The G 0 Experiment: Parity Violation in e-N Scattering

1. The G0 Experiment:Parity Violation in e-N Scattering Colleen Ellis The University of Maryland The G0 Collaboration: CalTech, Carnegie-Mellon,William & Mary, Hendrix, IPN-Orsay, LPSC-Grenoble, JLab, LaTech, NMSU, Ohio University,TRIUMF, U Conn, UIUC, U Manitoba, U Maryland, U Mass, UNBC, U Winnipeg, VPI, Yerevan, Zagreb Hall C Meeting 18 January 2008

2. G0 Graduate Students Carissa Capuano: W&M, USA Maud Versteegen: LPSC, France. Alexandre Coppens: Manitoba, Canada Mathew Muether: Illinois, USA Colleen Ellis: Maryland, USA John Schaub: NMSU, USA. Juliette MammeiVirginia Tech. USA.

3. Overview • Physics Introduction • G0 Forward Angle • G0 Backward Angle--Elastic Electron Scattering • Experimental Set-up • Analysis Overview • Preliminary Data • Detector Performance • Other Backward Angle Physics Topics • Inelastic e-p measurement to measure parity violation in N- transition • Elastic e-p scattering with transverse beam polarization to investigate 2 photon exchange • PV pion photoproduction on the  resonance

4. Strange Form Factors How does s quark contribute to electromagnetic properties of the nucleon? Electron scattering involves EM and Weak interactions Assume: Isospin symmetry Known G0 measures

5. Model Independent Form Factors PV asymmetries from EM and weak interference terms can be varied between zero and unity for a fixed Q2 by varying the beam energy and electron scattering angle. Two kinematics, two targets gives 3 linear combinations of EM and weak form factors

6. G0 Forward Angle Experiment • Forward angle measurement completed May 04 • LH2 target, detect recoil protons • Q2 = 0.12-1.0 (GeV/c)2, E=3.03GeV • Spectrometer sorts protons by Q2 in focal plane detectors (16 rings in total) • Detector 16: “super-elastic”, crucial for measuring the background • Beam bunches separated by 32 ns • Time-of-flight separates protons from pions • Results published in : • D.S. Armstrong, et al., • PRL 95, 092001 (2005)

7. G0 Forward Angle Results • Forward Angle • 700 hrs of data taking • 101 C. • 18 Q2 measurements • Good agreement with other experiments (HAPPEx and PVA4) • Backward Angle • Two Q2 measurements • 0.23 and 0.62 GeV2 • Required for complete separation of • and (GeV2)

8. CED+Cerenkov FPD e- beam target e- beamline G0 Backward Angle • Hydrogen and deuterium targets • Electron beam energy of : • 362 MeV : Q2=0.23 (GeV/c)2 • 687 MeV : Q2=0.62 (GeV/c)2 • Detection of scattered electrons ~ 108º • Particle detection and identification : • 16 Focal Plan Detectors • 9 Cryostat Exit Detectors elastic and inelastic electron separation • Additional Cerenkov detectors electron and pion separation Backward Angle Configuration

9. G0 Backangle Superconducting Magnet (SMS) Target Service Module G0 Beam Monitoring Detectors: Ferris Wheel (FPDs) Detectors: Mini-Ferris wheel (CEDs+Cerenkov)

10. Collected Data • Transverse • LH2 362MeV 3.6 C • LD2 362MeV 2.1 C • LH2 687MeV 1.0 C • Longitudinal • LH2 362MeV 90 C • LD2 362MeV 70 C • LH2 687MeV 120 C • LD2 687MeV 45 C • Special Runs Types • pion matrix • random matrix • magnet scans

11. G0 Backangle Analysis Approach Raw Yields and Blinded Asymmetries by target and Q2 Blinding Factor Rate Corrections for Electronics --Deadtime and Random Coincidences Helicity Correlated Beam Corrections Q2 Determination Forward Angle Corrections from inelastic electrons Background from target walls Pion Asymmetry Contamination Aphys Unblind Beam Polarization Correction EM Radiative Corrections (via Simulation)

