slac-pub-2605 - SLAC - Stanford University

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scale-constant A is relatively small: Aeff 5 100 to 300 MeV. [The larger
value is only possible if mass corrections are important.1
A more qualitative success of QCD is the fact that the pion form factor,
computed with the asymptotic wavefunction normalized to the pion decay con-
stant, is within a factor of -2 of the observed space-like data. The
definitive check of the predictions for F,(Q') will require an evaluation
of the order as(Q2) correction, as well as further constraints on the pion
distribution amplitude (p(x,Q). As we shall show in Sect. 5, measurements of
the scaling properties and angular dependence of the two-photon processes
+o
do,dt(yy+M%), with M = a-' , pt'i and their ratio to the corresponding
,
e+e- -+ M+M- cross sections can provide extraordinary checks on QCD and
important constraints on the form of the distribution amplitudes at non-
asymptotic momenta. These two-photon processes are the simplest non-trivial
hadronic scattering amplitudes computable in perturbative QCD. Pinch contri-
butions are power-law suppressed in this case. We also emphasize the import-
ance of experimentally checking the ratio of If to K+ to pL + form factors which
are predicted to asymptotically approach the ratios ff : fi: 2fi - 1:1.5 : 2.5.
The fact that the pion form factor has the same sign as its value at Q2=0
(i.e., no zeroes) is a non-trivial check of QCD; for scalar gluons, the meson
form factor would change sign as Q2 increases. Another qualitative success
of QCD is its apparent explanation of the surprisingly large normalization of
the pp+pp and xp+np scattering amplitudes and the magnitude of large momentum
transfer nuclear form factors. It remains an open question whether the large
spin polarization observed in large angle pp+pp scattering at Argonne can be
explained in terms of perturbative QCD mechanisms.