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Precision Measurements of Electric and Magnetic Form Factors in Backward Angle Scattering

The G0 experiment aims to measure the electric (GEs) and magnetic (GMs) form factors along with the axial form factor (GAe) across various momentum transfers with high precision. While forward angle measurements have been completed and published, this paper outlines the required backward angle measurements with hydrogen (H2) and deuterium (D2). Scheduled to begin in March 2006, the backward measurements will utilize the same setup as previous forward results, enabling a comprehensive study of form factors at low Q2 values.

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Precision Measurements of Electric and Magnetic Form Factors in Backward Angle Scattering

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  1. G0 Backward Angle Request: Q2 = 0.23, 0.48 GeV2 Main points • G0 goal is to measure GEs, GMs and GAe over range of momentum transfers with best possible precision • Forward angle measurements complete/published • PRL, Sept. 2, 2005; nucl-ex/0506021 • Requires backward angle H2 and D2 measurements • Q2 = 0.8 GeV2 run scheduled, now starting Mar. 2006 • Based on forward results choose Q2 = 0.23 GeV2 then Q2 = 0.48 GeV2 D. Beck, UIUC PAC28, Aug. 2005

  2. = = 0 • Physics from comparison with ANVS • “no vector strange” asymmetry, ANVS, calculated with for all Q2 GE GM s s G0: Forward Angle Results (1) • Measurement over wide range of Q2: 0.12 – 1.0 GeV2 • Measure elastic asymmetries (recoil protons) • asymmetry: 1 – 40 ppm

  3. Where Were We? • From HAPPEX H preprint nucl-ex/0506011

  4. G0: Forward Angle Results (2) PRL in press (Sept. 2), nucl-ex/0506021, http://www.npl.uiuc.edu/exp/G0/Forward

  5. Q2 = 0.1 GeV2 good agreement among all measurements • If has simple dipole falloff, rises monotonically to Q2zL2 • At Q2 = 0, , at low Q2, • Decrease of around Q2 = 0.2 GeV2 suggests hGM s s GM + h s GE < 0 GE GM GM s = +0.62  0.31 s s GM  Q2 GE GE GM = 0 + h + h s s s s G0: Forward Angle Results (3) • Summary of conclusions: non-trivial Q2 dependence • Remember s-quark charge is factored out: • contributions to charge and magnetization distributions are

  6. s GE s GM World Data @ Q2 = 0.1 GeV2 = -0.013  0.028 = +0.62  0.31  0.62 2s • Contours • 1s, 2s • 68.3, 95.5% CL • Theories • Leinweber, et al. PRL 94 (05) 212001 • Lyubovitskij, et al.PRC 66 (02) 055204 • Lewis, et al.PRD 67 (03) 013003 • Silva, et al.PRD 65 (01) 014016 http://www.npl.uiuc.edu/exp/G0/Forward

  7. GM GE s s World Data @ Q2 = 0.23 GeV2 • PVA4 measurement at Q2 = 0.23 GeV2 • consistent probable value for • supports negative http://www.npl.uiuc.edu/exp/G0/Forward

  8. Background Overview • Measure yield and asymmetry of entire spectrum • Correct asymmetry according to where Aelis the raw elastic asymmetry, • Actual analysis: f = f(t) • det. 1-14 • fit Yback(poly’l of degree 4), Gaussian for elastic peak • then fit Aback(poly’l of degree 2), constant Ael • uncertainties • statistical contribution: f/(1-f)2 in D2Astat (20% for f = 15%) • systematic contribution: ~ 0.5 DAstat

  9. Proposed Backward Measurements • Measurements at Q2 = 0.23, 0.48 GeV2 • motivated by present data: G0 + Mainz, G0 + HAPPEX, respectively • convincing picture at Q2 = 0.1 GeV2 • same setup as scheduled Q2 = 0.8 GeV2 run • new cryostat exit scintillators (CEDs), Cherenkov detector • regular beam structure (499 MHz) • higher beam current (80 mA) • requires lower beam energies scheduled

  10. Cherenkov FPD CED Electron incident Backward Measurements • Additional detectors complete – final testing • Target modifications complete • extension of support

  11. Backward Measurements • Additional detectors complete – final testing CED PMTs CEDs Cherenkov PMTs Cherenkov PMTs Cherenkov CEDs CED PMTs Backward Angle Detector Rotation Test

  12. Backgrounds • “Direct” • inelastic electrons, electrons from p0 decay • continuing development of MC • use of wire chamber to make careful separation of yields • measures angle near focal surface • “Indirect” • “hall background” - shower from target • main addition – lead insert downstream of target • careful shielding of exit beamline and dump tunnel

  13. Direct Backgrounds Q2 = 0.23 GeV2 • Asymmetries measured for combinations of CEDs and focal plane detectors (FPDs)

  14. Direct Backgrounds Q2 = 0.48 GeV2 • Asymmetries measured for combinations of CEDs and focal plane detectors (FPDs) • contamination from inelastic electrons few % for Q2 = 0.48 GeV2

  15. Direct Backgrounds Q2 = 0.8 GeV2 • Asymmetries measured for combinations of CEDs and focal plane detectors (FPDs) • contamination from inelastic electrons few % for Q2 = 0.48 GeV2 • electrons from p0 decay likely to dominate, especially at higher Q2 • measure trajectory angles with wire chamber at low beam current • understand components of background yields

  16. Direct Background Components Q2 = 0.8 GeV2

  17. Indirect Background • GEANT code based on that of P. Degtiarenko • Added detailed G0 geometry • Careful shielding of dump • Add lead insert downstream of target • With this configuration, Q2 = 0.23 GeV2 background ~ same as at 0.8 GeV2

  18. GE n Beam Polarization Measurement • Beam polarization measured with Møller polarimeter • forward angle: <Pe> = 73.71.0% • use <Pe> = 75 1.5% for backward angle estimates • Low energy running requires moving Q1 in Møller spectrometer • previous move by 6 in. successful ( ) Parity Quality Beam • Require ~ x2 looser specs compared to forward angle • Plan to use feedback for position differences • hope to improve damping in injector • very small damping in forward measurement • better matching in 1/4 cryo and injector cryomodule • promising solution tried recently (Y. Chao)

  19. Expected Results • Assumes single measurement 50 d LH2 • total background uncertainty 2% (stat. unc. 2.8%) stat stat + sys PVA4 stat + sys + model G0 Forward G0 Backward

  20. Expected Results • Assumes two measurements 30 d each: LH2, LD2 • total background uncertainty 3% (stat. unc. 3.3%) stat stat + sys HAPPEX stat + sys + model G0 Forward G0 Backward

  21. e GA Axial Form Factor • is important component of asymmetry at backward angles • no information yet about Q2 dependence

  22. Beam Request • Running periods • Breakdown of auxiliary measurement time • forward measurement required about 10% • expect same for backward measurement - periodically measure: • beam polarization • beam energy • charge monitor calibration • recall 10 d commissioning time for detector, target tuneup, background studies, etc.

  23. s s GM GE Summary • May have glimpse of physics picture from SAMPLE, forward angle measurements • may be negative • Most interesting physics around Q2 = 0.2 GeV2 • best to make backward angle measurements where there are other data • Q2 = 0.23 GeV2: G0 forward, PVA4 I • Q2 = 0.48 GeV2: G0 forward, HAPPEX I • Detectors, target, electronics ready for first run at 0.8 GeV2 = +0.62  0.31

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