Comparison of the isolated direct photon cross sections in p(p)over-bar collisions at root s=1.8 TeV and root s=0.63 TeV

We have measured the cross sections d(2)sigma/dP(T)deta for production of isolated direct photons in p (p) over bar collisions at two different center-of-mass energies, 1.8 TeV and 0.63 TeV, using the Collider Detector at Fermilab. The normalization of both data sets agrees with the predictions of quantum chromodynamics for a 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)(=2P(T)/roots). This disagreement in the x(T) ratio is difficult to explain with conventional theoretical uncertainties such as scale dependence and parton distribution parametrizations.