Isolated-photon production in polarized pp collisions

TL;DR

The paper advances the study of polarized prompt-photon production at RHIC by introducing a fragmentation-free photon isolation that enables the first fully consistent NLO calculation in this channel. It compares the new isolation to the traditional cone method, analyzes inclusive and photon+jet observables, and demonstrates reduced scale dependence at NLO. The work highlights the strong sensitivity of spin asymmetries to the polarized gluon density Δg, with predictions suggesting Δg can be constrained in the x-range 0.04–0.25, while also outlining future improvements such as completing NLO fragmentation calculations and potential resummations. Overall, the study provides a rigorous framework and phenomenological results to extract Δg from RHIC data using prompt photons.

Abstract

We perform a detailed study of the production of isolated prompt photons in polarized hadronic collisions, in the centre-of-mass energy range relevant to RHIC. We compare the results obtained for a traditional cone-isolation prescription, with those obtained by imposing an isolation condition that eliminates any contribution to the cross section from the fragmentation mechanism. The latter prescription will allow us to present the first fully consistent next-to-leading order calculation in polarized prompt-photon production. We will discuss the theoretical uncertainties affecting the cross section, addressing the issue of the reliability of the perturbative expansion, for both inclusive isolated-photon and photon-plus-jet observables. Finally, we will study the dependence of our predictions upon the polarized parton densities, and the implications for the measurability of the gluon density.

Figures (12)

• Figure 1: Transverse-momentum spectrum of the isolated photon, in the case of definition B, for polarized $pp$ collisions at $\sqrt{S}=500$ GeV. The polarized parton densities used are GRSV STD. The scale dependences of the Born and NLO results are also shown; see the text for details.
• Figure 2: Transverse-momentum spectrum of the isolated photon, in the case of definition A. The ratio of the cross section over that obtained with definition B is shown in the lower part. The rest of the figure displays the scale dependence, at the Born and NLO levels.
• Figure 3: Same as in fig. \ref{['fig:ptspectrumA']}, for non-isolated photons.
• Figure 4: Average transverse energy in the cone annuli around the photon.
• Figure 5: Pseudorapidity spectrum of isolated photons (definition B). The cross sections obtained by varying the renormalization and factorization scales are also shown.