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

TL;DR

This study measures isolated direct photon production cross sections in proton-antiproton collisions at two energies, revealing shape differences inconsistent with QCD predictions and challenging current theoretical models.

Contribution

It provides a comparative analysis of photon cross sections at different energies, highlighting discrepancies in shape and ratios not explained by standard uncertainties.

Findings

Normalization agrees with QCD predictions at 25 GeV/c

Shape differences between energies are observed

Discrepancy in cross section ratios challenges existing theories

Abstract

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.