The QCD Quark Propagator in Coulomb Gauge and - Institut für Physik

Chapter 6. Nucleon Form Factors in a Covariant Diquark-Quark model 85
1.2
1
/ G M
p
µ p
G E
p
0.8
0.6
0.4
0.2
set A
set B
SLAC
JLab 1
JLab 2
0
0 1 2 3 4 5 6
Q 2 [GeV 2 ]
Figure 6.5: Proton form factor ratio µ p G p E (Q2 )/G p M (Q2 ), [AHK + 05]. Calculated results:
lower band - set A in table 6.1; and upper band - set B. For both bands, G p E (Q2 ) was
calculated using the point-particle values: µ 1 + = 2 and χ 1 + = 1, equation(6.77), and κ T = 2,
equation(6.83); i.e., the reference values in tables 6.2–6.4. Variations in the axial-vector
diquark parameters used to evaluate G p E (Q2 ) have little effect on the plotted results. The
width of the bands reflects the variation in G p M (Q2 ) with axial-vector diquark parameters and,
in both cases, the upper border is obtained with µ 1 + = 3, χ 1 + = 1 and κ T = 2, while the
lower has µ 1 + = 1. The data are: squares - reference[J + 00]; diamonds - reference[G + 02]; and
circles - reference[W + 94].
G + 02]. It should be stressed that neither this nor the Rosenbluth [W + 94] data played any
role in the preceding analysis or discussion.
The extension of the calculation for set B to higher momenta is displays in figure 6.6.
The form factor ration passes through zero at Q 2 ≈ 6.5GeV 2 . This prediction still awaits
an experimental test. In our model the existence of the zero is robust but its location
depends on the model’s parameters.
We have also examined the proton’s Dirac and Pauli form factors in isolation. On
the domain covered, neither F 1 (Q 2 ) nor F 2 (Q 2 ) show any sign they have achieved the
asymptotic behaviour anticipated from perturbative QCD.
In figure6.7 we depict weighted ratios of these form factors. Our numerical results are