The Strange Form Factors of the Proton and the G 0 Experiment Jeff Martin University of Winnipeg Collaborating Institutions Caltech, Carnegie-Mellon, William&Mary, Grinnell College, Hampton, IPN-Orsay, LPSC-Grenoble, JLab, Kentucky, LaTech, NMSU, TRIUMF, UConn, UIUC, U Manitoba, U Maryland, UMass, UNBC, U Winnipeg, VPI, Yerevan

neutral weak form factors, axial form factor Projected precision of NW form factors in GeV 2 Q 2 range ~ 10% from the current generation of experiments Nucleon form factors measured in elastic e-N scattering Nucleon form factors From scattering theory: form factor ~ Fourier transform of density Well defined experimental observables provide an important benchmark for testing non-perturbative QCD structure of the nucleon electromagnetic form factors Measured precision of EM form factors in GeV 2 Q 2 range ~ 2 - 4%

Flavour Decomposition of the Form Factors  e p SpeciesChargeWeak Charge u d s  exchange: Z 0 exchange: Goal: Isolate G M s and G E s by isolating the Z 0 exchange graph

G 0 Program At a given Q 2, full decomposition of G s E, G s M, G e A requires 3 new parity-violation measurements: ← data-taking complete Forward angle e + p (elastic) ← data-taking complete ← data-taking Backward angle e + p (elastic) ← data-taking ← data-taking Backward angle e + d (quasi-elastic) ← data-taking G 0 will perform the separation at three different Q 2 values - 0.3, 0.5, 0.8 (GeV/c) 2

Parity-Violating Elastic Scattering where: Scatter polarized electrons from unpolarized protons, and measure the asymmetry: prevalent at forward angles prevalent at backward angles Note: A ~ = 1 ppm

Experiment General requirements: High statistics ( ) events –high current, high polarization beam –large acceptance –high rate capability Systematics –control of helicity-correlated beam properties –background characterization and rejection

Forward-Angle Measurements Elastic proton detection Toroidal focusing spectrometer Time-of-flight distinguishes elastic protons from pions, inelastic protons

G 0 beam monitoring girder superconducting magnet (SMS) detectors (Ferris wheel) cryogenic supply target service module G 0 Forward-Angle Configuration at Jefferson Laboratory Beam

Estimate of background yield under the elastic peak is performed by using - data with full and empty (gas H 2 ) targets, different pressures - data with dummy entrance and exit windows (Al) - Data with W radiator and dummy windows (electro/photo production)  Unfold backgrounds from target windows and inelastic LH 2 processes pions elastic protons inelastics Fractional background in elastic cut Dominant Systematic: Backgrounds

Yield asymmetry Detector 8 For smaller detectors, yields and asymmetries are measured on each side of the elastic peak and smooth interpolation is reasonable. For larger detectors, background asymmetry is large and varies across the elastic peak. Small increase of uncertainty in elastic asymmetry due to background for small detectors. Yield asymmetry Detector 13 Effect of backgrounds will likely dominate systematic uncertainty for larger detectors. Background Correction For preliminary result, used linear interpolation

Progress Background Correction Constrain yield with Monte Carlo motivated fit. Constrain asymmetry by enforcing constant elastic asymmetry. ppm Yield – Ring 8Asymmetry – Ring 8

Forward angle asymmetries - full statistics - detectors 13 to 15 not shown. - preliminary background corr. - 25% blinding factor applied Asymmetry (ppm) Increasing Q 2 Detector number Preliminary Results Systematic uncertainties included for preliminary result: - Deadtime corr2%  - Beam parameters corr0.01 ppm  - Leakage beam corr0.10 ppm  - Beam Polarization2%  - Background corrnot included - Q 2 determination1%  - Rad. Corr., EM FF’ssmall BLINDING FACTOR APPLIED G 0 Daily News Box Has Been Opened Blinding Factor Revealed Analysis complete, preparing for public release of results

Next Phase of G 0 : Backward-Angles Electron detection Add Cryostat Exit Detectors (“CED’s”) to define electron trajectory Add aerogel Čerenkov counter to reject  - Measurements on H and D to separate G M s, G A e E beam (MeV) Q 2 (GeV 2 ) beam target magnet FPD #1 FPD #16 CED #9 CED #1 Čerenkov inelastic e - or photo  - elastic e -

Canadian Contributions to Backward-Angle Expt. Čerenkov counters built by: –U Manitoba, TRIUMF, UNBC –Grenoble (France) Čerenkov/CED support structure (TRIUMF) and CED construction. Electronics for Canadian Čerenkov counters: –U Winnipeg Acquisition and Analysis of Data –Pion asymmetries (U Winnipeg) First run starts Dec. 3, 2005

Summary Forward angle production run complete. Analysis of forward angle data nearing completion. Preparations for back-angle run underway: –additional scintillators, Čerenkov counters being tested. –new electronics being tested. –new analysis software in preparation. Back angle running will begin in December 2005, will continue throughout 2006, 2007.

By 2008: Separated Form Factors Q 2 dependence of the strange electric and magnetic form factors below Q 2 = 1 GeV 2