Analysis of E94110 Elastic Cross Sections and Global Fits

1
Analysis of E94-110 Elastic Cross Sections and
Global Fits
M. Eric Christy Hampton University
Hall C Summer Workshop Sept 11, 2003
2
Proton Elastic Form Factors
Dse(1 t) eGE2 tGM 2 dW sMott tGE
slope GM intercept

• Cross Section and Polarization transfer
  measurements disagree on GE/GM !
• Accurate elastic cross sections are needed for
  normalization and radiative corrections in wide
  range of experiments! What is wrong?
• Problem with formalism or 1/both techniques?

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Sensitivity of Cross Section to GE

• Cross section measurements become
  insensitive to GE at large Q2

• ?

Polarization transfer technique provides
experimental method of determining GE/GM at large
Q2 with good precision.
4
Recent Global Fits
Question Can we extract GE and GM separately
from the entire data set, ie. including both the
cross section and polarization transfer (PT)
measurements? How to combine the data?

• There have been two recent attempts to do this
  by E. Brash et.al ( PRC 65, 2002),
  and J.R. Arrington (arXivnucl-ex 2003).
• Brash extraction uses GE/GM from PT as
  constraint and performs a 1 parameter
  Rosenbluth refit of the cross section data .
• Arrington extraction uses and equal weighting c
  2 fit to both cross section and PT data.

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Jlab Hall C Experiment E94-110

• Performed Rosenbluth type separations to extract
  sL and sT in resonance region. ?
• Typical e pt-pt Uncertainties
  lt 1.5 systematic
  ltlt 1 statistical.

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E94-110 Elastic Cross Sections

• Elastic eP cross sections measured
  at 28 kinematic settings for
  ( 0.5 lt Q2 lt 5.5
  GeV2/c2).
• Typical e pt-pt Uncertainties
  lt 1.2 systematic
  lt 1 statistical.
• Not optimized for elastic L-Ts but a
  large range covered in Q2 e.

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Hall C E94-110 Systematics

• Use of a single spectrometer (HMS) for both high
  and low e.
• HMS optics and acceptance are well understood.
  ?
• Kinematics are well determined. ?
• Relatively large acceptance spectrometer. This
  has distinct advantages over previous
  experiments
  
  
  
  
  
  
  
  
  
  
• 4 cm tuna can target provides minimal
  
  target length/energy loss/multiple scattering
  effects.
• Beam current kept constant to within 2 uA

• Cutoff point in the radiative tail integration
  is typically not determined by acceptance (same
  cut for most all kinematics)!
• Relatively insensitive to resolution matching
  between MC and data!

• Small pt-pt uncertainties due to Q
  normalization, target boiling.

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HMS Monte Carlo

• Comparison of MC to E99-118 data using
  E94-110 resonance region model.

• Spectrometer model is excellent.
• Excellent agreement between
  different experiments!
  
• Acceptance is determined to lt 1 pt-pt
  in the kinematics.

Acceptance
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E94-110 Elastic Kinematics

• Elastic calibrations (Wrec MP) allow a
  determination of the kinematics to dE dE'
  0.04 dq 0.3 mrad.
  
• HMS E' and q consistent with many Hall C
  experiments over 3-4 years .
• E determination consistent with recent
  remapping of Arc Magnet.

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Elastic Cross Section Extraction

• Apply background subtractions and acceptance
  corrections in each E'-q bin.
• Integrate radiative tail in each q bin.
• Apply radiative corrections (code from SLAC NE11,
  modified for current target).

• Use Model to remove q dependence.
• Do Weighted average over q.

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Comparison to Arrington fit of previous Cross
Section measurements.

• Current measurements compare well to Arrington
  fit of s only.
• No systematic deviation with kinematics is
  observed.

12
Study of Radiative Tail Integration

• No e dependent systematic with cut is observed.
• Further studies performed indicate that cut is
  large enough provide relative insensitivity to
  MC resolution matching in extracting acceptance.
• Standard cut applied in analysis was W2max 1.13.

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Rosenbluth Separated GE/GM

• Data are more consistent with previous cross
  section data than with the
  polarization transfer results.
  
  
• Any systematic in the cross section data is
  global and not isolated to any
  single data set, device, or facility! ...
  Radiative corrections?

return from detour
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Comparisons to Recent Fits
C2/pdf (Q2 gt 1)
1.06
1.442
2.387
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Summary/Conclusions

• E94-110 measured elastic cross sections with
  precision comparable to previous best precision
  measurements from SLAC.
• Measurements are consistent with previous cross
  section measurements/Rosenbluth separations.
  
  
  
  
  
  
• Global extractions/fits can not give us
  separated GE GM unambiguously since some ansantz
  of how to combine the inconsistent data sets is
  needed.
• Fits to cross sections alone should be used for
  normalization / radiative corrections for other
  experiments.
• Discrepency needs to be resolved Experiment
  Super Rosenbluth, Theory 2g exchange? ...
  Progress is being made.
  
  
  
  
  
  
  
  
  
  

• It is unlikely that there is some heretofore
  unknown systematic in the cross section
  measurements.

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HMS Spectrometer
HMS Properties (pt-pt tune)
Detector Stack (view from above)
Kinematic Range
Central Momentum
0.5 7.5 GeV/c
Vertically Segmented Hodoscope
4-layer Electromagnetic Calorimeter
Central Angle
10.5O - 80O
2 sets of vertical horizontal Drift Chambers
Acceptance
DW
Gas Cerenkov
6.5 msr
Dp/p
/-9
Resolution
Dp/p
lt 0.1
Q
1 mrad

• Resolution is easily good enough for
  1.5 cross sections if spectrometer is
  understood well.
  
• Cer Cal provide p rejection factor
  104/1 Above 1GeV.

horizontally Segmented Hodoscope