A Precision Measurement of GEp/GMp with BLAST

1
A Precision Measurement of GEp/GMp with BLAST

• Chris Crawford
• MIT Laboratory for Nuclear Science
• May 18, 2005

2
Outline

• Introduction
• Formalism
• World Data
• Experiment overview
• Experimental Setup
• LDS polarized target
• BLAST detector
• Calibrations

• Analysis
• Cuts Yields
• Asymmetry
• Extraction of mGE/GM
• Systematic errors
• Conclusion
• Results mGE/GM
• Separation of GE, GM

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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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Form Factors of the Nucleon

• Form Factor definition
• Nucleon current
• Breit frame

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Elastic Cross Section
b target spin angle w/r to the beam line
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World Data
World Unpolarized Data
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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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GE/GM World Data
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Theory and Models

• Direct QCD calculations
• pQCD scaling at high Q2
• Lattice QCD
• Meson Degrees of Freedom
• Dispersion analysis, Höhler et al. 1976
• Soliton Model, Holzwarth 1996
• VMD Chiral Perturbation Theory, Kubis et al.
  2000
• Vector Meson Dominance (VMD), Lomon 2002
• QCD based constituent quark models (CQM)
• LF quark-diquark spectator, Ma 2002
• LFCQM CBM, Miller 2002

Nucleon Electromagnetic Form Factors, Haiyan
Gao, Int. J. of Mod. Phys. E, 12, No. 1,
1-40(Review) (2003)
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Models Consistent with Polarized 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 0.9 (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 98 pb-1 D2 126 pb-12005 run

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Atomic Beam Source

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

Spin State Selection
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Laser Driven Source (LDS)

• Optical pumping Spin Exchange
• Spincell design
• Target and Polarimeter
• Results

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Spin-Exchange Optical pumping
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LDS Experimental Setup
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Pictures of the LDS
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Atomic Dissociation
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Atomic Polarization
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Comparison of Polarized Targets
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BLAST Detector Package

• Detector Requirements
• Definition of q
• ??e ? 2?, ??e ? .?, ?z ? 1 cm
• e/p/n/?? separation
• PID ?t ? 1?, Cerenkov
• Optimize statistics
• Large Acceptance
• Asymmetry Super-ratios
• Symmetric Detector
• Polarized targets
• 1 m diameter in target region
• Zero field at target
• B-gradients ? 50 mG/cm

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TOF Scintillators

• timing resolution s350 ps
• velocity resolution s 1

coplanarity cuts
ADC spectrum
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Cosmics TOF Calibration
L 15
L 12
channels
L 9
L 6
L 3
L 0
channels
R 0
R 3
R 6
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TOF Efficiency
green efficiency magenta non-bias red
misses
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TOF Scintillator Cuts
TOF paddle, proton
TOF paddle, electron
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Cerenkov Detectors

• 1 cm thick aerogel tiles
• Refractive index 1.02-1.03
• White reflective paint
• 80-90 efficiency

• 5" PMTs, sensitive to 0.5 Gauss
• Initial problems with B field
• Required additional shielding
• 50 efficiency without shielding

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Drift Chambers

• 2 sectors 3 chambers
• 954 sense wires
• 200µm wire resolution
• signal to noise ratio 201

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NSED (Online Display)
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Reconstruction

• Scintillators
• timing, calibration
• Wire chamber
• hits, stubs, segments
• link, track fit
• PID, DST

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Newton-Rhapson Track Fitter
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Hyperbolic time?dist function
D
TDC
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Linear T2D Calibration
72 33
1mm resolution
c2
28 MeV 12 MeV
D p (GeV/c)
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Wire Chamber Efficiency
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WC Offsets/Resolution/Cuts
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Tracking Efficiency
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Comparison of Yields with MC
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Experimental Spin Asymmetry
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i left,right sector j Q2 bin (1..n) b spin
angle

• Single-asymmetry Method
• measure P first, use to calculate R
• model-dependent
• Super-ratio Method
• 2 equations in P, R
• in each Q2 bin j
• independent measure of polarization in each bin!
• 2n parameters Pj, Rj

• Global Fit Method
• fit for P, R1, R2, from all Aij together
• model independent
• better statistics
• n1 parameters
• can also fit for b

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Extractions of m GE/GM
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Single AsymmetryExtraction
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Systematic Errors

• DQ2 (1.8)
• comparison of qe and qp
• difference between left and right sectorsmost
  problematic
• appeal to TOF timing !

• Db (0.8)
• fieldmap 47.1 1
• Hohler 47.5 0.8
• Fit Method 42 3
• (1st 7 bins) 48 4
• T20 analysis 46.5 3

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GE/GM Results
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Extraction of GE and GM
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GE and GM Results
BLAST World Data
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Q2 Corrections from TOF
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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 0.1 0.5 GeV2
• sensitive to the pion cloud
• is dip in GE around Q20.3 GeV2 real?
• systematic errors are being reduced