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Hadron structure with electromagnetic probes

Hadron structure with electromagnetic probes. Marc Vanderhaeghen Johannes Gutenberg Universität, Mainz. Graduate School HANUC Jyväskylä, August 25-29, 2008. TexPoint fonts used in EMF. Read the TexPoint manual before you delete this box.: A A A A. nucleon form factors. (generalized)

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Hadron structure with electromagnetic probes

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  1. Hadron structurewith electromagnetic probes MarcVanderhaeghen Johannes Gutenberg Universität, Mainz Graduate School HANUC Jyväskylä, August 25-29, 2008 TexPoint fonts used in EMF. Read the TexPoint manual before you delete this box.: AAAA

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

  3. Outline lectures Ia. Introduction : some “key” questions in hadron structure physics Ib. Nucleon electromagnetic form factors, two-photon exchange, quark transverse charge densities II. Generalized Parton Distributions (GPDs) III. Nucleon excitation spectrum, chiral EFT in the Δ(1232) region, and time-like processes

  4. Strong QCD : from quarks to hadrons nucleon : many body system of quarks, anti-quarks, gluons quarks come in 3 color charges dynamics : non-abelian gauge theory ( QCD ) ’independent’ nucleon motion in nuclei ( BE ≈ 1 % ) collectivenuclear behavior QCD phase diagram :quark-gluon plasma

  5. Ia. Some “key” questions in hadron structure research 1) Why can quarks not be isolated (confinement) ? 2) How can we ‘see’ quarks (quark-gluon structure) ? 3) What is the origin of the mass of the hadrons ? 4) Which symmetries govern hadron structure ? Role of effective degrees of freedom : (pion cloud, constituent quarks, flux tube, …)

  6. - + 1) Why can quarks not be isolated ? color confinement : quarks & gluons are not observed as free particles QED electric dipole field analogy : behavior when quarks are at short distance QCD behavior when quarks are taken apart vacuum acts as a dual superconductor ( ‘t Hooft, …) electric field lines are trapped (confined) in a flux tube ( Nambu)

  7. Numerical evidence for confinement : potential between quarks lattice QCD simulations with dynamical quarks pure glue mq = ∞ SU(3)f T= 0.97 Tc Tc ≈ 154 ± 8 MeV mπ= 800 MeV mπ= 380 MeV flux tube Karsch et al. QCD string tension r in fm 0.3 0.7 potential is screened string breaking

  8. Lattice QCD Simulation of Vacuum Structure Leinweber, Signal et al.

  9. 2) How can one ‘see’ quarks ?

  10. Inclusive Deep Inelastic Scattering of leptons from nucleon x structure functions F1, F2 (unpolarized) g1, g2 (polarized) Bjorken scaling

  11. u u d * c c p X How does the nucleon get its spin ? Quark helicity contribution CERN, SLAC, DESY quark spin explains 20–30 % of nucleon spin Gluon contribution ΔG HERMES, COMPASS, RHIC Spin orbital contribution Lq accessible through Generalized Parton Distributions (GPD) x1 x2 JLab, HERMES, COMPASS

  12. 3) Origin of mass of the nucleon & spectrum of hadrons MASS GAP mu , md ≈ 5 – 10 MeV, ms ≈ 150 MeV QCD ground state : large condensates

  13. Nucleon mass : lattice QCD calculations MILC (2001) : domain wall valence quarks, staggered sea ETMC (2008) : dynamical Wilson quarks physical world nucleon mass in a world where the quark mass is zero : MN0 = 0.861 – 0.868 GeV ETMC (2008)

  14. 3) Origin of mass of the nucleon & spectrum of hadrons spectroscopy : meson ( qq ) baryon (qqq) MAMI, ELSA, JLab,… penta-quark ( qqqqq ) MΘ+ = 1.54 GeV ?? MASS GAP glueball PANDA@FAIR lattice : M0++ ≈ 1.5 GeV hybrid ( qqg ) e.g. 0+- , 1-+ mu , md ≈ 5 – 10 MeV, ms ≈ 150 MeV COMPASS , JLab (12 GeV) QCD ground state : large condensates q = u, d, s : q = c (ccg) : PANDA@FAIR

  15. QCD: Exotics QCD predicts the existence of exotic mesons Glueballs are mesons without valence quarks “pure gluon” states emblematic of non-Abelian nature of QCD (lattice QCD predicts mass for 0++ around 1.5 GeV) Other exotics involve excitation or vibration of gluons

  16. Glueballs • Glueballs are “pure gluon” states emblematic of non-Abelian nature of QCD • Aim to compute masses of lowest few states of given quantum number Morningstar and Peardon PRD60, 034509 lattice glueball calculations provide road-map (N.B. r0-1 ~ 0.4 GeV)

  17. Exotics : BNL / GSI (PANDA@FAIR) / JLab (Hall D) • Computations in heavy-quark sector- insight into excitations of the string • For heavy quarks, energy associated with “excited string” of around 1 GeV • Lowest 1-+ state around 1.8-2.0 GeV

  18. Glueballs and hybrid mesons

  19. u u d d 4) Which symmetries govern nucleon structure ? mu , md << MN mu = md ≈ 0 L R left and right quarks decouple SU(2)L x SU(2)R chiral symmetry / ms small : approximate SU(3)L x SU(3)R chiral symmetry NOT manifest in spectrum(no parity doublets) spontaneously broken into SU(2)V (massless) Goldstone bosons Goldstone bosons : weakly interacting at low energies low energy effective field theory (ChPT) : expansion in mπ/Λχ ,pπ/Λχ role of pion cloud surrounding nucleon many precision experiments

