Comparison of the isolated direct photon cross sections in $p\bar{p}$ collisions at $\sqrt{s}=$ 1.8-TeV and $\sqrt{s}=$ 0.63-TeV

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.

Note:

34 pages, submitted to Physical Review D Report-no: FERMILAB-PUB-01/390-E

13.85.Qk
12.38.Qk
anti-p p: colliding beams
anti-p p: inclusive reaction
photon: direct production
photon: transverse momentum
differential cross section: transverse momentum
rapidity dependence
quantum chromodynamics
cross section: ratio

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