1 / 48

Elastic Form Factors of the Proton, Neutron and Deuteron

INPC2007, Tokyo, June 3-8, 2007. Elastic Form Factors of the Proton, Neutron and Deuteron. Michael Kohl. MIT-Bates, Middleton, MA 01949 USA. Outline. Introduction, motivation and formalism Traditional and new techniques Overview of experimental data Theoretical calculations Proton

lafayette
Download Presentation

Elastic Form Factors of the Proton, Neutron and Deuteron

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. INPC2007, Tokyo, June 3-8, 2007 Elastic Form Factors of the Proton, Neutron and Deuteron Michael Kohl MIT-Bates, Middleton, MA 01949 USA

  2. Outline • Introduction, motivation and formalism • Traditional and new techniques • Overview of experimental data • Theoretical calculations • Proton • Low Q2: Pion cloud effect • High Q2: Proton form factor ratio and two-photon exchange Neutron • Electric and magnetic form factors Deuteron • Unpolarized structure functions A and B • Separation of charge and quadrupole form factor Nucleon Deuteron

  3. Elastic Electromagnetic Form Factors … • Fundamental quantities • Describe internal structure of the nucleons and the deuteron • Related to spatial distribution of charge and magnetism • Rigorous tests of nucleon and deuteron models • Ultimately calculable by Lattice-QCD • Input for nuclear structure and parity violation experiments • 50 years of ever increasing activity • Tremendous progress in experiment and theory over last decade • New techniques / polarization experiments • Unexpected results

  4. (Hadronic) Structure and (EW) Interaction Factorization! Structure Interaction s(structured object) |Form factor|2 = s(pointlike object) → Interference! Probe Object →Utilize spin dependence of electromagnetic interaction to achieve high precision Born Approximation Inelastic Elastic Structure Hadronic object Electroweak probe Interaction Lepton scattering

  5. The Beginnings R. HofstadterNobel prize 1961 ep-elastic Finite size of the proton ed-elastic Finite size + nuclear structure

  6. Nucleon Elastic Form Factors • General definition of the nucleon form factor • Sachs Form Factors • In One-photon exchange approximation above form factors are observables of elastic electron-nucleon scattering

  7. Rosenbluth Separation GE2 tGM2 θ=180o θ=0o

  8. GpE and GpM from Unpolarized Data • charge and magnetization density • Dipole form factor within 5% for Q2 < 10 (GeV/c)2

  9. Nucleon Form Factors and Polarization • Double polarization in elastic/quasielastic ep or en scattering:Recoil polarization or (vector) polarized target • Polarized cross section • Double spin asymmetry = spin correlation • Asymmetry ratio (“Super ratio”)independent of polarization or analyzing power 1,2H(e,e’p), 1,2H(e,e’p), 2H(e,e’n), 2H(e,e’n)

  10. Recoil Polarization Technique • Pioneered at MIT-Bates • Pursued at Hall A and MAMI A1 • In preparation for Hall C V. Punjabi et al., Phys. Rev. C71 (2005) 05520 Focal-plane polarimeter Secondary scattering of polarizedproton from unpolarized analyzer Spin transfer formalism to account for spin precession through spectrometer

  11. Proton Form Factor Ratio Jefferson Lab Dramatic discrepancy! • All Rosenbluth data from SLAC and Jlab in agreement • Dramatic discrepancy between Rosenbluth and recoil polarization technique • Multi-photon exchange considered best candidate

  12. Proton Form Factor Ratio Iachello 1973: Drop of the ratio alreadysuggested by VMD F. Iachello et al., PLB43 (1973) 191F. Iachello, nucl-th/0312074 1 mpGpE/GpM 0 0 2 4 6 8 10 Q2/(GeV/c)2 A.V. Belitsky et al., PRL91 (2003) 092003 G. Miller and M. Frank, PRC65 (2002) 065205 S. Brodsky et al., PRD69 (2004) 076001 Quark angular momentum Helicity non-conservation  Q F2/F1 = const. (contradicting pQCD)

  13. Two-Photon Exchange Two-photon exchange theoretically suggested • P.G. Blunden, W. Melnitchouk, and J.A. Tjon, PRC72 (2005) 034612, PRL91 (2003) 142304 • P.A.M. Guichon and M. Vanderhaeghen, PRL91, 142303 (2003) • M.P. Rekalo and E. Tomasi-Gustafsson,EPJA22 (2004) 331 • Y.C. Chen et al., PRL93 (2004) 122301 • A.V. Afanasev and N.P. Merenkov, PRD70 (2004) 073002 Experiment proposals to verify hypothesis: e+/e-: CLAS/PR04-116 Novosibirsk/VEPP-3 BLAST@DORIS/DESY SSA: PR05-15 e-dep.: PR04-119 (pol.), PR05-017 (unp.)