  20. Ib. Nucleon e.m. form factors, two-photon exchange and quark charge densities data : nucleon e.m. form factors two-photon exchange processes lattice QCD calculations fornucleon FFs quark transverse charge densities fromnucleon e.m. FFs and deuteron e.m. FFs

  21. Pun05 Gay02 proton e.m. form factor : status green : Rosenbluth data (SLAC, JLab) JLab/HallA recoil pol. data new JLab/HallC recoil pol. exp. (spring 2008) : extension up to Q2 ≈ 8.5 GeV2 new MAMI/A1 data up to Q2 ≈ 0.7 GeV2

  22. 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 new JLab/HallA double pol. exp. (spring 07) : extension up to Q2 ≈ 3.5 GeV2 completed

  23. neutron GE at largerQ2 new JLab/HallA double pol. exp. E02-013 : preliminary results at Q2 ≈ 1.7 and 3.5 GeV2

  24. 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)

  25. 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)

  26. Elastic eN scattering beyond one-photon exchange approximation Kinematical invariants : (me = 0) equivalently, introduce

  27. Observables including two-photon exchange Real parts of two-photon amplitudes

  28. Phenomenological analysis Guichon, Vdh (2003) 2-photon exchange corrections can become large on the Rosenbluth extraction,and are of different size for both observables relevance when extracting form factors at large Q2

  29. Normal spin asymmetries in elastic eN scattering directly proportional to the imaginary part of 2-photon exchange amplitudes spin of beam OR target NORMAL to scattering plane OR on-shell intermediate state order of magnitude estimates : target : beam :

  30. Beam normal spin asymmetry New MAMI A4 data at backward angles Ee = 0.300 GeV Θe = 145 deg Ee = 0.570 GeV Θe = 35 deg Ee = 0.855 GeV Θe = 35 deg data : MAMI A4 theory : Pasquini & Vdh (2004) also : SAMPLE, Happex, G0, E-158

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

  32. 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; Olympus/Doris with refurbished BLAST detector Hall C 05-017; being analyzed by-product of 04-019/04-108? Hall A 05-015 (3He ) whether two-photon exchange is entirely responsible for the discrepancy in the FF extraction is to be determined experimentally

  33. test ofε-dependence of Pt / Pl new JLab/Hall C data (2008) PRELIMINARY, not to be quoted 1γ result for Pt / Pl The preliminary data for Q2=2.5 GeV2 show no ε-dependence of GEp/GMp at the 0.01 level

  34. 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

  35. LHPC results ( Lattice 2008 ) valence DWF on Asqtad staggered sea GEV new mπ = 293 MeV factor 4 reduction in error modest mπ dependence Puzzle: no strong chiral behavior expected around Q2 ≈ 1 GeV2 , however more than factor 2 deviation with data ! <r12>V

  36. RBC results ( Lattice 2008 ) mπ= 0.330 GeV mπ= 0.420 GeV mπ= 0.560 GeV Nf = 2 + 1 degenerate dynamical flavors of DWF mπ= 0.670 GeV F1V F2V

  37. Nucleon densities and relativity rest frame Breit frame - q / 2 qL q + q / 2 qL relative velocity between frames : Lorentz contraction factor :

  38. Nucleon densities and relativity intrinsic FF rest frame density non-rel : importance of relativity limit : k = 2 M (Compton wavelength) Kelly (2000)

  39. quark transverse charge densities in nucleon (I) light-front q+ = q0 + q3 = 0 z p p’ photon only couples to forward moving quarks quark charge density operator longitudinally polarized nucleon Miller (2007)

  40. quark transverse charge densities in nucleon (II) transversely polarized nucleon transverse spin e.g. along x-axis : dipole field pattern Carlson, Vdh (2008)

  41. empirical quark transverse densities in proton ρT ρ0 induced EDM : dy= F2p (0) . e / (2 MN) data : Arrington, Melnitchouk, Tjon (2007) densities : Miller (2007); Carlson, Vdh (2007)

  42. empirical quark transverse densities in neutron ρT ρ0 induced EDM : dy= F2n (0) . e / (2 MN) data: Bradford, Bodek, Budd, Arrington (2006) densities : Miller (2007); Carlson, Vdh (2007)

  43. empirical quark transverse densities in deuteron λ= ± 1 λ= 0 separated data up to 2 GeV2 : Abbott et al. (2000) densities : Carlson, Vdh (2008)

  44. longitudinally polarized deuteron ρ1 ρ0 λ= 0 λ= ± 1 deuteron equidensity surfaces ( ρd = 0.24 fm-3 ) from Argonne v18 : Forest et al. (1996)

  45. transversely polarized deuteron ρT0 ρT1 data : Abbott et al. (2000) densities : Carlson, Vdh (2008)

  46. transversely polarized spin-1 electric dipole moment for spin-1 point particle (e.g W, Z bosons) : g = 2GM (0) = 2 electric quadrupole moment for spin-1 point particleGM (0) = 2 and GQ(0) = -1 [exp. test by D0 Coll. (2008) ] transverse charge densities depend only on anomalous values of e.m. moments reflect internal structure

  47. transversely polarized deuteron experiment :

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