Precision Measurement of G E p /G M p with BLAST

1. Precision Measurement of GEp/GMp with BLAST Chris Crawford MIT Laboratory for Nuclear Science Thesis Committee Review Meeting 2003/12/4

2. Outline • Introduction and Motivation • Previous methods, world data • Asymmetry super-ratio method • Status of BLAST • Hardware: ABS, wire chambers • Software, asymmetries • Progress • Contributions • Outlook

3. Rosenbluth Separation • Elastic e-p cross section • At fixed Q2, fit dσ/dΩ vs. tan2(θ/2) • Measurement of absolute cross section • Dominated by either GE or GM

4. Polarization Transfer • Recoil proton polarization • Focal Plane Polarimeter • recoil proton scatters off secondary 12C target • Pt, Pl measured fromφ distribution • Pb, and analyzing powercancel out in ratio

5. World Data • Unpolarized Data • Polarization Transfer • Milbrath et al. (BATES) 1999 • Jones et al. (JLAB), 2000 • Dieterich et al. (MAMI), 2001 • Gayou et al. (JLAB), 2002 • Super-Rosenbluth • JLab Hall A, preliminaryresults expected soon

6. Super Rosenbluth Separation

7. Form Factor Ratio @ BATES • New technique: polarized beam and target • Exploits unique features of BLAST • internal target: low dilution, fast spin reversal • large acceptance: simultaneous measurement of all Q2 points • symmetric detector: ratio measurement • Different systematics • insensitive to Pb and Pt • Q2 = 0.07 – 0.9 (GeV/c) 2 • overlap with JLab dataand RpEX (future exp.at Bates to measure rp)

8. Asymmetry Super-ratio Method • Polarized cross section • Super-ratio

9. Projected Results • Statistics • A1, A2 • Systematics • θ*1, θ*2 Δp, Δθ, Δβ Errors are minimized as a function of β (target spin angle)

10. ep-elastic event generator RC studies w/Mascarad Newton-Rhapson fitter rapid convergence Lazy track fitter fast and efficient, poor res. TOF cal. – position, timing WC cal. – in progress Harmonic interpolation of BLAST field map Maintained BlastLib2, CVS standard program interface blastrc – config file Nsed enhancements port to GCC-3.2 Control software Elog–BLAST electronic logbook Trigger, HV enhancements ROOT enhancement improved histogram drawing Contributions

11. Radiative Corrections • MASCARAD code • A. Afanasev et al., Phys.Rev.D 64,113009 • Covariant calculation with no cutoff parameter • small corrections (<1%) to asymmetry

12. BlastLib2 Software • BlastLib2 v3.0 soon to be ready • Geometry: full 6 degrees of freedom • DST: for second pass reconstruction • Improved handling of WC noise • Debugged event-by-event

13. Position – coplanarity Timing – cosmics TOF Calibrations

14. Garfield drift time simulation Newton-Rhapson track fitter 200 μm resolution 20:1 signal to noise ratio Wire Chamber Tracking

15. Newton-Rhapson Track Fitter

16. Tracking Resolution

17. very low dilution due to problems with ABS Asymmetry left sector right sector 45 degree holding field left sector right sector left sector right sector 0 degree holding field 180 degree holding field

18. 1 1 1 1 MFT (2->3) 3 3 2 2 nozzle 4 6-pole 6-pole Atomic Beam Source • Standard technology • Dissociator & nozzle • 2 sextupole systems • 3 RF transitions Spin State Selection:

19. ABS Enhancements Sextupole Damage MEASURED FIELD ON THE POLE TIPS: Magnet 1 8.3 kG Magnet 2 9.1 kG Magnet 3 9.9 kG Magnet 4 11.6 kG Magnet 5 15.6 kG Magnet 6 15.4kG Magnet 7 15.2 kG BLAST Field Effect

20. New Asymmetry Results previous results previous results left sector right sector left sector 0 degree holding field, ABS improvements target z-dependence

21. Outlook • Work on hardware/software complete • Now taking high polarization data • Will concentrate on A, not GEp/GMp • Schedule • December: asymmetry analysis • January: start writing thesis • April: thesis defense