12. Forming Asymmetry • measure raw yield for each helicity state (+ or -) apply rate corrections (electronic deadtime and random coincidences): • correct for beam correlated effects : • form asymmetry : • correct for background contribution : • correct for beam polarization (P) LH2 LD2 Afalse< 4 ppb Am ~ 10 ppm fb < 10 %

13. LH2, 362 MeV LD2, 362MeV Electron Yields (Hz/uA) Elastic Elastic Inelastic 90 C 120 C LH2, 687 MeV Quasi Elastic Inelastic 45 C 70 C Inelastic LD2, 687 MeV

14. IHWP IN OUT Elastic Electron Asymmetries LH2 362 PRELIMINARY RAW BLINDED LD2 362

15. Elastic Electron Asymmetries LH2 687 PRELIMINARY RAW BLINDED LD2 687

16. G0 BackwardAngle : Beam Specifications • Beam parameters specifications were set to assure: • Helicity correlated beam properties false asymmetry Correction : linear regression All Møller measurements during run) P=85.78 +/- 0.07 (stat) +/-1.38 (sys) %

17. LD2 687 Field Scan (Octant 1) Random subtracted Electron Yield vs SMS Current (2 sample cells) • Ramped SMS from 1900A to 4900A • Cell by cell fits made using a Gaussian (blue) for low momentum “background” and 2 Gaussians (with shared width) (red) for the elastic peak. A constant (lt. green) is also added to the fit to remove any field independent rate. Cell by Cell dilutions extracted as:

18. Cerenkov Efficiencies • Electron detection efficiency • Determined using three different techniques • Does not change asymmetry Four Cerenkov Detectors CED/FPD Coincidence electron pion

19. Measured Cerenkov Efficiencies

20. Net effect is to reduce the energy of the scattered electron so elastic peak now has a low energy tail due to events which have “radiated” out of the peak. EM Radiative Effects • Follow process of Tsai [SLAC=PUB-848] 1971. • Compute asymmetry [ ] based on the kinematics at the reaction vertex after the radiative emission. • This is compared to Born asymmetry calculation • [ ] with

21. LH2 687 RC Yield Simulations Without RC Effects Without RC Effects With RC Effects

22. Expected G0 Results

23. Elastic Region: G0 Inelastic Region: N D G0: N →  • Measurement: Parity-violating asymmetry of electrons scattered inelastically • ANΔ gives direct access to GANΔ • Directly measure the axial (intrinsic spin) response during N →Δ+ transition • First measurement in neutral current process IN Asymmetry (ppm) vs Octant (LH2 @ 687MeV) Data: Inelastic electrons Scattered from both LH2 and LD2, each at two energies (362MeV & 687MeV) OUT BLINDED Asymmetry (ppm) Octant Raw Asymmetry (averaged over inelastic region)

24. Transverse Polarization 2-Exchange • When a transversely polarized electron scatters from a proton, the scattering rate has an azimuthal dependence arising from two-photon exchange contributions • This beam normal single spin asymmetry is of the same order of magnitude as the PV asymmetry; it can introduce a background asymmetry if the beam polarization has a transverse component

25. G0 362 MeV LH2 Transverse Asymmetry • BLINDED ---no corrections for helicity correlated beam parameters, deadtime, … Raw Preliminary Blinded Octant

26. Parity Violating Photoproduction of - on the Delta Resonance • PV asymmetry for pion photoproduction may be as large as 5 ppm (based on hyperon model) with several ppm statistical uncertainty • Can access this from inclusive - asymmetries at kinematics. (Zhu et al, Phys. Rev. Lett.) • Electroweak radiative corrections generate a non-zero asymmetry at Q2 = 0. (Siegert’s theorem)

27. LD2, 687 MeV Pion Yields CED Hz/uA FPD Pion Yield Measurement Rate corrections : • fr ~15% (2/3 deadtime, 1/3 random coincidences) • Longitudinal A is small Analysis well underway

28. Summary • G0 Forward Angle and G0 Backward Angle Measurement allows model independent determination of • Analysis underway; good progress • Above specification beam and well-understood detector performance • Other Backward Angle Physics Topics Analysis well underway

29. The End