1. Elton S. Smith 1 JLab: Probing Hadronic Physics with Electrons and Photons Elton S. Smith Jefferson Lab V Latinamerican Symposium on Nuclear Physics Santos, Brazil, September 2003 Introduction to JLab The shape of the proton Pentaquarks

2. Elton S. Smith V Latinamerican Symposium, Sep 1-5, 2003 2 Why use electron and photon probes? e e’ p p Electromagnetic interaction is well-known F(Q 2 ) Elastic Form FactorsInelastic transitions Size probed ~ 1/√Q 2

3. Elton S. Smith V Latinamerican Symposium, Sep 1-5, 2003 3 CEBAF @ 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  A ─ e polarization80%  Simultaneous operation in 3 halls L [cm -2 s -1 ] ─ A: Two High Resolution Spectrometers (p max =4 GeV/c10 39 ─ B: Large Acceptance Spectrometer for e and  induced reactions10 34 ─ C: Two spectrometers (p max = 7 and 1.8 GeV/c) + special equipment10 39

4. Elton S. Smith V Latinamerican Symposium, Sep 1-5, 2003 4 CEBAF accelerator site

5. Elton S. Smith V Latinamerican Symposium, Sep 1-5, 2003 5 Three Experimental End-Stations

6. Elton S. Smith V Latinamerican Symposium, Sep 1-5, 2003 6 G Ep : Electric form factor of the proton Goal: Determine the charge and current distributions inside the proton NY Times “Is a proton round?” from G.A. Miller u u d Naïve expectation is that the charge and currents are determined from the spatial distribution of quark charges and spins.

7. Elton S. Smith V Latinamerican Symposium, Sep 1-5, 2003 7 Charge distribution and Form Factors Radius (fm)Q 2 (GeV 2 /c 2 )  (r) F(Q 2 )  =0.94 GeV  =0.84 GeV  =0.74 GeV r   )( r e    8 3

8. Elton S. Smith V Latinamerican Symposium, Sep 1-5, 2003 8 Charge Distributions and Form Factors point exponential Yukawa Gaussian unity dipole pole Gaussian

9. Elton S. Smith V Latinamerican Symposium, Sep 1-5, 2003 9 Decomposition of the elastic cross section )( )2/(tan2 1 22 22 eMp Ep ns G GG d d           )()( )1( 2222 QGQG d d MpEp ns R         

10. Elton S. Smith V Latinamerican Symposium, Sep 1-5, 2003 10 Proton Form Factors pre-1998

11. Elton S. Smith V Latinamerican Symposium, Sep 1-5, 2003 11 Spin transfer reaction e p → e p e e’ p p’ N ˆ T ˆ L ˆ  )2/tan( 2 ' M EE P P G G ee l t Mp Ep   e

12. Elton S. Smith V Latinamerican Symposium, Sep 1-5, 2003 12 Transport through magnet

13. Elton S. Smith V Latinamerican Symposium, Sep 1-5, 2003 13 Azimuthal asymmetry in the polarimeter

14. Elton S. Smith V Latinamerican Symposium, Sep 1-5, 2003 14 G Ep from polarization transfer E93-027, E99-007 Perdrisat, Punjabi, Jones, Brash

15. Elton S. Smith V Latinamerican Symposium, Sep 1-5, 2003 15 World data for G Ep

16. Elton S. Smith V Latinamerican Symposium, Sep 1-5, 2003 16 Interpretation of new data F 2 (Q 2 ) is a spin-flip transition In the absence of quark angular momentum Quark orbital angular momentum essential to describe data

17. Elton S. Smith V Latinamerican Symposium, Sep 1-5, 2003 17 Pentaquark: Baryon with five quarks Goal: Determine quark content of colorless hadrons Expectation from the quark model is that the properties of baryons are determined by three valence quarks (qqq)

18. Elton S. Smith V Latinamerican Symposium, Sep 1-5, 2003 18 Hadron multiplets Baryons qqq Mesons qq Baryons built from meson-baryon basis ── ++ N  

19. Elton S. Smith V Latinamerican Symposium, Sep 1-5, 2003 19 Production and decay of  ─ →    ─

20. Elton S. Smith V Latinamerican Symposium, Sep 1-5, 2003 20 What are pentaquarks?  Minimum quark content is 5-quarks.  Anti-quark has different flavor than any of 4-quarks ( ).  Quantum numbers can not be defined by 3-quarks.  General idea of a five-quark states has been around since late 60’s.  However, searches did not give any conclusive results.  PDG dropped the discussion on pentaquark searches after 1988. Qqqqq

