Recent Results from Jefferson Lab

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1 Recent Results from Jefferson Lab Strange quarks in the nucleon N- Deformation Latest on Pentaquarks Elton S. Smith Jefferson Lab XI International Conference on Hadron Spectroscopy Centro Brasilero Pesquisas Fisicas Rio de Janeiro, Brazil 1

2 Why use electron and photon probes? Electromagnetic interaction is well-known e e p F(Q 2 ) p Elastic Form Factors Inelastic transitions Size probed ~ 1/ Q 2 2

3 Why Hadronic Physics with EM Probes π resolution of probe Central question: What are the relevant degrees of freedom at varying distance scales? N low q high e.m. probe Diakonov hep-ph/

4 JLab Today Main physics programs nucleon electromagnetic form factors (including strange form factors) N N* electromagnetic transition form factors spin structure functions of the nucleon form factors and structure of light nuclei Superconducting recirculating electron accelerator max. energy 5.7 GeV max current 200 µa e polarization 80% Simultaneous operation in 3 halls L[cm -2 s -1 ] 2 High Resolution Spectrometers (p max =4 GeV/c) spectrometers (p max =7 and 1.8 GeV/c) + special equipment Large Acceptance Spectrometer for e and γ induced reactions

5 CEBAF accelerator site 5

6 Three Experimental End-Stations High Resolution Spectrometers Electron or Hadron Arm Q1 Q2 Dipole Detector Q3 HALL A Pair of identical High Resolution Spectrometers (HRS2) 53 m HALL B CEBAF's Large Acceptance Spectrometer (CLAS) and Bremsstrahlung Photon Tagger Short Orbit Spectrometer High Momentum Spectrometer HALL C 30 m High Momentum Spectrometer (HMS) and Short Orbit Spectrometer (SOS) 46 m 6

7 Strange quarks in the nucleon G 0 (forward) Q 2 dependence HAPPEX H HAPPEX 4 He Parity-violating Asymmetries in ep elastic scattering 7

8 EM and weak contributions γ-exchange Z-exchange e e e e 2 γ + Ζ p p p p u u γ γ γ u d d s g γ = +2/3 g γ = 1/3 g γ = 1/3 Ζ Ζ Ζ u d d s s s g Z = 1 8/3 sin 2 θ w g Z = 1+4/3 sin 2 θ w g Z = 1+4/3 sin 2 θ w 8

9 Parity-violating electron scattering 9

10 G0 experiment overview 10

11 G0 experimental setup 11

12 HAPPEX II - 1 H Results Raw Parity Violating Asymmetry 9.5 M pairs, total width ~620 ppm A raw correction ~ 0.06 ppm Helicity Window Pair Asymmetry Q 2 = (GeV/c) 2 A raw = ppm ± 0.20 ppm (stat) Measurements are dominated by statistical errors 12

13 Experimental asymmetries 13

14 Strange quark contribution to asymmetry (Measured - No Vector Strange) p 2 p 2 s s 4πα 2 εge + τg E + η M = 2 p (0) phys NV G F Q ε G E M ( ) ( A ) S 1 + RV G G A Known electromagnetic Form Factors 14

15 Strange quark contribution to proton 15

16 World Q 2 =0.1GeV 2 16

17 N- Deformation Experiments at JLAB Hall A High luminosity, recoil polarization. Hall B Large acceptance and kinematic coverage, polarized target and beam. Hall C High Q 2, search for onset of pqcd scaling (E2/M1 1). See parallel session on Baryons III γ*p (1232) πn e e γ v λ = 0, ± 1 γ M1, E2, C2 SU(6): E 1+ =S 1+ =0 (1232) N N π 0, π + 17

18 What do E2/M1 and C2/M1 ratios measure? e / γ * e / γ * π 0 e M1, E2, C2 e E0+,S0+,M1-,S1- E1+,S1+,M1+, Short Range Physics? Long Range Physics? Gluon exchange D-state admixtures χsb Light pion mass Deformation of N / Excite qq pairs from vacuum Shape of pion cloud 18

