The Isolated Photon Cross Section in p pbar Collisions at sqrt(s) = 1.8 TeV

Abstract

We report a new measurement of the cross section for the production of isolated photons, with transverse energies (ET) above 10 GeV and pseudorapidities |eta| < 2.5, in p pbar collisions at sqrt{s} = 1.8 TeV. The results are based on a data sample of 107.6 pb-1 recorded during 1992--1995 with the D0 detector at the Fermilab Tevatron collider. The background, predominantly from jets which fragment to neutral mesons, was estimated using the longitudinal shower shape of photon candidates in the calorimeter. The measured cross section is in good agreement with the next-to-leading order (NLO) QCD calculation for ET > 36 GeV.

Figures (4)

• Figure 1: Distribution of the discriminant variable for $21< E_T^\gamma< 26\,{\rm GeV}$ central photon candidates (points with error bars), and the fitted distribution (solid curve) composed of Monte Carlo photons (curve labelled $(a)$) and jets with and without charged particles (curves labelled $(c)$ and $(b)$, respectively). The Monte Carlo curves shown here were smoothed for clarity (this was not done in the fitting itself). Results of these fits provide the purity ${\cal P}$ of the signal: for this bin, ${\cal P}= 0.58\pm0.07$.
• Figure 2: The fraction of photon candidates that are direct photons as a function of $E_{T}^{\gamma}$, for central and forward photons.
• Figure 3: The cross section $d^2\sigma/dE_T^\gamma\,d\eta$ for isolated photons as a function of transverse energy $E_T^\gamma$, for central and forward regions. The curves show the NLO QCD calculated cross sections.
• Figure 4: Difference between the measured differential cross section for isolated photon production and the prediction from NLO QCD, using CTEQ4M parton distributions.