Hadron Spectroscopy at CLAS: present status and perspectives

1. Hadron Spectroscopy at CLAS: present status and perspectives R. De Vita Istituto Nazionale di Fisica Nucleare Genova, Italy

2. p p Why hadron spectroscopy? • QCD is responsible for most of the observed mass in the universe • Precise determination of the spectrum and internal structure of hadrons is necessary to reach a deep understanding of QCD • Revealing the nature of the mass of the hadrons • Identify the relevant degrees of freedom • Understand the origin of confinement • Validate LQCD predictions > 1 fm 0.1 – 1 fm << 0.1 fm Effective Degree of Freedom Quark and Gluons Mesons & Baryons

3. Jefferson Lab The core of Jefferson Lab is the CEBAF accelerator CEBAF is a superconductive electron accelerator • continuous beam (RF cavities at 1.5 GHz) • high longitudinal polarization (85%) • energy range  0.75 –5.9 GeV • max current  200mA • simultaneous delivery to 3 halls C A B

4. The CLAS Spectrometer CEBAF Large Acceptance Spectrometer • magnetic spectrometer based on six-coil toroidal field • large kinematical coverage • high luminosity: 1034cm-2s-1 • simultaneous measurement of exclusive and inclusive reactions • central field-free region well suited for the insertion of a polarized target

5. Spectroscopy at CLAS • Measurement of the magnetic form factor of the neutron • Study of nucleon resonances: • Measurement of the transition form factors • Search for missing states • Strangeness production • L and S production • Cascade spectroscopy • Search for exotic states • high statistics search of pentaquark states • search for hybrid mesons

6. e’ e N g* Data from the CLAS detector at Jefferson Lab m Counts N*,D p m Nucleon Resonances Excited states of the nucleon were first observed in pN scattering Nucleon resonances are also evident in reaction induced by electro-magnetic probes Use of electron beam allows the probe resonance transition at different distance scale

7. D N p ND(1232) In framework of the quark model the ND transition involves the spin flip of a constituent quark dominated by the magnetic M1+ multipole At low Q2, large distances, the coupling of the pion cloud with the quark core can contribute to small values of E1+ and S1+ E1+=M1+ M1+ dominance E1+= S1+=0 E1+/M1+  0.3 S1+/M1+ 0.1 At high Q2, helicity conservation in pQCD leads to E1+=M1+

8. ND(1232) Transition multipoles measured at CLAS with high precision up to Q2  6GeV2 E/M remains negative with no indication of the transition to E/M=1 expected from pQCD Good description of both E/M and S/M by models that include contributions from the pion cloud G*M decreases with Q2 faster than elastic magnetic form factor (dipole) M. Ungaro et al. (CLAS Collaboration), Phys. Rev. Lett. 97 (1006),112003

9. Quark model classification of N* D13(1520) S11(1535) |q2q |q3  Missing States D(1232) Roper P11(1440) The missing states Constituent Quark models describe well the lowest mass states of the spectrum but predict more states than what observed experimentally Two explanations have been proposed • 1- quark-diquark configuration • Some of the internal degrees of freedom are frozen, limiting the number of possible configuration • 2- we haven’t been able to see them yet • Limitation of the experimental technique • new measurement are performed with large acceptance detector and polarized beam and target • Resonance decouple from pN decay mode • search has to be extended to other final states (ppN, wN, KL) These measurements are in progress at CLAS

10. The missing states M.Ripani et. al. (CLAS Collaboration), Phys. Rev. Lett.91, 022002 (2003) Evidence for the missing resonances is being searched in various decays modes as Npp, Nh, Nw Results from ppp decay mode show a strong peak at a mass of 1730 MeV Comparison of cross section with expectations from previous measurements gives indication for missing strength New resonance??

11. pentaquark pentaquark q q q q q q q q q q q q q q q q tetraquark glueball meson hybrid meson Beyond the Quark Model: Hybrids and Exotics Quarks combine to “neutralize” color force q q q q q mesons baryons Other quark-gluon configurations can give colorless objects

12. T. Nakano et al., Phys.Rev.Lett. 91 (2003) 012002 M(Q+)=1530 MeV G15 MeV S = +1  uudds state The Pentaquark First evidence for a pentaquark state was reported by the LEPS Collaboration in 2003 Q+ This state named Q+ appeared as a narrow structure in the nK+ invariant mass spectrum Scientists from several labs and collaborations tried to reproduce these initial findings

13. The Pentaquark The CLAS Collaboration searched for evidence of this state in very high statistics experiments Photons were scattered over proton and deuterontargets looking for hints of a narrow resonance decaying to a neutron and a kaon Q+ ? M. Battaglieri et al. (CLAS Collaboration), Phys.Rev.Lett.96 (2006) 042001 B. McKinnon et al. (CLAS Collaboration), Phys.Rev.Lett.96 (2006) 212001 No sign of the narrow peak observed in the previous experiment was found

14. q q q q q q q q q q q q q tetraquark hybrid meson hybrid meson Hybrids and Exotics Quarks combine to “neutralize” color force q q q q q mesons baryons Other quark-gluon configurations can give colorless objects pentaquark glueball meson

15. Linear potential Looking for the glue Flux tubes realized in LQCD Gluons possess colorcharge: they couple to each other!! Theoretical calculations show that the quark-antiquark pair forming a meson interact through the exchange of gluons that form a flux tube D. Leinweber G. Bali

16. Lattice QCD calculations predict masses around 2 GeV, a range that can be explored at JLab Normal meson:flux tube in ground state Pion Beam Photon Beam q q Hybrid meson:flux tube in excited state q q Quark spinsalready aligned JPC = 0+- , 1-+ , 2+- Quark spins anti-aligned JPC = 1-- , 1++ Hybrid Mesons How can we find hints of the gluons that bind the quarks?? Flux tube JPC=1-+ , 1+- Excitation of the flux tube leads to a new spectrum of hadrons that can have exotic quantum numbers JPC = 0+- , 1-+ , 2+- ...

17. Analysis of existing CLAS data shows clear peaks associated to known meson in ep(e’)pp0p0 ep(e’)pp0h f0(980) a0(980) f2(1270) Search for Exotics at CLAS • g12: Search for new forms of hadronic matter in photoproduction on the proton • search for exotics in pp+p0p-, np+p+p-, pK+K-h,... final states • presently running • eg6: Meson spectroscopy in coherent production on 4He with CLAS • quasi-real photo-production on high pressure gas target • search for exotics in ph, ph’ final states • production on isospin-zero target leads to great simplification of PWA • data taking in 2009

18. Summary • Hadron spectroscopy is a key tool for the understanding of QCD • The Spectroscopy program in progress at CLAS addresses basic questions about the origin of the mass of hadrons, the internal structure of baryons and mesons and the identification of relevant degrees of freedom, the origin of quark confinement, ... • New data on production of the first excited state of the nucleon indicate that the proton behaves like a small three-quark core surrounded by a cloud of pions • Data including polarization observables with a variety of meson-baryon final states are essential to provide information on new or poorly studied states and to clarify the missing state puzzle • Search of exotic states in the baryon and meson sector is in progress • The high statistics search for the Q+(1540) gave no evidence of the existence of such states • New experiments looking for evidence of hybrid mesons will provide new high statistics data sets in the near future