High-pTproduction of direct photons and jets in quantum chromodynamics

Abstract

The production of high-p, direct photons is studied using quantum chromodynamics. The theoretical predictions are shown to be compatible with the existing data over the energy interval 20 < fi< 63 GeV. In addition, the possibility of using a high-p, direct-photon trigger in hadronic-jet experiments is studied. By simultaneously triggering on a direct photon and on the away-side hadronic jet it is possible to obtain more precise information concerning the underlying dynamical subprocesses than if a two-jet trigger is used. In particular, one can, by using a combination of n * , p , and p beams, measure both the pion quark and gluon distribution functions as well as the gluon distribution function for the proton. Furthermore, if particle identification is available for the away-side jet, one can separately measure various quark and gluon fragmentation functions. A method of measuring the Q 2 dependences of these various functions is also discussed.

Parton energy loss in heavy-ion collisions via direct-photon and charged-particle azimuthal correlations

“…The leading-order (LO) production processes of direct photons, quark-gluon Compton scattering (q + g → q + γ ) and quark-antiquark annihilation (q +q → g + γ ), are free from the uncertainties accompanying fragmentation. In the LO, the outgoing high-p T γ balances the p T of the partner parton separated by π in azimuth ("away-side"), modulo corrections owing to parton intrinsic p T [5]. The study of the spectra of the away-side jet particles associated with a high-p T γ dir trigger, as a function of p trig T , can constrain the dependence of E on E. The mean-free path of the γ in the medium is large enough that its momentum is preserved, regardless of the position of the initial scattering vertex.…”

Multiple parton emission effects in next-to-leading-order direct photon production

“…Direct photon production [1][2][3][4][5] in hadronic collisions has long been recognized as an important testing ground for perturbative QCD since many of the ambiguities involved in measuring jets are not present when analyzing photons. Direct photon production in lowest order QCD takes place via annihilation and Compton scattering Feynman graphs.…”

Relativistic Heavy-Ion Collider (RHIC) physics overview

“…For example, if we trigger on a direct photon and look at away side hadrons or identified particles, the fragmentation function can be precisely studied and compared to theoretical calculations [136]. Both STAR and PHENIX measured the fragmentation functions in p+p and Au+Au collisions with respect to a direct photon trigger, and the results are consistent with most of the theoretical calculations [137,138].…”