  14. New Measurements at high Q2 • Extension proposed at Jefferson Lab • Hall C PR01-109/PR04-108 recoil polarization to run in fall 2007 • Hall C PR05-017 Super-Rosenbluth Q2 = 0.9 - 6.6 (GeV/c)2 now running M.K. Jones et al., PRC74 (2006) 035201 Polarized Target: Independent verification crucial Polarized internal target / low Q2: BLAST

  15. BLAST at MIT-Bates Bates Large Acceptance Spectrometer Toroid • Symmetric, large acceptance, general purpose detectorDetection of e±, p±, p, d, n • Longitudinally polarized electrons in SHR850 MeV, 200 mA, Pe = 65% • Highly polarized internal gas target of pure H and D (Atomic Beam Source)6 x 1013 atoms/cm2, L = 6 x 1031/(cm2s), PH/D = 80%

  16. Separately prepare mI = +½, -½ (hydrogen) and mI = +1, 0, -1 (deuterium)with sextupoles and RF transitions • Switch between states every 5 minutes ABS polarized target Hydrogen Deuterium

  17. * Proton Form-Factor Ratio mpGpE/GpM C.B. Crawford et al., PRL98 (2007) 052301 • Impact of BLAST data combined with cross sections on separation of GpE and GpM • Errors factor ~2 smaller • Reduced correlation • Deviation from dipole at low Q2! *Ph.D. work of C. Crawford (MIT) and A. Sindile (UNH)

  18. New Measurements on Proton at low Q2 Hall A PR07-004 Recoil polarization LEDEX PR05-004, to be published soon Mainz A1 Rosenbluth

  19. Neutron Electric Form Factor GnE BLASTMC (e,e’n) (e,e’p) • GnE small, hard to measure,least known only to 10-20% • Amplification by interference with GnM • No free neutron target → quasi-elastic scattering 2H(e,e’n) • GnE(0) = Z = 0distribution of net-zero charge→ role of pion cloud • Input for interpretation of parity violation electron scattering

  20. * Neutron Electric Form Factor GnE • GnE world data fromunpolarized ed-elastic • GnE world data fromdouble pol. Experimentsincluding BLAST 2007 • BLAST fit<r2n> = -0.115 fm2→ Pion cloud effect? • Theoretical models • Dispersion theory+ error band PRELIMINARY *Ph.D. work of V. Ziskin (MIT) and E. Geis (ASU)

  21. Transverse radiusfrom GPD G. Miller, nucl-th/0705.2409 Neutron Electric Form Factor GnE Charge form factor ↔ Charge distribution PRELIMINARY Breit frame PRELIMINARY Forbidden interpretation asrest charge distribution r (fm)

  22. Future Measurements of GnE xMAMI 3He(e,e’n) GEn x

  23. d(e,e’n) d(e,e’p) Neutron Magnetic Form Factor GnM • Pre-polarization era • GnM world data fromunpolarized experiments • Cross section ratioquasielastic • Polarization era • GnM world data + 3He • + CLAS preliminary • VMD/Dispersion Theory + BLAST preliminary

  24. Nucleon Elastic Form Factors Before JLab and recent non-JLab Data

  25. Nucleon Elastic Form Factors Today, with Available JLab Data Data taken; analysis underway

  26. Nucleon Elastic Form Factors Today, with Available JLab Data and Planned GEp Extension

  27. Nucleon Elastic Form Factors EM form factors provide a testing ground for theories constructing nucleons from quarks and glue

  28. Deuteron

  29. Deuteron Degrees of Freedom e’ Nucleon-nucleon potential: from one-pion exchange to short-range repulsion p e’ e n p Beyond nucleons and mesons: Isobar configurations, 6-quarks cluster (hidden color)? p,r,w e’ e n Isoscalar meson-exchange currents e e’ e Asymptotic regime: perturbative QCD Low Q Intermediate Q High Q

  30. NN Interaction Potential Ms=0 Ms= 1 Structure ↔ Interaction • Phenomenological NN potentials: OPE + 2-body + … PD=4-6% ↔ tensor force • Bound state (structure) generatedby interaction potential minimum • Repulsive core • Qd problem / pNN coupling Tensor force generates bound deuteron state

