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Meson Spectroscopy and Partial Wave Analysis at CLAS

Workshop on Amplitude Analysis in Hadron Spectroscopy ECT* Trento, Italy January 24-28 2011. Meson Spectroscopy and Partial Wave Analysis at CLAS. Raffaella De Vita INFN – Genova for the CLAS Collaboration. Meson Production at CLAS.

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Meson Spectroscopy and Partial Wave Analysis at CLAS

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  1. Workshop on Amplitude Analysis in Hadron Spectroscopy ECT* Trento, Italy January 24-28 2011 Meson Spectroscopy and Partial Wave Analysis at CLAS Raffaella De Vita INFN – Genova for the CLAS Collaboration

  2. Meson Production at CLAS • Study of meson properties and production represents a large fraction of the physics program at CLAS • vector meson production at large momentum transfer • vector meson modification in the nuclear medium • meson spectroscopy and search for exotics • …

  3. The first CLAS result E.Anciant et al. PRL 85 4682 (2000) g p  p j  pK+ K- Study of photoproduction at large momentum transfer • POMERON  2- gluons • Nucleon wave function • Correlations between quarks • u-exchange around -tmax

  4. Meson Production at CLAS • Study of meson properties and production represents a large fraction of the physics program at CLAS • vector meson production at large momentum transfer • vector meson modification in the nuclear medium • meson spectroscopy and search for exotics • … • Search for exotic mesons • Study of scalar mesons • Study of strangeonia • … • Reveal the nature of the mass of hadrons • Identify the relevant degrees of freedom • Understand the origin of confinement and the role of gluons • …

  5. LINAC LINAC A C B Jefferson Lab • Continuous • Electron • Beam • Accelerator • Facility • E: 0.75 –6 GeV • Imax: 200mA • RF: 1499 MHz • Duty Cycle:  100% • s(E)/E: 2.5x10-5 • Polarization: 80% • Simultaneous distribution to 3 experimental Halls Injector Experimental Halls

  6. The CLAS detector at JLab

  7. Electromagnetic Calorimeter lead/plastic scintillator, 1296 PMTs Torus Magnet 6 Superconductive Coils Target +  start counter e mini-torus Drift Chamber 35,000 cells Time of Flight Plastic Scintillator, 684 PMTs Cherenkov Counter e/ separation, 256 PMTs CEBAF Large Acceptance Spectrometer

  8. Hall-B Photon Tagger • Photon beam produced from the primary electron beam via Bremsstrahlung • Gold and diamond radiator for In/Coherent Bremsstrahlung • Energy coverage: 0.2-0.95 E0 • Efficiency ~ 80% • Energy Resolution ~ 10-3 • Timing Resolution ~100 ps • Maximum photon energy of 5.7 GeV • - Wmax 3.4 GeV • Beam intensity 107g/s

  9. Search for the Exotic π1(1600) • Candidate for an exotic hybrid meson • Should belong to JPC=1-+multiplet • Observed in pion beam experiments and pp annihilation but with controversial results • Observed by VES initially in ηπ and then in other decays • Strongest signal observed by E852 in ρπ, f2π, and others • E852 results not confirmed by a later analysis of a larger data sample

  10. Search for the Exotic π1(1600) • Candidate for an exotic hybrid meson • Should belong to JPC=1-+multiplet • Observed in pion beam experiments and pp annihilation but with controversial results • Observed by VES initially in ηπ and then in other decays • Strongest signal observed by E852 in ρπ, f2π, and others • E852 results not confirmed by a later analysis of a larger data sample • Recently positive results reported by COMPASS

  11. Search for the π1(1600) at CLAS • First results from a photoproduction experiment: • Bremsstrahlung photon beam produced from 5.744 GeV electrons • Photon flux of 5×107γ/s • Liquid-hydrogen target • Search in the γp®nπ+π+π- • Final state selected detecting the 3 pions and using the missing mass technique • Selection of low –t region and forward-going pions to reduce baryon-resonance background M. Nozar et al. (CLAS Collaboration),PRL 102, 102002 (2009)

  12. PWA for the 3π channel • Full PWA of selected 83000 events from G6C data set • PWA based on isobar model developed from E852 tools • Choice of waves based on observed dominance of ρ and f2 in the ππ invariant mass • Different sets of waves used for low mass an high mass portion of the 3 π spectrum • Spin-density matrix reduced to Rank-2 matrix because of limited statistics • Unbinned log-likelihood fit with different sets of waves Final wave set: 13 waves 31 waves

  13. Results from the 3π channel • Dominance of the 2++ state a2(1320): clear signal at correct mass and width • a2(1320) cross section (0.81±0.25) μb in agreement with previous measurements • No-evidence of exotic 1-+ state π1(1600), upper limit on c.s. of 13.5 nb(95%CL) CLAS-g6C • Possible distortion/leakagesbecause of limited wave set forced by low statistics • Results are consistent with expectations from previous measurements • Demonstrate that PWAs in CLAS are feasible • Higher statistics data samples are necessary for thorough study

