Precision Measurement of G E p /G M p with BLAST Chris Crawford MIT Laboratory for Nuclear Science Thesis Committee Review Meeting 2003/12/4

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

Rosenbluth Separation  Elastic e-p cross section  At fixed Q 2, fit dσ/dΩ vs. tan 2 (θ/2) »Measurement of absolute cross section »Dominated by either G E or G M

Polarization Transfer  Recoil proton polarization  Focal Plane Polarimeter »recoil proton scatters off secondary 12 C target »P t, P l measured from φ distribution »P b, and analyzing power cancel out in ratio

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, preliminary results expected soon

Super Rosenbluth Separation

Form Factor 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 Q 2 points »symmetric detector: ratio measurement  Different systematics »insensitive to P b and P t  Q 2 = 0.07 – 0.9 (GeV/c) 2 »overlap with JLab data and RpEX (future exp. at Bates to measure r p )

Asymmetry Super-ratio Method  Polarized cross section  Super-ratio

Projected Results  Statistics »A 1, A 2  Systematics »θ * 1, θ * 2 Δp, Δθ, Δβ Errors are minimized as a function of β (target spin angle)

Contributions  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

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

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

TOF Calibrations  Position – coplanarity  Timing – cosmics

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

Newton-Rhapson Track Fitter

Tracking Resolution

Asymmetry  very low dilution due to problems with ABS 180 degree holding field0 degree holding field 45 degree holding field left sector right sector

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

ABS Enhancements MEASURED FIELD ON THE POLE TIPS: Magnet kG Magnet kG Magnet kG Magnet kG Magnet kG Magnet kG Magnet kG Sextupole Damage BLAST Field Effect

New Asymmetry Results 0 degree holding field, ABS improvementstarget z-dependence left sectorright sectorleft sector previous results

Outlook  Work on hardware/software complete  Now taking high polarization data  Will concentrate on A, not G E p /G M p  Schedule »December: asymmetry analysis »January: start writing thesis »April: thesis defense