1 / 49

March 2006

March 2006. Nikolas Meitanis. Outline. Theoretical Framework Experimental Apparatus Data Analysis Results and Conclusion. THEORETICAL FRAMEWORK. Motivation. Importance of Nucleon Form Factors. Fundamental quantities: describe electro-magnetic structure

tanuja
Download Presentation

March 2006

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. March 2006 Nikolas Meitanis

  2. Outline • Theoretical Framework • Experimental Apparatus • Data Analysis • Results and Conclusion

  3. THEORETICAL FRAMEWORK

  4. Motivation Importance of Nucleon Form Factors • Fundamental quantities: describe electro-magnetic structure • Tests of theoretical calculations • Necessary for parity-violation experiments NOTE: Due to the absence of free neutron targets: Neutron form factors known less precisely compared to proton.

  5. Electron-Nucleon Elastic Scattering Unpolarized target: Rosenbluth cross section Mott cross section: (spin ½ electron, spinless+structureless nucleon) Polarized target:

  6. Electron-Nucleon Elastic Scattering Forming asymmetries: Asymmetry in terms of Sachs form factors: ( θ*, φ* ) : Angles between the target polarization and the momentum transfer vector. θ* = 0 deg. : parallel kinematics θ* = 90 deg. : perpendicular kinematics.

  7. Electron-Deuteron Elastic Scattering Deuteron Properties: Simplest nuclear system in nature Spin 1, Isospin 0 Proton and Neutron loosely bound, spins aligned Ground state: admixture of S and D Unpolarized Cross Section Polarized Cross Section

  8. Electrodisintegration EXCLUSIVE: both scattered electron and hadron detected. Scattering off either proton or neutron. Quasi-Elastic • QE: Special case of electrodisintegration • QE: Electron scatters off single nucleon • PWBA: No excited states, no FSI Polarized cross section Beam-target vector asymmetry

  9. Inclusive Electron-Deuteron Quasi-elastic Scattering Only electron detected. Cross section derived from integral of exclusive structure functions over n-p phase space. 10 Structure Functions remaining after integration. Beam-target vector asymmetry This asymmetry exhibits sensitivity to GMn in the inclusive electro-disintegration reaction of polarized electrons and polarized deuterium. NEW MEASUREMENT.

  10. Sensitivity to GMn ASYMMETRIES Polarized deuteron: Incoherent sum of pol. neutron and pol. proton NOTE: Asymmetries vary with opposite sign in the two sectors. This can be exploited in a linear combination of the two.

  11. Sensitivity to GMn(2) Same sensitivity across W spectrum.

  12. Sensitivity to GMn(3) RATIO OF ASYMMETRIES The ratio enhances sensitivity to the form factor. The form factor enters the ratio squared.

  13. Calculation by H. Arenhovel A model of Deuteron structure using the Bonn potential. Incorporated in Monte Carlo. Friedrich & Walcher form factors for proton, Galster form factor for GEn. Incorporates: 1. Final State Interactions (FSI) 2. Meson Exchange Currents (MEC) 3. Isobar Configurations (IC) 4. Relativistic Corrections (RC)

  14. Friedrich&Walcher Parametrization Expressed form factors as “smooth“ part plus “bump” smooth bump

  15. F&W Parametrization (2) The effect of changing the proton form factors from dipole to the FW parametrization. The Galster parametrization is used for Gen and the dipole for GMn.

  16. Previous GMn Measurements • Unpolarized electron-deuteron quasi-elastic. Inclusive. Proton contributions subtracted (model dependent). • Ratio of cross sections D(e,e’n) and D(e,e’p) in QE kinematics. Less sensitive to nuclear structure. Needed to know neutron detection efficiency. • Quasi-elastic scattering of polarized electrons off polarized Helium-3. Inclusive. Nuclear structure model an issue

  17. World’s Data for GMn THEORETICAL CALCULATIONS • Holzwarth B1, B2: Soliton • Simula: CQM • Lomon: VMD model • Miller: Cloudy Bag model • FW: Friedrich & Walcher par. • Faessler: ChPT

  18. EXPERIMENTALAPPARATUS

  19. MIT-Bates Linear Accelerator Siberian Snakes • Polarized Source • Linac • Recirculator • South Hall Ring (SHR) • Siberian Snakes • BLAST detector in SHR • ABS: BLAST target embedded in the beamline

  20. The BLAST Detector • ABS target • Wire Chambers • Cerenkov Counters • TOFs • Neutron Counters • Magnetic Coils

  21. The BLAST Detector + Coils • ABS target • Wire Chambers • Cerenkov Counters • TOFs • Neutron Counters • Magnetic Coils

  22. Atomic Beam Source (ABS) • RF Dissociator • Sextupole System • RF Transition Units • Storage Cell • Breit-Rabi Polarimeter

  23. RF Dissociator Dissociates molecules into atoms Consists of an RF coil, connected to an RF power supply and wrapped around a glass tube. Critical parameters for optimizing performance: Nozzle temperature Gas through-put Vacuum Matching Network – RF power Oxygen Admixture

  24. Dissociation Results HYDROGEN DEUTERIUM

  25. Sextupole System Used to focus atoms with pos. atomic electron spin and de-focus the rest. 24 segments glued together. Create radial field. RAYTRACE simulations used to optimize location / opening of apertures, location of sextupoles.

