We have measured the cross sections d2 sigma/dP(t) d_eta for
production of isolated direct photons in p anti-p colllisions 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)/s**(1/2)). This
disagreement in the x_T ratio is difficult to explain with conventional
theoretical uncertainties such as scale dependence and parton distribution
parameterization.
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Updated: Friday, 2002 June 21 13:59:30 CDT automatically from input from Carol Picciolo