21. Elton S. Smith V Latinamerican Symposium, Sep 1-5, 2003 21 The Anti-decuplet predicted by Diakonov et al.

22. Elton S. Smith V Latinamerican Symposium, Sep 1-5, 2003 22 Reactions on deuterium pK nKnp nK  p  pn    K      )1520( )() ()( )()( * *   n  K─K─ K+K+ n ++ pp p  K+K+ K─K─ p  nn  N  (1020) N  K + K - N

23. Elton S. Smith V Latinamerican Symposium, Sep 1-5, 2003 23 CEBAF Large Acceptance Spectrometer Liquid D 2 (H 2 )target +  start counter; e minitorus Torus magnet 6 superconducting coils Drift chambers argon/CO 2 gas, 35,000 cells Time-of-flight counters plastic scintillators, 684 photomultipliers Gas Cherenkov counters e/  separation, 256 PMTs Electromagnetic calorimeters Lead/scintillator, 1296 photomultipliers

24. Elton S. Smith V Latinamerican Symposium, Sep 1-5, 2003 24 Exclusive measurement using  d reactions CLAS Collaboration (S. Stepanyan, K. Hicks, et al.), hep-ex/0307018  Requires FSI – both nucleons involved ─ No Fermi motion correction necessary ─ FSI puts K - at larger lab angles: better CLAS acceptance ─ FSI not rare: in ~50% of   (1520) events both nucleons detected with p > 0.15 GeV/c

25. Elton S. Smith V Latinamerican Symposium, Sep 1-5, 2003 25  d → p K + K ─ (n) in CLAS K-K- K+K+ p

26. Elton S. Smith V Latinamerican Symposium, Sep 1-5, 2003 26 Kaon times relative to proton pp  - p+-p+-  t (p-K ─ ) (ns)  t (p-K + ) (ns) pK + K ─

27. Elton S. Smith V Latinamerican Symposium, Sep 1-5, 2003 27 Reaction  d→pK + K - (n) Clear peak at neutron mass. 15% non pKK events within ±3  of the peak. Almost no background under the neutron peak after event selection with tight timing cut. Reconstructed Neutrons

28. Elton S. Smith V Latinamerican Symposium, Sep 1-5, 2003 28 Identification of known resonances  Remove events with IM (K  K  )   (1020) by IM > 1.07 GeV  Remove events with IM(pK  )   (1520)  Limit K + momentum due to  d  p K    phase space p K  < 1.0GeV/c  C. Meyer (CLAS note 03-009): checked narrow structure impossible in  d  K + Y*N  K + (K - N)N, + KN rescattering

29. Elton S. Smith V Latinamerican Symposium, Sep 1-5, 2003 29 nK + invariant mass distribution Distribution of  (1520) events ++

30. Elton S. Smith V Latinamerican Symposium, Sep 1-5, 2003 30  + : experimental status  Experimental evidence for  + have been reported at four laboratories. ─ LEPS collaboration at Spring-8 (Japan), January 2003 - peak in the invariant mass of the nK + at 1.54 GeV with statistical significance of 4.6 . ─ DIANA collaboration at ITEP (Moscow), April 2003 – peak in the invariant mass of pK 0 at 1.538 GeV, statistical significance 4.4 . ─ CLAS collaboration at JLAB, July 2003 – peak in the invariant mass of the nK + at 1.542 GeV, statistical significance 5.3 . ─ SAPHIR collaboration at ELSA (Bonn), August 2003 – peak in the invariant mass of the nK + at 1.54 GeV, statistical significance 4.8 .  All experiments observe a narrow width.  Spin, isospin and parity not yet established.  Subject of intense interest and research. ─ Penta-Quark 2003 Workshop at JLab in November.

31. Elton S. Smith V Latinamerican Symposium, Sep 1-5, 2003 31 Summary  We have presented two examples which highlight the physics program at Jefferson Lab.  The electromagnetic interaction can be used to probe deep into the structure of nucleons. ─ From measurements of G Ep up to a Q 2 = 5.6 GeV 2 we have gained new insights into the shape of the proton. ─ Orbital angular momentum of quarks is a key ingredient in our understanding of proton structure.  A key question in non-perturbative QCD is the structure of hadrons ─ We have presented evidence for an exotic baryon with S = +1, which would have a minimal quark content of five quarks (uudds). ─ This baryon represents a new class of colorless hadrons.

32. Elton S. Smith V Latinamerican Symposium, Sep 1-5, 2003 32 Scaled F 2 /F 1 ratio