  26. Hyperfine Structure HYDROGEN Quantum mechanics of spin ½ - spin ½ system. Two Zeeman multiplets: Symmetric triplet, anti-symmetric singlet. DEUTERIUM Spin 1 – spin ½ system: Quadruplet + Doublet.

  27. RF Transition Units To induce transitions between the hyperfine states. MFT UNIT DEUTERIUM TRANSITIONS

  28. Storage Cell Used to increase target thickness for internal target. 60 cm long, 15 mm diameter, Al. De-polarization effects: Recombination Spin Relaxation To limit de-polarization: Cooled to 100 K. Coated with Dryfilm.

  29. Other Components Target Holding Field : To maintain and control the orientation of the target polarization. Electromagnet with two pairs of coils. Covers ± 20 cm of the cell. Breit-Rabi Polarimeter (BRP) : To monitor transitions. A dipole magnet with a gradient field for electron-spin separation.

  30. Polarization Results Target Vector Pol. ≈ 85% (deuterium) From (e,e’p) analysis off deuterium. Beam: Average Pol. ≈ 65% Measured with Compton Pol. Target Tensor Pol. ≈ 80% (deuterium) From ed-elastic analysis.

  31. DATA ANALYSIS

  32. Data Sets

  33. Inclusive Electron Selection 1. Particles with inbending Wire-Chamber Track (negative charge). 2. Correlated TOF – Cerenkov signals. 3. Invariant mass cuts: essentially limit events to QE regime. 4. The data were divided into four Q2 bins.

  34. Inclusive Electron Selection Data from 3 triggers Trigger 1 : (e,e’p), (e,e’d) Trigger 2: (e,e’n) Trigger 7: (e,e’) singles prescaled by 3 INCLUSIVE: ADD TRIGGERS NOTE: In forming the inclusive cross section, the individual detection efficiencies cancel out when trigger 7 is taken into account. Electron detection efficiency is not crucial when forming asymmetries.

  35. Data Spectra Sample of experimental spectra for first Q2 bin and 2004 data. Bin 1 Bin 2 Bin 1 Bin 2 Bin 3 Bin 4 Bin 3 Bin 4

  36. Data Spectra (2) Sample of experimental spectra for first Q2 bin and 2004 data. Bin 1 Bin 2 Bin 2 Bin 1 Bin 4 Bin 3 Bin 3 Bin 4

  37. Experimental Background • Empty target background From cell-wall scattering etc. Uniform across W spectrum, 1-3% Dilutes individual sector asymmetries • Pion contamination Only past pion-threshold (high-W edge) Expected to be negligible • Electro-deuteron elastic scattering Only at low-W edge Varies between 1-5% Sizeable effect on asymmetries and ratio

  38. Electron-Deuteron Elastic Events • Monte Carlo of ed elastic versus disintegration events. Resolution convolutes peaks • Mostly at low-Q2 • Only at low-W edge • Varies between 1-5% • Effect on asymmetries and ratio QE elastic

  39. Electron-Deuteron Elastic Events The contamination was accounted for in the MC.

  40. Extraction of GMn The following analysis process was performed for each data set independently: • Divide the data into the Q2 bins and form the asymmetries in both perpendicular and parallel kinematics. • Within each Q2 bin, divide the data in W bins. • Correct the asymmetries for empty target background. Q2 = 0.189 (GeV/c)2

  41. Extraction of GMn (2) Q2 = 0.189 (GeV/c)2 2. Divide the asymmetries to form the ratio. 2004 2005 • Vary GMn value wrt the dipole • form factor in the Monte Carlo. 4. Obtain χ2 for each calculation.

  42. Extraction of GMn (3) Q2 = 0.189 (GeV/c)2 • Find minimum of χ2 for each Q2 bin • using a parabolic shape. 2004 6. Calculate error by varying χ2for each Q2 bin by 1. 2005

  43. Systematic Uncertainties

  44. Target polarization angle uncertainty

  45. Uncertainty in GEn A 20% uncertainty in GEn contributes 0.5% uncertainty in GMn .

  46. False Asymmetries Negligible Effect.

  47. RESULTS & CONCLUSION

  48. Final Results

  49. Final Results (2) 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.

More Related