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

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

Where Were We? From HAPPEX H preprint nucl-ex/

G0: Forward Angle Results (2) PRL in press (Sept. 2), nucl-ex/ ,

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 =  0.31 GMGM s Q 2 = 0.1 GeV 2 good agreement among all measurements

World Q 2 = 0.1 GeV 2 =  GEGE s GMGM s =  0.31   Contours 1 , 2  68.3, 95.5% CL Theories 1.Leinweber, et al. PRL 94 (05) Lyubovitskij, et al. PRC 66 (02) Lewis, et al. PRD 67 (03) Silva, et al. PRD 65 (01)

PVA4 measurement at Q 2 = 0.23 GeV 2 –consistent probable value for –supports negative World Q 2 = 0.23 GeV 2 GMGM s GEGE s

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 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

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) 1H1H H2H H1H H2H H1H H2H scheduled

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

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

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

Direct Backgrounds Asymmetries measured for combinations of CEDs and focal plane detectors (FPDs) Q 2 = 0.23 GeV 2

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

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

Direct Background Components Q 2 = 0.8 GeV 2

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

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

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

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

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

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)(%) 1H1H H/ 2 H /2.7

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 =  0.31 GEGE s