  31. * * Elastic Electron-Deuteron Scattering • Spin 1 ↔ three elastic form factors GdC, GdQ, GdM • Quadrupole momentM2dQd = GdQ(0) = 25.83 • GdQ↔ Tensor force, D-wave • Unpolarized elastic cross section • Polarized cross section

  32. A and B

  33. Deuteron Structure Function A(Q2) Gilman and Gross, JoPG28 (2002) R37 High Q2: Hall A / C results compatible but shifted by ~10% Hall A Hall C Low Q2: Mainz/Saclay discrepancy: 8% in A, 100% in GnE Jlab Hall A / PR05-004: Data 2006, analysis in progress Simon 1981 LEDEX Platchkov 1990

  34. Deuteron Structure Function B(Q2) • High-Q: First minimum unconfirmed • Location of node and of second maximumquite uncertain Measurements on B to be extended up to ~12 fm-1 (Petratas, PAC 2008) Low-Q: Same Mainz data consistent

  35. Deuteron Polarization Experiments The deuteron cross section depends on the deuteron alignment Polarized target (T20) Recoil polarization (t20) d d e e e’ e’ Atomic gas in storage ring Novosibirsk NIKHEF Bates/BLAST Cryogenic solid target in extracted beam Bonn Bates/Argonne3He(d,p) Bates/AHEADH(d,p) JLab/POLDER H(d,pp) External calibration required Polarimeter analysis challenging Target polarization measurement challenging Data analysis “straightforward”

  36. Tensor-pol. Elastic ed Scattering • Tensor asymmetry and tensor analyzing powers • T20dominant, T21 significant, T22small • Global fit analysis to determine GdC, GdQand GdMfrom world data + BLAST

  37. * Tensor Analyzing Power T20 PRELIMINARY *Ph.D. work of C. Zhang (MIT)

  38. * Tensor Analyzing Powers T20 and T21 PRELIMINARY PRELIMINARY *Ph.D. work of C. Zhang (MIT)

  39. GQ(Q) GC and GQ PRELIMINARY GC(Q)

  40. Node of GC PRELIMINARY GC(Q)

  41. Node of GC GC(Q) Node of GC discovered 1994 at MIT-Bates Location confirmed with Jlab, VEPP-3, and BLAST 4.19 ± 0.05 fm-1 PRELIMINARY

  42. Vector-pol. Elastic ed Scattering Unpolarized cross section Polarized cross section Polarization observables Can extract GM independently of back angles measurements (B) GC GQ GM Spin 1

  43. Vector-pol. Elastic ed Scattering • Beam-target vector asymmetry and vector spin correlation parameters • Te10small, Te11 dominant • Determine GdM at low Q2 from Te11,T20 and world data on A(Q2)

  44. * Vector observables Te10, Te11, and GdM PRELIMINARY PRELIMINARY *Ph.D. work of P. Karpius (UNH)

  45. Nonrelativistic potential models From the impulse approximation (NRIA) to the inclusion of meson-exchange currents

  46. Relativistic calculation framework Quantum Field Theory (explicitly covariant) Quantum Mechanics (equation of motion from different representations of Poincaré group) Bethe-Salpeter (4D) 3D-reductions Light-front dynamics Light-front form Point form Instant form Chung…, Strikman… Lev, Pace, Salme Spectator nucleon on-shell Integration over relative energy (ET) Allen, Klink, Polyzou… Gross, Van Orden … Devine, Phillips, Wallace Carbonell, Karmanov... Cooke, Miller… Forest, Schiavilla 2 nucleons equally off mass shell Hummel, Tjon…

  47. Chiral Effective Field Theory The Qd problem D. Phillips, J. Phys. G 34, 365 (2007) BLAST (preliminary)

  48. Summary • Nucleonand Deuteron electromagnetic elastic form factors • Tremendous progress during last decade High precision, low systematics through polarization experimentsWorldwide activity at its peak • Nucleon: • Deuteron: • High-Q2 surprise in GpE/GpM; strong impact on theoretical pictureEvidence for two-photon exchange effects • New precise picture of GnE for Q2<1.5 (GeV/c)2, GnM <5 (GeV/c)2 • Evidence for structure beyond GDipole at low Q2in all form factors • Precision measurement of T20allows to separate GCand GQ • First measurement of Te11 allows to determine GM at low Q2 • Some problems in data unresolved • Progress in theory: Relativistic calculations, Chiral EFT

More Related