Precision Measurement of GEpGMp with BLAST

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Precision Measurement of GEp/GMp with BLAST

• Chris Crawford
• MIT Laboratory for Nuclear Science
• Thesis Committee Review Meeting
• 2003/12/4

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Outline

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

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Rosenbluth Separation

• Elastic e-p cross section
• At fixed Q2, fit ds/dO vs. tan2(?/2)
• Measurement of absolute cross section
• Dominated by either GE or GM

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Polarization Transfer

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

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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

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Super Rosenbluth Separation
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Form Factor Ratio _at_ 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)

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Asymmetry Super-ratio Method

• Polarized cross section
• Super-ratio

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Projected Results

• Statistics
• A1, A2
• Systematics
• ?1, ?2 ?p, ??, ?ß

• Errors are minimized as a function of ß (target
  spin angle)

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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
• ElogBLAST electronic logbook
• Trigger, HV enhancements
• ROOT enhancement
• improved histogram drawing

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Radiative Corrections

• MASCARAD code
• A. Afanasev et al., Phys.Rev.D 64,113009
• Covariant calculation with no cutoff parameter
• small corrections (lt1) to asymmetry

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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

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TOF Calibrations

• Position coplanarity

• Timing cosmics

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Wire Chamber Tracking

• Garfield drift time simulation
• Newton-Rhapson track fitter

• 200 µm resolution
• 201 signal to noise ratio

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Newton-Rhapson Track Fitter
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Tracking Resolution
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Asymmetry
left sector
right sector

• very low dilution due to problems with ABS

45 degree holding field
left sector
left sector
right sector
right sector
180 degree holding field
0 degree holding field
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Atomic Beam Source

• Standard technology
• Dissociator nozzle
• 2 sextupole systems
• 3 RF transitions

Spin State Selection
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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 15.4
kG Magnet 7 15.2 kG
BLAST Field Effect
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New Asymmetry Results
previous results
previous results
left sector
right sector
left sector
0 degree holding field, ABS improvements
target z-dependence
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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