A Precision Measurement of GEp/GMp with BLAST

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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
• Probe the pion cloud
• QED Lamb shift

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World Data
World Unpolarized Data
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m GE/GM World Data
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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
• input for P.V. experiments
• structure of pion cloud

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Asymmetry Super-ratio Method

• Beam-Target Double Spin Asymmetry
• Super-ratio
• b 45?

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Polarized Beam and Target

• Storage Ring
• E 850 MeV
• Imax225 mA
• Pb 0.65

• Internal ABS Target
• 60 cm storage cell
• t 4.9?1013 cm-2
• Pt 0.80

• isotopically pure internal target
• high polarization, fast spin reversal
• L 3.1 ? 1031 cm-2s-1
• H2 94 pb-1 D2 170 pb-12005 run

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BLAST
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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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Preliminary Results m GE/GM
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Extraction of GE and GM
BLAST World Data
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Conclusion

• 1st measurement of mGE/GM using double spin
  asymmetry
• 2 3.5 improvement in precision of mGE/GM at Q2
  0.10.8 GeV2
• sensitive to the pion cloud
• narrow dip structure in GE around Q20.3 GeV2
• systematic errors are under investigation