Measurement of direct photon production at Tevatron fixed target energies

TL;DR

This paper presents precise measurements of high-p_T direct-photon production in fixed-target collisions using π−Be, pBe, and pp interactions. It combines detailed event reconstruction with two independent Monte Carlo simulations to estimate backgrounds and acceptance corrections, and compares results to NLO PQCD with and without k_T enhancements. The data show that standard NLO PQCD underpredicts the cross sections, while k_T-enhanced predictions with softer gluon PDFs better describe the observations, highlighting the role of initial-state soft-gluon radiation. These measurements offer valuable constraints on the large-x gluon distribution and motivate incorporating direct-photon data into global PDF fits, complementing Tevatron jet data. Overall, the study advances the understanding of direct-photon production mechanisms and the gluon structure of hadrons at fixed-target energies.

Abstract

Measurements of the production of high transverse momentum direct photons by a 515 GeV/c piminus beam and 530 and 800 GeV/c proton beams in interactions with beryllium and hydrogen targets are presented. The data span the kinematic ranges of 3.5 < p_T < 12 GeV/c in transverse momentum and 1.5 units in rapidity. The inclusive direct-photon cross sections are compared with next-to-leading-order perturbative QCD calculations and expectations based on a phenomenological parton-k_T model.

Figures (30)

• Figure 1: Leading-order diagrams for direct-photon production.
• Figure 2: A drawing of the liquid argon electromagnetic calorimeter with some components pulled away in one quadrant to reveal a view of the internal details.
• Figure 3: The longitudinal distribution of reconstructed vertices for 1990 (top) and 1991-1992 (bottom) target configurations. The data are represented by the histogram, the results from the detailed detector simulation by the points.
• Figure 4: Fraction of energy deposited in the front section of the EMLAC for identified electromagnetic showers (electrons from photon conversions) and hadronic showers (charged pions from $K_s$ decays). The data are represented by the histograms, the results from the detailed detector simulation by the points. The distributions have been normalized to unit area.
• Figure 5: Distribution of the distance squared between the reconstructed positions of electromagnetic showers and the nearest charged-particle track in data (histogram) and in the detailed detector simulation ($\bullet$). To enhance the electron fraction in these samples, only showers with $>0.75$ of their energy deposited in the front section of the EMLAC contribute to this plot. The distributions have been normalized to the same area.