Measurements of the Electromagnetic Form Factor of the Proton

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Measurements of the Electromagnetic Form Factor
of the Proton

• Huihong Li
• June 9th 2004

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Outline

1. Introduction
2. Event selection
3. Luminosity
4. Efficiency
5. Result
6. Summary

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1. Introduction

• ? The nucleon is not pointlike, but by far there
  is no perfect
• theory to describe its complex structure. We
  can only
• empirically use electromagnetic form factor to
  describe
• the distributions of inner charge and magnetic
  moment.
• ? Four form four form factors , ,
  ,
• ?The early measurements of proton form factor
  were in the
• space-like region, and there are a lot of
  results which
• show that empirical dipole formula can work
  well for
• proton form factors.

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• ? While in the time-like
• region, the each
• experiment group's
• data is very limited .
• Especially for 6 lt s lt9 ,
  there is no data at present.
• ? Two processes,
• and , are used to
• measure proton form
• factor. In the figure and
• table, the studies of these
• two processes from other
• experiments are shown.

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2. Event selection
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3. Luminosity

• ? The integrated luminosity at each data point
  was calculated
• via an analysis of wide-angle Bhabha events.
• ? The method was as follows We first select
  signal Bhabha
• events using BSC information only. The
  efficiency of this
• selection procedure is determined by using a
  non-BSC
• selection method to find Bhabha events in the
  data and
• comparing efficiencies. The motivation for
  using a data-
• based efficiency deterimation was to avoid
  known
• discrepancies between the MC and data angular
  distributions
• due to ribs in the BSC.
• ? The final calculation was

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4. Efficiency

• ? With M.C data the efficiency can be calculated.
  Here we use
• v10402 SIMBES to generate M.C data. In the
  generator of
• p2bb( ), some corrections about
  the initial state
• radiation, Coulomb effect and final state have
  been taken
• into account.
• ? Initial state radiation correction
• In 1985, Kuraev and Fadin presented to use
  the structure function of the electron and
  positron to calculate the initial state radiative
  correction.
• This scheme is a universal component of the
  corrections to processes with an arbitrary final
  state(such as , ).
  

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• ? Coulomb effect correction
• ? Final state correction

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? Efficiency With the generator taken
account of initial radiative correction,
Coulomb effect, and final state correction, 10
thousand M.C events for each energy point are
generated. The detection efficiency is
calculated by submitting M.C events to the
same cut criterion used for real data.


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5. Result

• ? Form factor

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? Error analysis
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6. Summary

• ? The proton form factors have been presented for
  10 center-
• of-mass energies between 2.00 and 3.07 GeV
  about 2
• batches data R scan data taking in 1999 which
  are
• reconstructed by v103 and other 3 points data
  in 2004.
• ? There is no one signal at 2.9GeV, so the upper
  limits of the
• cross section and form factor are presented at
  90 C.L.
• ? The result is consistent to the PQCD
  prediction,
• the express in experiment is,
• in which 0.3GeV is the QCD scale
  parameter and C is a free parameter.