  14. Photoproduction of Pion Pairs • Study of π+π- production on the proton and study of scalar mesons • Bremsstrahlung photon beam: 1.6-3.8 GeV • 40 cm long liquid hydrogen target • ~7∙109 triggers • Integrated Luminosity~ 80 pb-1 • Proton and π+detected in CLAS • Reactionπ γppπ+π-isolated via missing mass • Analysis focused on high energy (3.0-3.8 GeV) and low –t (0.4-1.0 GeV2) region

  15. Short range (QCD) production Meson formation Photoproduction of Pion Pairs 1) Extraction of Moments of the Di-Pion Angular Distribution • Moments are expanded on a set of basis function and weighted with Monte Carlo events to account for detector acceptance and efficiency • Moments are fitted to the data using an unbinned likelihood procedure 2) Evaluation of Partial Waves from Experimental Moments • Moments can be expressed as bilinear combination of partial wave amplitudes • +… • Partial Waves are parameterized using Dispersion Relations in term of known pp phase shifts and unknown coefficients and fitted to the experimental moments • Partial Wave cross sections are derived and can be compared with models e.g. 2p system

  16. Moments of the Di-Pion Angular Distribution 3.2 GeV < Eg < 3.4 GeV 0.5 GeV2 < -t< 0.6 GeV2 M. Battaglieri et al. (CLAS Collaboration), PRD 80, 072005

  17. P wave r(770) D wave f2(1270) f0(980) S wave Partial Wave Amplitudes gppp+p- 3.4 GeV < Eg < 3.6 GeV 0.5 GeV2 < -t< 0.6 GeV2 • M(p+p-) spectrum below 1.5 GeV: • P-wave: r meson • D-wave: f2(1270) • S-wave: s, f0(980) and f0(1370) • Moments of the 2-pion angular distribution extracted via likelihood fit of data • Partial Wave fitted to experimental moments • Known states well reproduced, e.g. r(770) First observation of f0(980) in photoproduction M. Battaglieri et al. (CLAS Collaboration), PRL 102, 102001

  18. New High Statistics Photon run: g12 • Search for new forms of hadronic matter in photoproduction on the proton: • Data taking completed in 2008 • Photon Energy up to 5.5 GeV • More than 26 billion triggers (2-prong + 3-prong) • Total Luminosity: 68 pb-1 (~50 pb-1 for E> 4.4 GeV) • 10 times more statistics than previous CLAS run in this energy range • Data processing completed and physics analysis in progress • Several exclusive channels are being analyzed: Meson Spectroscopy • Search for exotic mesons • Study of Strangeonia • … Baryon Spectroscopy • Cascades • … γp π+π+π-(n) γp pπ+π-η γp pK+K- (η/π) γp K+K+ X*- (1530) γp π+K+K-(n) γp e+e-p ...

  19. D. Schott (Florida International U.) G12: search for the π1(1400) π- g • γp➝Δ++ηπ- • All particles detected • By requiring a Δ++ exchanged particles is constrained to be a pion: • X resonance is constrained to be m=1 state • From VMD π:ρ exchanges are 9:1 • Structure corresponding to known states are clearly visible in πη mass spectrum • PWA started !! X η π π+ Δ++ p p PRELIMINARY PRELIMINARY

  20. C. Bookwalter(Florida State U.) G12: search for the π1(1600) π- g π+ X π+ π,ρ p n • γp➝nπ+π+π- • 3 pions detected and neutron reconstructed via missing mass • Low –t region and forward-going pions selected to suppress baryons-resonance background • ∼850 000 events after all selection cuts PRELIMINARY

  21. C. Bookwalter(Florida State U.) G12: search for the π1(1600) π- g π+ X PWA in progress.. π+ π,ρ p n • γp➝nπ+π+π- • 3 pions detected and neutron reconstructed via missing mass • Low –t region and forward-going pions selected to suppress baryons-resonance background • ∼850 000 events after all selection cuts PRELIMINARY

  22. Study of strangeonia • Mesons containing ss pairs • Can be regular states in the quark model or hybrids with/without exotic quantum numbers • Experimental data still quite sparse, only 7 of the 22 states expected below 2.2 GeV are widely accepted • Model prediction for width and decays available • Experimental search would require measurement of many different final states 3P0 model, Barnes, Black and Page (2002)

  23. M. Saini(Florida State U.) G12: study of strangeonia • Several different final states are being investigated: • KK, KK*, ηφ,… γp➝pφη ➝pK+K-η PWA started PRELIMINARY

  24. r D++ gp pp+p- Coherent Meson Production on 4He • Eliminates-channel resonance background • Simper PWA: S=I=0 target acts as spin and parity filter for final state mesons • Production cross section expected ~ e-bt|A FA(t)|2low -t kinematic Meson spectroscopy on 4He g4He4Hep0 hg4He4Hep0 h’ • Strongest evidence of JPC=1-+ p1(1400) exotic meson p-p nhp0in E852-Brookhaven • Search for a resonance in P-wave in p0h and p0h’ • Known (non-exotic) resonances can be used as a benchmark (e.g. JPC=2++ a2(1232))

