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Outline. Introduction FF from Rosenbluth and recoil polarization The proton Form Factor “discrepancy” New Form Factor data: Proton and Neutron Recently completed experiments in Hall C and A at JLab Conclusions

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Outline

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  1. Outline Introduction FF from Rosenbluth and recoil polarization The proton Form Factor “discrepancy” New Form Factor data: Proton and Neutron Recently completed experiments in Hall C and A at JLab Conclusions (see also review in “Progress in Particle and Nuclear Physics”, C.F.P., V. Punjabi and M. Vanderhaeghen, Prog. Part. Nucl. Phys. 59, 694-764, 2007) and other reviews: J. Arrington et al.(06); C. Hyde-Wright and K. de Jaeger (04); H. Gao (03)

  2. Introduction • At large Q2 electromagnetic Form Factors contain structure information on the many-body system of quarks and gluons of the nucleon. At low Q2 they inform us about the pion cloud. • When obtained from experiment, the Form Factors are relativistic invariants only to the extent that the probe is asinglevirtualphotonexchanged between electron and nucleon; higher order contributions destroy this invariance, which one might regain after applying a number of radiative corrections; the current status of these corrections is unsatisfactory.

  3. ep elastic in Born approximation J=<p’||p> j=<e’||e> Nucleon vertex: Dirac Pauli F1 helicity conserving , F2 helicity non-conserving form factors Alternately, the Sachs form factors GE(Q2)=F1(Q2)- F2(Q2) GM(Q2)=F1(Q2)+F2(Q2) Traditionally, it is assumed that in the Breit frame, and for very small Q2, GE and GM are Fourier transforms of charge- and current distributions.

  4. Rosenbluth vs. Recoil Polarization Cross section with Reduced cross section: Recoil polarization components Form Factor ratio:

  5. all Rosenbluth separation data Divided by the dipole form factor GD=(1-Q2/0.71)-2

  6. Recoil Polarization Results

  7. So what is the cause for the different results? First, radiative corrections at large Q2 are large and strongly ε-dependent. R=[(1+)/][exp/Mott]= =G2Mp+ G2Ep/ green for 1.75 GeV2 blue for 3.75 GeV2 redfor 5 GeV2 Data from Andivahis et al. (1994)

  8. Second, there is a scatter in size of calculated corrections Andivahis et al: based on Mo and Tsai, with improvements from Walker et al. Vdh: code similar to Maximon and Tjon: with realistic energy cut, external radiative correction included. Bystritskiy, Kuraev, Tomasi-Gustafsson:with Drell-Yan structure structure function (Drell-Yan parton picture). RC for electron to all orders. 5 GeV2 Afanasev et al: two-photon correction Interpolation from Hall A recoil polarization, GMp from Kelly fit

  9. Third, two-(hard)photon exchange might play a role. Afanasev, Brodsky,Carlson, Y.C. Chen, Vanderhaeghen, GPDs fitted to FF data, Guidal et al.(04) Blunden Melnitchouk and Tjon (05): intermediate state a proton, includes finite size effects: effect on Pt order ≤ 3 %, increases with Q2

  10. Whether two-photon exchange is entirely responsible for the FF “crisis” is of course to be determined experimentally For example: Real part of Y2γ • ε-independence of GEp/GMp in recoil polarization • cross section difference in e+ and e- proton scattering • non-linearity of Rosenbluth plot Also imaginary part • from induced out-of-plane polarization • single-spin target asymmetry Hall C 04-019, completed Hall B 07-005; also Olympus/Doris with refurbished BLAST detector Hall C 05-017; being analyzed by-product of 04-019/04-108? Hall A 05-015 (3He )

  11. Two-Gamma 04-019 PRELIMINARY, not to be quoted Some model calculations predict a greater sensitivity to 2γ for the polarization than for the cross section data The preliminary data for Q2=2.5 GeV2 show no ε- dependence of GEp/GMp at the 0.01 level

  12. Electric FF of the neutron Two of the Hall A preliminary data shown (02-013), anticipated error for two more (5/2008) All polarization results, including new Bates/BLAST data (Geis 2008)

  13. Magnetic FF of the neutron PRELIMINARY Preliminary Bates/BLAST results d(e,e’), Meitanis et al

  14. The form factors of the proton GEp/GD from selected polarization experiments, compared to the Kelly fit GMp from polarization GEp/GMp, (Brash et al) Magnification of the small Q2 region of GEp/GD

  15. Proton: F2 /F1 and pQCD Guidal et al (05): 3 parameters model of GPDs Brodsky and Farrar (75): Belitsky, Ji and Yuan (03): Q2F2/F1 constant Q2F2/F1 ln2(Q2/2) ! !

