  We have measured the cross sections $d^2\sigma/dP_T d\eta$ for production of
isolated direct photons in \pbarp collisions at two different center-of-mass
energies, 1.8 TeV and 0.63 TeV, using the Collider Detector at Fermilab (CDF).
The normalization of both data sets agree with the predictions of Quantum
Chromodynamics (QCD) for photon transverse momentum ($P_T$) of 25 GeV/c, but
the shapes versus photon $P_T$ do not. These shape differences lead to a
significant disagreement in the ratio of cross sections in the scaling variable
$x_T (\equiv 2P_T/\sqrt{s}$). This disagreement in the $x_T$ ratio is difficult
to explain with conventional theoretical uncertainties such as scale dependence
and parton distribution parameterizations.