19 G M */3G D γ*p (1232) Magnetic Dipole Dynamical pion models Quenched lattice Pion Cloud Sato,Lee PRC 63, (2001) C. Alexandrou et al, PRL, 94, (2005) G M * (e/2m N ) Q 2 (GeV 2 ) Fits of dynamical pion models to π photoproduction data suggest 30-50% of M1 photocoupling strength near Q 2 =0 due to meson rescattering at EM vertex. Extrapolation of lattice G M * to Q 2 =0 is within 10% of experiment (where are the pions?). Predicted form factor falls with Q 2 more slowly than data. Q 2 (GeV 2 ) Lattice shows a decreasing G M * coupling strength as quark mass decreases. Uncertainties: size of lattice volume (too small?), effects of unquenching (qq loops, decay) 19

20 γ*p (1232) Quadrupole transitions Pion-cloud models Lattice γ*p (1232) π 0 p C. Alexandrou et al, PRL, 94, (2005) S 1+ / M 1+ (%) E 1+ / M 1+ (%) C2 / M1 (%) E2 / M1 (%) Quenched lattice consistent with E2/M1 data! Low Q 2 behavior of C2/M1 strong test of calculations! 20

21 Pion Electroproduction Structure Functions d d 2 σ Ω p * π 2 * * * * = σ + ε σ + εσ θπ φπ + ε ε + σ θπ φ * * T L L TT 2 L( 1) LT π π k γ ( sin cos2 sin cos ) M 1+ dominance Re(M 1+ )=0 Q 2 = 0.16 W = 1.22 peep (, ) π 3 2 e / 0 * ( 1 1 ) σ + + * LT = 6Re S M cosθπ e γ * * ( M1 E1 ) 2 σ = M1 + 3Re + + TT * ( ( )) ( ) 1 + Re M E θ σ = 2 M 2 2 * 1+ M P cos π T p θ * φ * π o * ϕπ = π ϕ * π = π /2 ϕ = d 2 σ(maid) vs θ* and φ* at (1232) peak. * π 0 Grid shows experimental (cosθ*,φ*) bins. 21

22 Typical pπ 0 Cross Sections Near Pion Threshold Q 2 = 0.2 GeV 2 W= GeV dσ (µb) dσ (µb) φ* cos θ* 22

23 W Dependence of Legendre Coefficients dσ (µb) σ T LσL σtt σ + ε = A + AP + AP LT = C = D + D P (M 1+ dominance) Resonant Multipoles M = A o 1+ ( ) Re( E M ) = A 2 C / 3 / 8 * Re( S M ) = D / 6 2 * /2 W (GeV) Disagreement with MAID for S 1+ over full W range down to threshold 23

24 Preliminary CLAS Results for E 1+ /M 1+ and S 1+ /M 1+ S 1+ / M 1+ (%) E 1+ / M 1+ (%) preliminary Preliminary E1+/M1+ is in good agreement with MAID, while S1+/M1+ continues to show strong Q2 dependence. Consistency between different data sets at low Q 2 needs to be understood. Reanalysis of ELSA low Q2 points in progress. Q 2 (GeV 2 ) 24

25 Latest on Pentaquarks New Results from CLAS Θ + Ν 5 See parallel session on Exotics I Σ 5 Ξ Ξ + 25

26 Quarks and QCD Established quark configurations include baryons (3q) and mesons (qq) Quantum Chromo Dynamics allows hadronic states with different quark configurations (e.g. 4q+q pentaquarks). When the antiquark has a different flavor than the other 4 quarks, the pentaquark has exotic quantum numbers. Rotational excitations of the soliton [rigid core surrounded by chiral (meson) fields] (ud) L=1 s Spin ½ antidecuplet uudds Θ + (1530) N(1710) I=1,S=0 I=0,S=1/2 (ud) L=1 (uds) Karliner, Lipkin, hep-ph/ (ud) JW hep-ph/ JM hep-ph/ SZ hep-ph/ ddssu Σ(1890) Ξ(2070) uussd 26

