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Nucleon Form Factors and the BLAST Experiment at MIT-Bates

Nucleon Form Factors and the BLAST Experiment at MIT-Bates. Probing Nucleon Structure with Spin-Dependent Electron Scattering at Low Q 2. Introduction/Motivation BLAST Experiment Results and Discussion. Spin 1/2: Elastic electron scattering from nucleons. t = Q 2 /4M 2

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Nucleon Form Factors and the BLAST Experiment at MIT-Bates

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  1. Nucleon Form Factors and the BLAST Experiment at MIT-Bates Probing Nucleon Structure with Spin-Dependent Electron Scattering at Low Q2 • Introduction/Motivation • BLAST Experiment • Results and Discussion R. Alarcon @ PAV06

  2. Spin 1/2: Elastic electron scattering from nucleons t = Q2/4M2 e = [1+2(1+t)tan2(q /2)]-1 GEp(0) = 1, GEn(0) = 0 Sach’s ff’s: charge magnetism GMp(0) = 2.79 GMn(0) = –1.91

  3. p p n Polarized e-N scattering • Polarized beam + polarized target: Donnelly + Raskin, Ann. Phys. 169 (1986)247 p neutron charge ff n neutron magnetic ff • Polarized beam + polarization of recoil nucleon: neutron charge ff proton GE/GM Akhiezer+Rekalo, Sov.JPN 3 (1974) 277 Arnold,Carlson+Gross, PRC 21 (1980) 1426

  4. Why study hadron form factors? • They give us the ground state properties of (all visible) matter: • size and shape • charge and magnetism distributions • spin and angular momentum • They are required for knowledge of many other things: • structure of nuclei at short distances • Proton charge radius and Lamb shift • precision tests of Weak interaction at low Q2 • They should give clues on how to connect QCD to the NN force cPT pQCD form factors quarks/gluons p cloud lattice QCD They can be measured NOW with high precision!

  5. Scientific Motivation for BLAST • Comprehensive study of spin observables from few-body nuclei at low Q2. • Nucleon form factors provide basic information on nucleon structure. • Low-Q2 region is a testing ground for QCD and pion-cloud inspired and other effective nucleon models. • GnE is the least known among the nucleon form factors, with errors of typically 15-20%. • GnE related to neutron charge distribution. • Precise knowledge of GnE is essential for parity violation experiments.

  6. MIT-Bates Linear Accelerator Center • Linac: 2500 MeV • Beam: 850 MeV / Imax = 225 mA / Pe = 65 % • SHR: Siberian Snake + Compton Polarimeter • Target: Internal Target = Atomic Beam Source

  7. Internal Target Physics at MIT-Bates Ee  1 GeV, Pe= 40-80 % Im= 200 mA,   10 min • pure species • thin • high polarization • thin cell • low holding field South Hall Ring L = 1031-1033 atoms cm-2 s-1 Novosibirsk, AmPS, HERMES, IUCF, COSY

  8. Atomic Beam Source • Isotopically pure H or D • Vector Polarized H • Vector and Tensor D • Target Thickness/Luminosity • Flow 2.2 x 1016 atoms/s • Density 6 x 1013 atoms/cm2 • Luminosity 6 x 1031 cm-2s-1 • Target Polarization typically 70-80%

  9. Atomic Beam Source

  10. Atomic Beam Source • Quasielastic • Beam-Target Asymmetry • Aved(exp) = h·Pz ·Aved(th) • <hPz> = 0.567 ± 0.006 <Pz> = 0.85 ± 0.04 <h> = 0.67 ± 0.04

  11. Bates Large Acceptance Spectrometer Toroid • Left-Right symmetry • Large Acceptance • 0.1 < Q2/(GeV/c)2 < 1.0 • Coils: B = 3.8 kG • Drift Chambers PID,tracking •  ≈ 0.5º, • Cerenkov Counters • e,  separation • Scintillators • TOF, PID, trigger • Neutron Counters • Neutron ToF

  12. The BLAST Collaboration

  13. Experimental Program High quality data for nucleon and deuteron structure by means of spin-dependent electron scattering

