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Marc Vanderhaeghen Jefferson Laboratory / College of William & Mary

Overview of nucleon structure studies. Marc Vanderhaeghen Jefferson Laboratory / College of William & Mary. INPC07 Tokyo, June 3-8, 2007. nucleon form factors. (generalized) parton distributions spin, tomography. nucleon resonances Δ (1232),…. Pun05. Gay02.

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Marc Vanderhaeghen Jefferson Laboratory / College of William & Mary

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  1. Overview of nucleon structure studies Marc Vanderhaeghen Jefferson Laboratory / College of William & Mary INPC07 Tokyo, June 3-8, 2007

  2. nucleon form factors (generalized) parton distributions spin, tomography nucleon resonances Δ(1232),…

  3. Pun05 Gay02 proton e.m. form factor : status green : Rosenbluth data (SLAC, JLab) JLab/HallA recoil pol. data new JLab/HallC recoil pol. exp. (2008) : extension up to Q2 ≈ 9 GeV2

  4. neutron e.m. form factor : status MAMI JLab/HallC JLab/CLAS JLab/HallA new MIT-Bates (BLAST) data for both p and n at low Q2 : see talk M. Kohl (D1-1) new JLab/HallA double pol. exp. (spring 07) : extension up to Q2 ≈ 3.5 GeV2 completed

  5. nucleon form factors : pion cloud Friedrich, Walcher (2003) phenomenological fit : “smooth” part (sum of 2 dipoles) + “bump” (gaussian) 6 parameter fit for each FF pion cloud pronounced structure in all FF around Q  0.5 GeV/c

  6. nucleon FF : lattice prospects F1V state of art : connected diagrams -> OK for isovector quantities LHPColl. full QCD lattice calculations Pion masses down to less than 300 MeV √(r2)1V chiral extrapolation to physical mass Leinweber, Thomas, Young (2001) next step : inclusion of disconnected diagrams see talks S. Ohta (D4-5), A. Schäfer

  7. Two-photon exchange effects Rosenbluth vs polarization transfer measurements of GE/GM of proton SLAC, Jlab Rosenbluth data Jlab/Hall A Polarization data Jones et al. (2000) Gayou et al. (2002) Two methods, two different results ! 2γ exchangeproposed as explanation Guichon, Vdh (2003)

  8. Two-photon exchange calculations partonic calculation elastic contribution N GPDs Chen, Afanasev, Brodsky, Carlson, Vdh (2003) Blunden, Melnitchouk, Tjon (2003, 2005)

  9. Generalized Parton Distributions * Qhard2 large t = Δ2 low –t process : -t << Qhard2 Ji , Radyushkin (1996) x + ξ x - ξ P - Δ/2 P + Δ/2 GPD (x, ξ ,t) (x + ξ) and(x - ξ): longitudinal momentum fractions of quarks at large Q2 : QCD factorizationtheorem hard exclusive process can be described by 4 transitions(GPDs) : ~ Vector :H (x, ξ ,t) Tensor :E (x, ξ ,t) Axial-Vector :H (x, ξ ,t) Pseudoscalar :E (x, ξ ,t) ~

  10. Δ P - Δ/2 P + Δ/2 known information on GPDs forward limit : ordinaryparton distributions unpolarized quark distr polarized quark distr : do NOT appear in DIS new information first moments : nucleonelectroweak form factors Dirac Pauli axial ξ independence : Lorentz invariance pseudo-scalar

  11. Why GPDs are interesting Unique tool to explore the internal landscape of the nucleon : 3D quark/gluon imaging of nucleon Access to static properties : constrained (sum rules) by precision measurements of charge/magnetization orbital angular momentum carried by quarks

  12. GPDs yield 3-dim quark structure of nucleon Burkardt (2000, 2003) Belitsky, Ji, Yuan (2004) Elastic Scattering transverse quark distribution in coordinate space DIS longitudinal quark distribution in momentum space DES (GPDs) fully-correlated quark distribution in both coordinate and momentum space

  13. electromagnetic form factors PROTON NEUTRON modified Regge GPD parametrization 1 : Regge slope -> protonDirac (Pauli) radius 2, 3 :large x behavior of GPD Eu, Ed ->large Q2 behavior of F2p, F2n 3-parameter fit Guidal, Polyakov, Radyushkin, Vdh (2005) world data (2006) also Diehl, Feldmann, Jakob, Kroll (2005)

  14. proton Dirac & Pauli FFs : GPD framework PQCD modified Regge GPD model data : SLAC data : JLab/HallA data : JLab/HallA Belitsky, Ji, Yuan (2003) Guidal, Polyakov, Radyushkin, Vdh (2005)

  15. GPDs : transverse image of the nucleon (tomography) Hu(x, b? ) x b?(fm)

  16. quark contribution to proton spin X. Ji (1997) with parametrizations for E q : GPD : based on MRST2002 μ2 = 2 GeV2 lattice : full QCD, no disconnected diagrams so far

  17. e e e e  Z p p p p e e 2  p p NucleonstrangenessFFs PV e-scattering APV = Q2 = 0.1 GeV2 re-analysis Young et al. (2006) GMs = 0.28 ± 0.20 GEs = -0.006 ± 0.016 GMs = -0.01 ± 0.25 GEs = +0.002 ± 0.018

  18. NucleonstrangenessFFs : Q2 dependence Forward angle e-p data • Rapid variation at low Q2 unlikely • Await backward angle measurements from A4, G0 • Deuterium running will provide constraints on GA • One high precision point at Q2~0.6 (HAPPEX III: run in 2009)