  16. New Theoretical Results De Melo, Frederico, Pace, Pisano, Salmè: light front CQM with qqbar from Z-diagram, the source of the zero crossing. Zero crossing still near 9 GeV2 when GEp data taken out of fit

  17. Zero crossing of GEpis natural! Argument of F. Gross and collaborator (Gross, Ramalho and Peňa, 2008): zero crossing is quite natural, unlike the defunct “scaling” behavior. Simple argument: as long as F1 and F2 are positive, and Q2F2/F1 behavior supports that, GE=F1-τF2 must become negative somewhere! Gross al et al: oversimplifying GE/GM = (f1 - τf2)/(f1 + f2)= (1- τκ)/ (1 + τκ). f1 and f2 are quark Dirac and Pauli FF, κ is quark anomalous magnetic moment, approximately 2. The zero crossing is then at Q2=2 GeV2!

  18. New Theoretical Results: 2γ Kondratyuk and Blunden added 5 low lying resonances to previous calculation with nucleon and Δ using polarization data as “Born” FF: higher resonances ~ cancel each other. Bystricky, Kuraev, Tomasi-Gustafsson, use structure function (Drell-Yan parton picture) method: includes RC for electron to all order. Say two-photon contribution negligible! Borisyuk and Kobushkin evaluate 2γ exchange in dispersion relation approach; different formalism, results similar to Arrington, Melnitchouk and Tjon (2007) Jain, Joglekar and Mitra use formalism of nonlocal field theory; one free parameter: predict an ε dependence for the GEp /GMp ratio of polarization transfer (Definitively not seen)

  19. GEp(III), 04-108 8.54 GeV2point: 181.3 Cb, 1.63 billion triggers collected New equipment worked beautifully: BigCal and FPP Analyzing power at 5.4 GeV/c close to Dubna value 6.8 GeV2point: 160 million triggers 5.2 GeV2point: A test of the spin transport at χ=180o

  20. Recent and future FF 04-108 (GEp(III)) has just been completed in Hall C; promising data at 5.2, 6.8 and 8.5 GeV2 04-019 (2-gamma) also just completed in Hall C; very small error bars on GEp/GMp at 2.5 GeV2, 3 values of ε. Very recent Hall A investigation of GEp/GMp by recoil polarization, in range 0.3<Q2<0.7 GeV2, with small statistical uncertainty Last year GEn in Hall A, Q2 of 1.20, 1.65, 2.48 and 3.43 GeV2; data are currently being analyzed! • After the 12 GeV upgrade • LOI for Hall C GEp/GMp to 13 GeV2, with SHMS and BigCal, LOI 12-06-103, to be submitted at next PAC • Approved experiment E12-07-109, GEp/GMp to 15 GeV2 with new large acceptance Multi Purpose Spectrometer (MPS), to be built with single dipole and GEM trackers

  21. GEp/GMp with 12 GeV at JLab

  22. FPP and BigCal in Hall C 2001….2008 F. Wesselman, M. Meziane (WM student) L. Pentchev, A. Puckett (MIT student), W. Luo (Lanzhou U. student)

  23. Actual performance of FPP and BC Q2=6.8 GeV2 From P HMS σ=0.3o From θHMS σ=0.56o Angular resolution from the Calorimeter BigCal Azimuthal dependence of asymmetry in FPP at a Q2 of 6.8 GeV2

  24. Conclusion, Perspective Both experimental characterization and phenomenological understanding of the structure of the proton, have changed drastically since 1998, year of the first recoil polarization experiment in Hall A. Rapid decrease of GEp with Q2 not a surprise: predicted in at least 3 papers: Iachello Jackson and Lande (73) with VMD, Frank, Jennings and Miller (96) withCQM, and Holzwarth (96) with chiral soliton. Full understanding of implications of the JLab finding not in yet. Where does the pQCD behavior start? are the nucleons spherical in their ground state? Are we “really” seeing the effect of quark orbital angular momentum? Also under development is a full understanding of two-photon effects, and revision of standard radiative correction calculation codes. The hope is that lattice QCD predictions will soon catch up with the data!

  25. The End

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