A GLOBAL QCD STUDY OF DIRECT PHOTON PRODUCTION

TL;DR

This study performs a global NLO QCD analysis of direct photon production by combining fixed-target and collider data to test perturbative QCD and constrain parton distributions across x from 0.01 to 0.6. It finds that most data show a steeper photon p_t (x_t) dependence than predicted by NLO QCD, and that neither new PDFs nor improved fragmentation functions can fully account for the observed pattern since deviations occur at different x for different experiments. The authors show that introducing k_t broadening from initial-state radiation or nonperturbative effects improves agreement but does not fully resolve the discrepancies, suggesting a need for resummation or kt-dependent approaches. They conclude that robust gluon determination requires multi-process data (DIS, direct photons, jets) and that future work should refine the treatment of k_t physics in global analyses.

Abstract

A global QCD analysis of the direct photon production process from both fixed target and collider experiments is presented. These data sets now completely cover the parton $x$ range from 0.01 to 0.6, thereby providing a stringent test of perturbative QCD and parton distributions. Previous detailed studies of direct photons emphasized fixed target data. We find most data sets have a steeper $p_t$ distribution than the QCD prediction. Neither global fits with new parton distributions nor improved photon fragmentation functions can resolve this problem since the deviation occurs at different $x$ values for experiments at different energies. A more likely explanation is the need for additional broadening of the $k_t$ of the initial state partons. The magnitude and the possible physical origin of this effect are investigated and discussed.

Figures (6)

• Figure 1: Direct photon production data from WA70 is compared to the NLO QCD fit of ABFOW using optimized scales. The fractional difference is shown in order to display details of the comparison. (see text)
• Figure 2: Fixed target direct photon experiments WA70, UA6, and E706 are compared to a NLO QCD results. Fractional differences between data points and the CTEQ2M fit are shown. (see text)
• Figure 3: Direct photon data from the CDF experiment is compared to NLO QCD predictions. (see text)
• Figure 4: Compilation of direct photon experiments compared to NLO QCD predictions using CTEQ2M parton distributions. (see text)
• Figure 5: Compilation of direct photon experiments compared to a NLO QCD prediction using parton distributions fit using all the data shown. (see text)