

33 GlueX – Exotic meson program at 12GeV To meet these goals GlueX will:

34 GlueX – Mass reach Other facilities: PANDA (GSI/Darmstadt). cc and cc-hybrid production in pp annihilation.

35 Summary Transition form factors of the NΔ(1232) measured in large Q 2 range. –no sign of approaching asymptotic QCD limit, needs 12 GeV upgrade –pion dressing of vertex needed to describe form factors Roper P 11 transition form factor determined for the first time. –zero-crossing of magnetic form factor –behaves like a Q 3 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 Mesons …in 2-3 slides, 5 mins, concentrate on future developments. 1. Definition(s) of exotic mesons 2. Current theoretical expectations (< 2.5 GeV) 3. Future developments Ted Barnes Physics Div. ORNL Dept. of Physics and Astronomy, U.Tenn. JLAB-RIA Workshop Wash. DC Oct 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. q 2 q 2. Controversial! (recall q 4 q) 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 J PC can be formed from qqg. ____________________________________________________________________________________________________________________________________________________________ The lightest hybrid multiplet is predicted to contain J PC = 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? M H = 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. f 1  b 1  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 collaborators. ____________________________________________________________ Theory - Improved LGT studies of hybrid spectroscopy. LGT H strong decays. Understanding photoproduction of meson resonances (CEX vs diffractive).

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 p 4 (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, f 0, a 0 ; f 2 Effective Lagrangian projected onto the exotic channel (Marco,Bass) reproduces the E852 ηπ and η’π signals (AS et al.) P-wave P -wave  ’  2 coupled channels S, D -wave  ’  3 coupled channels _

...but can we say something about the what is the quark content... Yes, study the large-N c dependence of S-matrix poles resonance pole moves deep into the complex plane resonance pole moves towards the real axis

The N c dependence of the E852 exotic is currently under study (J.R.Pelaez, AS) like a quark model state complicated !

Partial Wave Analysis at CMU Curtis A. Meyer, Carnegie Mellon University In the CLAS experiment at Jefferson Lab, there significant photoproduction data that can be used to provide information on baryon resonances. Now: Future: The GlueX experiment will study photoproduction of mesons to search for exotic-quantum-number hybrids. States that involve the confining gluonic field 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. 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 Procedure For 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 Events 8,000,000 Events 1,200,000 Events ¼ 1 Events These channels have not been extensively studied and 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. They are 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 and se how well they describe the data.

Preliminary Results pp pp  and  0 showing similar coupling to the same partial wave.

49 Exotic Mesons