Large-momentum-transfer production of direct photons, jets, and particles

Abstract

The theoretical framework for describing the production of direct photons in hadronic collisions is reviewed. A detailed comparison between the theoretical predictions and existing data is presented along with a critical evaluation of the various sources of theoretical uncertainty. The information available from direct-photon experiments is contrasted with that learned from jet or single-hadron production. Prospects for new types of measurements in future experiments are also presented.

Quantitative constraints on the gluon distribution function in the proton from collider isolated-photon data

“…It is thus important to find new collider observables that provide independent information on g(x, Q 2 ) and help improve the accuracy of its determination and reduce its associated uncertainties. Prompt photon production in hadronic collisions, defined as the production of photons not issuing from the electromagnetic decays of hadrons, appears as an excellent observable to determine the gluon PDF, since at leading order (LO) it probes the gluon directly through the quark-gluon "Compton" process qg → γq [7][8][9]. As a matter of fact, up to about 12 years ago, prompt photon data were used to constrain g(x, Q 2 ) in global PDF fits [10,11], as well as in independent determinations of the strong coupling constant [12].…”

Effect of running coupling on photons from jet–plasma interaction in relativistic heavy-ion collisions

“…3 the results for jet-photons corresponding to the RHIC energies are shown, where we have taken T i = 446 MeV and t i = 0.147 fm/c. The individual contributions from hard and bremsstrahlung processes [38] are also shown for comparison. These are estimated using the formalism given in Ref.…”

“…These are estimated using the formalism given in Ref. [38]. The total yield comprises of photons from jet-plasma interaction (with energy loss), hard and bremsstrahlung processes, thermal Compton and annihilation processes.…”

“…Besides the thermal photons from QGP and hadronic matter we also calculate photons from initial hard scattering from the reaction of the type h A h B → γ X using perturbative QCD. We include the transverse momentum broadening in the initial state partons [34,35]. The crosssection for this process can then be written in terms of elementary parton-parton cross-section multiplied by the partonic flux which depends on the parton distribution functions (PDF) for which we take CTEQ parameterization [36].…”

The charged-hadron/pion ratio at the Relativistic Heavy Ion Collider

“…This can be accomplished via unintegrated parton distribution functions or, more phenomenologically, via a product assumption and a Gaussian transverse momentum distribution g(k T ) (characterized by the width k 2 T ) [22,23]. We apply the latter procedure in the present paper.…”