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
CDF Collaboration, D. Acosta
TL;DR
The paper reports a two-energy measurement of isolated direct photon production in p-pbar collisions, using the CDF detector to test QCD predictions and constrain the gluon distribution. It employs two complementary photon-background subtraction methods and cross-energy cross-section ratios to minimize systematics. While the overall normalization agrees with NLO QCD near a reference Pt, the shapes with Pt and especially the x_T ratio between 0.63 and 1.8 TeV deviate significantly from theory, indicating missing physics in standard calculations. A simple Gaussian k_T recoil is shown to partially restore agreement, pointing to the importance of initial-state soft-gluon radiation and motivating more complete higher-order treatments. The study highlights the efficacy of cross-section ratios in challenging QCD predictions and guiding theoretical refinements for prompt-photon production.
Abstract
We have measured the cross sections $d^2σ/dP_T dη$ 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.