  14. Event Selection

  15. Neutron: Invariant Mass and Time of Flight • Very clean quasi-elastic 2H(e,e’n)p spectrum • Highly efficient proton veto (Wire Chambers)

  16. Neutron: Invariant Mass and Time of Flight • Very clean quasi-elastic 2H(e,e’n)p spectrum • Highly efficient proton veto (Wire Chambers)

  17. Target Spin Orientation

  18. Target Spin Orientation nucleons Electron - Right Sector “Parallel” Kinematics * ≈ 0 electrons

  19. Target Spin Orientation Neutrons electrons Electron - left sector “Perpendicular” Kinematics * ≈ 90 nucleons electrons

  20. Kinematics and Observables • Electrodisintegration of the Deuteron • Quasi-elastic 2H(e,e’n) • Beam + Target Polarized

  21. BLAST Data Collection

  22. BLAST Data Collection

  23. BLAST Data Collection

  24. Nucleon Form Factors Parameterizations

  25. Proton Form Factors

  26. Neutron Magnetic Form Factor GMn

  27. Neutron Magnetic Form Factor GMn

  28. Neutron Magnetic Form Factor GMn 1. New measurement technique. 2. Includes full deuteron structure. 3. Consistent with recent polarization and other data. 4. Provides a tighter fit to form factor in the low Q2 region.

  29. Preliminary BLAST GMn Data N. Meitanis

  30. Extraction of GnE • Quasielastic • Full Monte Carlo Simulation of the BLAST experiment • Deuteron Electrodisintegration cross section calculations by H. Arenhövel • Accounted for FSI, MEC, IC, RC • Spin-perpendicular beam-target asymmetry AedV(90º,0º) shows high sensitivity to GnE

  31. Extraction of GnE • Quasielastic • Full Monte Carlo Simulation of the BLAST experiment • Deuteron Electrodisintegration cross section calculations by H. Arenhövel • Accounted for FSI, MEC, IC, RC • Spin-perpendicular beam-target asymmetry AedV(90º,0º) shows high sensitivity to GnE • Compare measured AedV with BLASTMC, varying GnE

  32. Systematic Errors • Uncertainty of target spin angle 5% • 12% per degree • Beam-target polarization product 2.5% • Radiative effects <1.0% • Small helicity dependency • Uncertainty of GnM 1.5% • Model dependency <3% • Effect of potential negligible • Final state interaction reliable Total: 6.6%

  33. GnE World Plot PRELIMINARY

  34. GnE World Plot • Preliminary result • Only 50% of data, final data should reach 0.5 (GeV/c)2 • Use Arenhovel’s calculations for GnM and contribution of GnE • Need to combine with other BLAST measurements for global fit • Provide low Q2 data • Check bump • Pion cloud PRELIMINARY V. Ziskin and E. Geis

  35. BLAST Fit to World Polarization Data • Remarkable consistency of all modern polarization experiments! Global fit determines GEn to better than ±7%

  36. Nucleon Models • Dispersion theory gives the best description • No theory describes GnEat low and high Q2 simultaneously

  37. Conclusions & Outlook • BLAST at MIT-Bates has measured at low Q2 the nucleon form factors using polarized electrons from vector-polarized hydrogen/deuterium. • Very small systematic errors • GnEoverall known to ≈ 5% at Q2 < 1 (GeV/c)2 • Dispersion theory gives the best description • No theory describes GnEat low and high Q2 simultaneously • Evidence for enhancement at low Q2 - role of pion cloud? • With new precision data of T20 from BLAST and with improved A(Q2) new attempt to GnE determine from GQ • ed elastic analysis: Mainz-Saclay discrepancy 8% in A  factor of 2 in GnE • New measurements of A(Q2) at JLab (E-05-004) and MAMI • Extension of GnE at high Q2 < 3.5 (GeV/c)2 (E-02-13) • Proposal of BLAST@ELSA/Bonn Measure GnEfor Q2 = 0.04-1.5 (GeV/c)2 with both vector-polarized 2H and polarized 3He

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