Beyond leading order effects in photon pair production at the Tevatron

We discuss effects induced by beyond leading order contributions to photon pair production. We point out that next to leading order contributions to the fragmentation component of the signal lead to a change in the shape of distributions. This is already mildly visible in present Tevatron data though stringent isolation criteria tend to suppress the fragmentation component considerably. We expect the effect to be experimentally confirmed in future data samples with higher statistics which would serve as a precision test for QCD.Comment: 11 pages, 5 eps figures, uses feynmf.st

A next-to-leading order study of photon-pion and pion pair hadro-production in the light of the Higgs boson search at the LHC

We discuss the production of photon-pion and pion pairs with a large invariant mass at collider energies. We present a study based on a perturbative QCD calculation at full next-to-leading order accuracy, implemented in the computer programme DIPHOX. We give estimations for various observables, which concern the reducible background to the Higgs boson search in the channel H --> gamma gamma, in the mass range 80-140 GeV at the LHC. We critically discuss the reliability of these estimates due to our imperfect knowledge of fragmentation functions at high z and a subtle interplay between higher order corrections and realistic experimental cuts. We conclude that, whereas the invariant mass spectrum of photon-pion pairs is theoretically better under control, in the dipion case large uncertainties remain.Comment: 26 pages Latex, 14 eps figures, replaced by published versio

Stable One-Dimensional Integral Representations of One-Loop N-Point Functions in the General Massive Case: I - Three Point Functions

In this article we provide representations for the one-loop three point functions in 4 and 6 dimensions in the general case with complex masses. The latter are part of the GOLEM library used for the computation of one-loop multileg amplitudes. These representations are one-dimensional integrals designed to be free of instabilites induced by inverse powers of Gram determinants, therefore suitable for stable numerical implementations.Comment: 40 pages, 1 figur

A full Next to Leading Order study of direct photon pair production in hadronic collisions

We discuss the production of photon pairs in hadronic collisions, from fixed target to LHC energies. The study which follows is based on a QCD calculation at full next-to-leading order accuracy, including single and double fragmentation contributions, and implemented in the form of a general purpose computer program of "partonic event generator" type. To illustrate the possibilities of this code, we present the comparison with observables measured by the WA70 and D0 collaborations, and some predictions for the irreducible background to the search of Higgs bosons at LHC in the channel $h \to \gamma \gamma$. We also discuss theoretical scale uncertainties for these predictions, and examine several infrared sensitive situations which deserve further study.Comment: 45 pages Latex, 16 eps files plus some metafont files; replaced by the version to appear in Eur. Phys. J.

A critical phenomenological study of inclusive photon production in hadronic collisions

We discuss fixed target and ISR inclusive photon production and attempt a comparison between theory and experiments. The dependence of the theoretical predictions on the structure functions, and on the renormalization and factorization scales is investigated. The main result of this study is that the data cannot be simultaneously fitted with a single set of scales and structure functions. On the other hand, there is no need for an additional intrinsic $k_{_T}$ to force the agreement between QCD predictions and experiments, with the possible exception of one data set. Since the data cover almost overlapping kinematical ranges this raises the question of consistency among data sets. A comparative discussion of some possible sources of experimental uncertainties is sketched.Comment: 22 pages, 3 tables, 10 figures, Late

Photon - Jet Correlations and Constraints on Fragmentation Functions

We study the production of a large-pT photon in association with a jet in proton-proton collisions. We examine the sensitivity of the jet rapidity distribution to the gluon distribution function in the proton. We then assess the sensitivity of various photon + jet correlation observables to the photon fragmentation functions. We argue that RHIC data on photon-jet correlations can be used to constrain the photon fragmentation functions in a region which was barely accessible in LEP experiments.Comment: 23 pages, 9 figure