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

Abstract

A recent global analysis of direct photon production at hadron collider and fixed target experiments has noted a disturbing trend of disagreement between next-to-leading-order (NLO) calculations and data. The conjecture has been made that the discrepancy is due to explicit multiple parton emission effects which are not accounted for in the theoretical calculations. We investigate this problem by merging a NLO calculation of direct photon production with extra multiple parton emissions via the parton shower (PS) algorithm. Our calculation maintains the integrity of the underlying NLO calculation while avoiding ambiguities due to double counting of multiple parton emissions. We find that the NLO+PS calculation can account for much of the theory/CDF data discrepancy at √ s = 1.8 TeV. It can also account for much of the theory/UA2 discrepancy if a very large virtuality is assumed to initiate the initial state parton shower. For lower energy data sets (e.g. √ s < 63 GeV), NLO+PS calculations alone cannot account for the data/theory discrepancy, so that some additional non-perturbative k T smearing is needed.

Associated production of prompt photons and heavy quarks in off-shell gluon–gluon fusion

“…At the leading order of perturbative Quantum Chromodynamics (pQCD), prompt photons can be produced via quark-gluon Compton scattering or quark-antiquark annihilation and so, the cross sections of these processes are strongly sensitive to the parton (quark and gluon) content of a proton 1 . The perturbative QCD calculations [11] in the next-to-leading order (NLO) approximation agree with the recent high-p T measurements [6] within uncertainties (see also discussions in [12][13][14][15][16]). However, there are still open questions.…”

Isolated photons at hadron colliders at O(ααs2) (I): Spin-averaged case

“…All calculations now agree fully for the direct contribution, and it is therefore now possible to study whether the use of modern parton distributions coupled with the inclusion of the NLO fragmentation contributions will improve the agreement between theory and experiment. This will have a direct bearing on recent studies [17] which suggested that initial state gluon radiation can be used to explain the difference in slope between theory and data. A reduction of the discrepancy would suggest that less initial state radiation would be needed.…”

“…This will have a direct bearing on recent studies [17] which suggested that initial state gluon radiation can be used to explain the difference in slope between theory and data. A reduction of the discrepancy would suggest that less initial state radiation would be needed.…”

QCD at high energy

“…In the context of the slicing method, an approach has been proposed [ 162,163,164,165] which exploits an idea of refs. [ 166,167] (see also [ 168]). The slicing parameter δ in eq.…”