  25. Coherent Production of π0η, π0η’ on 4He • Resonances decaying to π0η (π0η’) in a P wave is exotic (JPC= 1-+) because of the quantum numbers of the final state (I=1, C=+1, G=-1, and P=(-1)L ) • Scattering off a spin and isospin zero target works as a spin and parity filter • Only C=-1 ω-exchanges is allowed, Natural Parity Exchange (NPE) • The helicity of the produced state will be that of the incoming photon, λ=λγ, production of π0η (π0η’) in S-wave is forbidden Test for above assumptions

  26. Analysis of existing CLAS data shows clear peaks associated to known mesons in ep(e’)pp0p0 ep(e’)pp0h The Technique works!! f0(980) a0(980) f2(1270) EG6: Meson spectroscopy in coherent production on 4He • search for exotics in ph, ph’ final states • 6 GeV electron beam on target on high pressure gas target • scattered electron at “0” degrees  quasi-real photo-production • recoiling nucleus detected in Radial TPC • hadronic final state detected in CLAS • data taking completed in fall 2009 • analysis in progress

  27. EG6: Meson spectroscopy in coherent production on 4He • search for exotics in ph, ph’ final states • 6 GeV electron beam on target on high pressure gas target • scattered electron at “0” degrees  quasi-real photo-production • recoiling nucleus detected in Radial TPC • hadronic final state detected in CLAS • data taking completed in fall 2009 • analysis in progress

  28. Radial TPC with 7atm. He4 Target • Solenoid for forward-focusing of Moeller electrons and bending of recoil nucleus in the TPC • PbWO4 calorimeter for improved photon acceptance at forward angles EG6: Meson spectroscopy in coherent production on 4He • search for exotics in ph, ph’ final states • 6 GeV electron beam on target on high pressure gas target • scattered electron at “0” degrees  quasi-real photo-production • recoiling nucleus detected in Radial TPC • hadronic final state detected in CLAS • data taking completed in fall 2009 • analysis in progress

  29. EG6: Meson spectroscopy in coherent production on 4He • search for exotics in ph, ph’ final states • 6 GeV electron beam on target on high pressure gas target • scattered electron at “0” degrees  quasi-real photo-production • recoiling nucleus detected in Radial TPC • hadronic final state detected in CLAS • data taking completed in fall 2009 • analysis in progress RTPC calibration using elastic events on 4He

  30. Summary Rich physics program in meson spectroscopy with CLAS: • Meson spectrum investigated in photoproduction to determine properties of regular states and search for unconventional configurations • Large statistics data sets are now available • Several decay modes have been/are being investigated • Different PWA frameworks have been developed: • Isobar model • Moments + Dispersion relations • First results published, several new analyses in progress... • Present program will be extended with CLAS12 using quasi-real photoproduction • Better understanding of hadrons structure • Progress in understanding confinement in QCD and the role of constituent quarks and gluons in the description of the non-perturbativeregime

  31. Normal meson:qq state m=0, PC=(-1)S+1 S1 q L q S2 Hybrid meson:qqg gluonic field contribute to quantum numbers PC≠(-1)S+1 q q Hybrids and Exotics Hybrids (qqg) are the ideal system to study qq interaction and the role of gluons The existence is not prohibited by QCD but not yet firmly established. A possibility to identify unambiguously a meson as an hybrid state is to look for exotic quantum numbers • Normal mesons (qq) are classified according to their JPC where • P=(-1)L+1 • C=(-1)L+S JPC= 0-+ (π,K,η,η’) 1--(ρ,K*,ω,Φ) 1+- (b1,K1,h1,h1’) ... • Some combinations of JPC are not allowed for conventional qq systems but can exist for “unconventional” states as hybrids • Excitation of the flux tube leads to a new spectrum of hadrons that can have exotic quantum numbers JPC = 0+- , 1-+ , 2+- ... • Lattice QCD calculations predict masses around 2 GeV, a range that can be explored at JLab

  32. Pion Beam Photon Beam Quark spins anti-aligned JPC = 1-- , 1++ Quark spinsalready aligned JPC = 0+- , 1-+ , 2+- regular mesons @ Eg = 5GeV X = a2 Exotic meson @ Eg = 8GeV X = p1(1600) Search for Exotics in Photoproduction • Signals for exotic hybrids have been reported by various experiments but evidence is not yet fully convincing: more data are needed and experiments with different production modes (different probes) are crucial • Photoproduction: exotic JPC are more likely produced by S=1 probe A. Szczepaniak and M. Swat, Phys. Lett. B516 (2001) 72 • Production rate for exotics is expected to be comparable to regular mesons Few data (so far) but expected similar production rate as regular mesons

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