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G0 Backward Angle Request: Q 2 = 0.23, 0.48 GeV 2

G0 Backward Angle Request: Q 2 = 0.23, 0.48 GeV 2. Main points G0 goal is to measure G E s , G M s and G A e over range of momentum transfers with best possible precision Forward angle measurements complete/published PRL, Sept. 2, 2005; nucl-ex/0506021

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G0 Backward Angle Request: Q 2 = 0.23, 0.48 GeV 2

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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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