Measurement of direct photon production in p + p collisions at sqrt(s) = 200 GeV

TL;DR

Next-to-leading order perturbative QCD (pQCD) describes the data well for pT >5 GeV/c, where the uncertainties of the measurement and theory are comparable and the observed fraction of isolated photons is well described by pQCD.

Abstract

Cross sections for mid-rapidity production of direct photons in p+p collisions at the Relativistic Heavy Ion Collider (RHIC) are reported for 3 < p_T < 16 GeV/c. Next-to-leading order (NLO) perturbative QCD (pQCD) describes the data well for p_T > 5 GeV/c, where the uncertainties of the measurement and theory are comparable. We also report on the effect of requiring the photons to be isolated from parton jet energy. The observed fraction of isolated photons is well described by pQCD for p_T > 7 GeV/c.

Figures (3)

• Figure 1: Correction for missing photon partners to the $\pi^0$ ($R^\mathrm{miss}_{\pi^0}$) vs. $p^\gamma_\mathrm{T}$. Dashed lines show the systematic uncertainty. Inset: Different contributions to the inclusive photon spectrum. Solid (dashed) lines represent all hadronic ($\pi^0$) decay contributions. The data points show the remaining photon contributions.
• Figure 2: (a) Direct photon spectra with NLO pQCD calculations for three theory scales, $\mu$. Brackets around data points show systematic errors. (b) Comparison to the NLO pQCD calculation for $\mu=\hbox{$p_\mathrm{T}\,$}$, with upper and lower curves for $\mu=\hbox{$p_\mathrm{T}\,$}/2$ and 2$p_\mathrm{T}\,$.
• Figure 3: Closed circles: Ratio of isolated direct photons to all direct photons from the $\pi^0$-tagging method. The statistical uncertainties are shown as black error bars and the systematic uncertainties are plotted as shaded bars. The solid and dashed curves are NLO pQCD calculations with three theory scales for BFGII Bourhis:1997yu and one scale for GRV Gluck:1992zx parton to photon fragmentation functions. Open circles: Ratio of isolated photons from $\pi^0$ decays to all photons from $\pi^0$ decays.