1. 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 –Requires backward angle H 2 and D 2 measurements Q 2 = 0.8 GeV 2 run scheduled, now starting Mar. 2006 Based on forward results choose Q 2 = 0.23 GeV 2 then Q 2 = 0.48 GeV 2 D. Beck, UIUC PAC28, Aug. 2005

2. G0: Forward Angle Results (1) Measurement over wide range of Q 2 : 0.12 – 1.0 GeV 2 Measure elastic asymmetries (recoil protons) –asymmetry: 1 – 40 ppm GEGE s GMGM s = = 0 Physics from comparison with A NVS –“no vector strange” asymmetry, A NVS, calculated with for all Q 2

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. G0: Forward Angle Results (3) Summary of conclusions: non-trivial Q 2 dependence If has simple dipole falloff, rises monotonically to Q 2 z  2 –At Q 2 = 0,, at low Q 2, Decrease of around Q 2 = 0.2 GeV 2 suggests GMGM s GMGM s +  GEGE s G M = 0 s +  GEGE s G M  Q 2 s +  GEGE s GMGM s G E < 0 s Remember s-quark charge is factored out: –contributions to charge and magnetization distributions are = +0.62  0.31 GMGM s Q 2 = 0.1 GeV 2 good agreement among all measurements

6. World Data @ Q 2 = 0.1 GeV 2 http://www.npl.uiuc.edu/exp/G0/Forward = -0.013  0.028 GEGE s GMGM s = +0.62  0.31  0.62 2  Contours 1 , 2  68.3, 95.5% CL Theories 1.Leinweber, et al. PRL 94 (05) 212001 2.Lyubovitskij, et al. PRC 66 (02) 055204 3.Lewis, et al. PRD 67 (03) 013003 4.Silva, et al. PRD 65 (01) 014016

7. PVA4 measurement at Q 2 = 0.23 GeV 2 –consistent probable value for –supports negative World Data @ Q 2 = 0.23 GeV 2 GMGM s GEGE s http://www.npl.uiuc.edu/exp/G0/Forward

8. Background Overview Measure yield and asymmetry of entire spectrum Correct asymmetry according to where A el is the raw elastic asymmetry, Actual analysis: f = f(t) –det. 1-14 fit Y back (poly’ l of degree 4), Gaussian for elastic peak then fit A back (poly’ l of degree 2), constant A el uncertainties –statistical contribution: f/(1-f) 2 in  2 A stat (20% for f = 15%) –systematic contribution: ~ 0.5  A stat

9. Proposed Backward Measurements Measurements at Q 2 = 0.23, 0.48 GeV 2 –motivated by present data: G0 + Mainz, G0 + HAPPEX, respectively convincing picture at Q 2 = 0.1 GeV 2 –same setup as scheduled Q 2 = 0.8 GeV 2 run new cryostat exit scintillators (CEDs), Cherenkov detector regular beam structure (499 MHz) higher beam current (80  A) –requires lower beam energies TargetQ2Q2 EnergyRateAsym. (GeV 2 )(GeV)(MHz)(ppm) 1H1H0.230.3603.1-13 2H2H0.230.3604.7-18 1H1H0.480.5850.72-32 2H2H0.480.5851.1-43 1H1H0.80.7990.19-54 2H2H0.80.7990.27-72 scheduled

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

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

12. Backgrounds “Direct” –inelastic electrons, electrons from  0 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 Asymmetries measured for combinations of CEDs and focal plane detectors (FPDs) Q 2 = 0.23 GeV 2

14. Direct Backgrounds Asymmetries measured for combinations of CEDs and focal plane detectors (FPDs) Q 2 = 0.48 GeV 2 –contamination from inelastic electrons few % for Q 2 = 0.48 GeV 2

15. Direct Backgrounds Asymmetries measured for combinations of CEDs and focal plane detectors (FPDs) Q 2 = 0.8 GeV 2 –contamination from inelastic electrons few % for Q 2 = 0.48 GeV 2 –electrons from  0 decay likely to dominate, especially at higher Q 2 –measure trajectory angles with wire chamber at low beam current understand components of background yields

16. Direct Background Components Q 2 = 0.8 GeV 2

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, Q 2 = 0.23 GeV 2 background ~ same as at 0.8 GeV 2

18. Beam Polarization Measurement Beam polarization measured with Møller polarimeter –forward angle: = 73.7  1.0% –use = 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) GEGE n

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

20. Expected Results Assumes two measurements 30 d each: LH 2, LD 2 –total background uncertainty 3% (stat. unc. 3.3%) HAPPEX G0 Forward G0 Backward stat stat + sys stat + sys + model

21. Axial Form Factor is important component of asymmetry at backward angles no information yet about Q 2 dependence GAGA e

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. TargetQ2Q2 EnergyRequest  A stat /A (GeV 2 )(GeV)(days)(%) 1H1H0.230.360502.8 1 H/ 2 H0.480.585603.3/2.7

23. Summary May have glimpse of physics picture from SAMPLE, forward angle measurements – – may be negative Most interesting physics around Q 2 = 0.2 GeV 2 –best to make backward angle measurements where there are other data –Q 2 = 0.23 GeV 2 : G0 forward, PVA4 I –Q 2 = 0.48 GeV 2 : G0 forward, HAPPEX I Detectors, target, electronics ready for first run at 0.8 GeV 2 GMGM s = +0.62  0.31 GEGE s