Proton Timelike Form Factors

1
Proton Timelike Form Factors

• The electromagnetic form factors of the proton in
  the time-like region
• can be extracted from the cross section for the
  process
• ?pp
  ? ee-
• First order QED predicts
• Data at high Q2 are crucial to test the QCD
  predictions for the
• asymptotic behavior of the form factors and the
  spacelike-timelike
• equality at corresponding values of Q2.

2

• Predictions of nucleon form factors are
  applicable up to high Q2 in both the
• spacelike and timelike regions.
• Perturbative QCD and analyticity relate timelike
  and spacelike form factors, predicting a
  continuous transition and spacelike-timelike
  equality at high Q2.
• At high Q2 PQCD predicts
• F1 and F2 are the Dirac and Pauli form
  factors respectively.
• PQCD and analyticity predict
• There are several unexpected features in the
  existing data which deserve further
• experimental investigation
• Threshold Q2 dependence.
• High Q2 predictions.
• Resonant structures.

3
Threshold Q2 Dependence
Steep behaviour near threshold observed by PS 170
at LEAR
4
High-Q2 predictions
The dashed line is the PQCD fit. The dot-dashed
line represents the dipole behaviour of the form
factor in the spacelike region for the
same values of Q2. The expected Q2 behaviour
is reached quite early, however there is a factor
of two between timelike and spacelike data
measured at the same Q2.
5
Resonant Structures
The dip in the total multihadronic cross section
and the steep variation of the proton form
factor near threshold may be fitted with a narrow
vector meson resonance, with a mass M ? 1.87 GeV
and a width ? ? 10-20 MeV, consistent with an N?N
bound state.
These considerations strongly support the
importance of a new measurement of the neutron
proton timelike form factors with much higher
statistics than previous work and with the
capability of separately determining the electric
and magnetic form factors.
6
Measurement of the Form Factor

• E835 statistics not sufficient to measure the
  angular distribution (and
• thus determine GE and GM separately. Calculate GM
  under two
• hypotheses
• (a) GE GM
• (b) GE contribution negligible

7
E835 Form Factor Measurement

• The dashed line is the PQCD fit

s (GeV2) 102?GM (a) 102? GM (b)
11.63
12.43
8
Future Measurements of the Proton Timelike FF

• Measurements at ee- machines (to be checked)
• BaBar
• Belle
• CLEO-III/CLEO-C
• BES
• Daphne
• VEPP
• Measurements at p?p machines
• PANDA