27 Pentaquark Scorecard (April 2005) Positive Results Experiment signal backgrd Λ Λ(1520) Spring Spring SAPHIR CLAS(d) CLAS(p) DIANA ν 19 8 HERMES COSY ZEUS * 193 SVD NOMAD signal backgrd Ξ D* NA H * Estimate from cross section Negative Results Experiment Λ Λ(1520) φ Λ c E ALEPH 2800 CDF BaBar HERA-B SPHINX HYPERCP COMPASS BELLE SELEX 2,800,000 Ξ Ξ(1530) D D * E ALEPH CDF BaBar HERA-B ZEUS WA FOCUS

28 Search for Pentaquarks at CLAS A comprehensive program to search for pentaquarks in photoproduction experiments at Jeffeson Lab were approved in with the goal of confirming previous results and explore new kinematics with at least a factor 10 increase in statistics. Relevant Publication g10 deuteron E γ ~ GeV data taking completed in 2004 CLAS(d) g Phys. Rev. Lett. 91, g11 eg3 proton E γ ~ GeV data taking completed in 2004 deuteron E γ ~ GeV data taking completed in 2005 SAPHIR 2003 Phys. Lett. B572, 127 NA Phys. Rev. Lett. 92, Super-g proton planned for 2006 E γ ~ GeV CLAS(p) g Phys. Rev. Lett. 92,

29 CLAS Cross Section Checks 6 dσ/dω CM (µb/sr) g10 preliminary (3375A) World data γn pπ < E γ < 1.15 GeV σ(µb) γp p ω CLAS g11 preliminary SAPHIR γp K + Λ CLAS g11 preliminary SAPHIR cosθ CM (π - ) E γ (GeV) 29

30 CLAS published data on γd p K K + (n) Events/10 MeV γ d Θ + n K Re-scattering n K + p K MM(K - p) (GeV/c 2 ) 30

31 High Statistics CLAS(d) result preliminary M(nK + )(GeV) Model-independent uppper limit 95% CL for Θ + is < 20nb. With assumptions about the spectator, we can set a modeldependent upper limit to the cross section of < 4-5 nb. 31

32 Search for the Θ + (1540) on proton target γ p Θ + K 0 The K 0 is detected via its K S component decaying into π + π - n K + π + π Final state neutron is identified using the missing mass technique Strangeness is tagged detecting the K + Using the full statistics (70 pb -1 ) a total of ~ events are selected Counts Counts K S n M(π + π )(GeV) M x (π + π K + X)(GeV) 32

33 Upper Limit on the Θ + Yield Counts/4 MeV Θ + (1540)? Counts/4 MeV Counts/4 MeV preliminary -0.8 < cosθ CM < < cosθ CM < 0.8 M(nK + )(GeV) M(nK + )(GeV) the nk + mass spectrum is smooth no structure is observed at a mass of ~1540 MeV M(nK + )(GeV) 33

34 Upper limit to the cross section γp Θ + K 0 preliminary 34

35 Summary and Outlook Parity violation experiments are on the edge of demonstrating that strange quarks contribute to the static properties of the nucleon. G G s E s M = ± = ± 0.31 Outlook: HAPPEX errors will be reduced by 1/3 this year G0 will take backward-angle measurements in 2006 The N- transition provides a testing ground of hadronic structure and continues to challenge our understanding of the simplest baryons. Outlook: Non-resonant contributions will be better constrained with π 0 and π + data polarization observables. New CLAS data find no Θ +, refuting original CLAS publication on deuterium and SAPHIR result on protons. But many positive claims of pentaquark states remain. Each must be evaluated on its own merits. Outlook: Hall A data has preliminary limits on Σ 5 and Θ + partners Data to search for Ξ 5 is being analyzed High-energy data on γp tentatively scheduled for