  19. DIS fit :ν - ν(CDHS data) x (s – s) NucleonstrangenessFFs : interpretation lattice GMs (0) = -0.046 ± 0.019 μNLeinweber et al. (2005) GEs (0.1) = 0.001 ± 0.004 ± 0.004Leinweber et al. (2006) + charge symm. constraints GPD F1s Regge slope :α’ ≈ 0.95 GeV2 data (Young et al.) : 0.002 ± 0.024 Barone, Pascaud, Zomer (2000) CCFR + NuTeV data (2001) Vdh ( PAVI2002 )

  20. t g* g,M,... x~xB x ~ ~ H,E,H,E p p’ Beam or target spin asymmetries contain only ImT, i.e. GPDs at x = x and -x Cross sections and charge asymmetries measurements (ReT) Integral of GPDs over x link GPDs and observables

  21. DVCS : beam spin asymmetry ALU = (BH) * Im(DVCS) * sin Φ GPDs Bethe-Heitler DVCS Q2 = 1 – 1.5 GeV2 , xB = 0.15 – 0.25, -t = 0.1 - 0.25 GeV2 Q2 = 2.6 GeV2 , xB = 0.11, -t = 0.27 GeV2 JLab/CLAS (2001) HERMES (2001) twist-2 + twist-3 : Kivel, Polyakov, Vdh (2000)

  22. Bethe-Heitler DVCS on protonJLab/Hall A @ 6 GeV DVCS GPDs Difference of polarized cross sections Q2 ≈ 2 GeV2 xB = 0.36 Unpolarized cross sections Muñoz-Camacho, Camsonne (2006) -> see talk P. Bertin (D4-2)

  23. DVCS on proton : JLab/Hall A @ 6 GeV • Twist-2terms dominate the cross section and are independent ofQ2in the explored kinematical domain • The contribution to the cross section oftwist-3terms issmalland isindependent of Q2in the limit of error bars indication in favor of factorization already from Q2=2 GeV2 in the valence region

  24. DVCS on proton : JLab/CLAS @ 6 GeV Girod, Jo (2007)

  25. DVCS on neutron Mazouz (2006) 0 because F1(t) is small 0 because of cancelation of u and d quarks n-DVCSgives access to the least known and constrained GPD,E JLab /Hall A (E03-106) : preliminary data

  26. H1, ZEUS Large phase space(x,t,Q2) and High luminosity required Valence region Sea/gluon region EIC JLab12 high xB only reachable with high luminosity JLab Upgrade at 12 GeV, CEBAF will be ideal for GPD studies in valence quark regime

  27. exclusive DVCS : BSA @ JLab 12 GeV e p epg L = 1x1035 T = 2000 hrs DQ2 = 1 GeV2 Dx = 0.05 CLAS12 in HallB Projected results E = 11 GeV increase luminosity tenfold to > 1035 cm-2s-1 DsLU~sinfIm{F1H+..}df selected kinematics

  28. Sphere: Prolate: Q20=0 Q20/R2 > 0 Oblate: Q20/R2 < 0 electromagnetic N -> Δ(1232) transition J P=3/2+ (P33), M' 1232 MeV,  ' 115 MeV N ! transition:  N !  (99%),  N !  (<1%) non-zero values for E2 and C2 : measure of non-spherical distribution of charges spin 3/2 Role of quark core (quark spin flip) versus pion cloud

  29. Q2 dependence of E2/M1 and C2/M1 ratios data points : M1 MIT-Bates (Sparveris et al., 2005) MAMI : Q2 = 0 (Beck et al., 2000) Q2 = 0.06 (Stave et al., 2006) Q2 = 0.2 (Elsner et al., 2005, Sparveris et al., 2006) E2/M1 EFT calculation predicts the Q2 dependence C2/M1 no pion loops pion loops included Pascalutsa, Vdh (2005) also Gail, Hemmert

  30. mπ dependence of E2/M1 and C2/M1 ratios Q2 = 0.1 GeV2 quenched lattice QCD results : at mπ= 0.37, 0.45, 0.51 GeV linear extrapolation in mq ~ mπ2 Nicosia – MIT group :Alexandrou et al. (2005) EFT calculation discrepancy with lattice explained by chiral loops(pion cloud)! Pascalutsa, Vdh (2005) data points : MAMI, MIT-Bates

  31. Summary Nucleon form factors : -> high precision data at low Q2 : map out pion cloud of nucleon -> difference Rosenbluth vs polarization data GEp /GMp : mainly understood as due to two-photon exchange effects (new expt. planned) -> PV e-scattering : strangeness contributions to E and M distributions very small -> lattice QCD : state-of-art calculations go down to mπ~ 300 MeV, into the regime where chiral effects are important GPDs : -> unifying theme in hadron physics (form factors, parton distributions) -> provide a tomographic image of nucleon -> access to angular momentum of quarks/gluons in nucleon -> encouraging experimental results coming out of HERMES, H1/ZEUS, JLab@6 GeV indicating twist-2 dominance -> future programs : COMPASS, dedictated JLab@12 GeV, EIC… Nucleon excitation spectrum : -> precision data on NΔform factors : shape of hadrons -> chiral EFT is used in dual role : describe both observables and use in lattice extrapolations strong non-analytic behavior in quark mass due to opening of πN decay channel (interplay of scales)

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