A Precision Measurement of GEpGMp with BLAST

1
A Precision Measurement of GEp/GMp with BLAST

• Chris Crawford
• MIT Laboratory for Nuclear Science
• for the BLAST collaboration

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Introduction

• GE,GM fundamental quantities describing
  charge/magnetization in the nucleon
• Test of QCD based calculations and models
• Provide basis for understanding more complex
  systems in terms of quarks and gluons
• QED Lamb shift

3
World Data

• Unpolarized Data
• CEA, Bonn, DESY, SLAC
• Christy et al. (JLab 2004)
• Arrington et al. (JLab )
• Polarization Transfer
• Milbrath et al. (BATES) 1999
• Jones et al. (JLab), 2000
• Dieterich et al. (MAMI), 2001
• Gayou et al. (JLab), 2002
• Theory
• quark orbital momentum
• 2g effects

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World Data

• Unpolarized Data
• CEA, Bonn, DESY, SLAC
• Christy et al. (JLab 2004)
• Arrington et al. (JLab )
• Polarization Transfer
• Milbrath et al. (BATES) 1999
• Jones et al. (JLab), 2000
• Dieterich et al. (MAMI), 2001
• Gayou et al. (JLab), 2002
• Theory
• quark orbital momentum
• 2g effects

5
World Data

• Unpolarized Data
• CEA, Bonn, DESY, SLAC
• Christy et al. (JLab 2004)
• Arrington et al. (JLab )
• Polarization Transfer
• Milbrath et al. (BATES) 1999
• Jones et al. (JLab), 2000
• Dieterich et al. (MAMI), 2001
• Gayou et al. (JLab), 2002
• Theory
• quark orbital momentum
• 2g effects

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MIT-Bates Exp. 01-01
Spokespersons H. Gao, J. Calarco, H. Kolster

• new technique
• precision test
• of theories
• at low Q2
• meson cloud structure

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Form Factor Ratio _at_ BATES

• Exploits unique features of BLAST
• internal target low dilution, fast spin reversal
• large acceptance simultaneously measure all Q2
  points
• symmetric detector ratio measurement
• Different systematics
• also insensitive to Pb and Pt
• no spin transport
• Q2 0.1 0.8 (GeV/c) 2
• overlap with JLab data

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Asymmetry Super-ratio Method

• Helicity Asymmetry
• Super-ratio
• b 45

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Polarized Beam and Target

• Storage Ring
• E 850 MeV
• Iinj175 mA
• Pb 0.65

• Internal ABS Target
• 40 cm storage cell
• t 2.7?1013 cm-2
• Pt 0.40

• isotopically pure internal target
• high polarization, fast spin reversal
• L 1.6 ? 1031 cm-2s-1

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Detector Package

• BLAST Coils
• Backward-Angle TOF
• TOF Scintillators
• Wire Chambers
• Cerenkov Detectors
• Neutron Detectors
• LADS Detectors

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BLAST
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Data Sets

• 1H run 1, December 2003
• reversed BLAST field electrons out-bending
• 20 kC beam (3.4 pb-1) 480k elastic events
• 1H run 2, April 2004
• nominal BLAST field electrons in-bending
• 57 kC beam (9.6 pb-1) 950k elastic events
• 2D run, May October 2004
• 450 kC beam (76 pb-1) Pz.75
• 1H run 3, November 2004
• 250 kC (42 pb-1) planned

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TOF Scintillator Cuts
TOF paddle, proton
TOF paddle, electron
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Resolution and Yields
preliminary
TOF paddle
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Experimental Spin Asymmetry
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GE/GM by Super-ratio Method
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GE/GM by Fit Method

• fit for
• take advantage of large acceptance
• optional also fit for b (spin angle)
• super-ratio method
• in each Q2 bin
• redundant measurement of polarization

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Preliminary Results

• combineddatasets(77 kC)
• preliminary results based on ¼ of the projected
  data

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Preliminary Results

• combineddatasets(77 kC)
• preliminary results based on ¼ of the projected
  data

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Conclusion

• new precision measurement of GEp/GMp
• exploits unique capabilities of BLAST
• 2 4 projected errors at low Q2