Study of partonkTsmearing effects in direct photon production at the Fermilab Tevatron

Abstract

Previous detailed studies of direct photon production from both fixed-target and collider experiments have witnessed a pattern of deviation between the measured inclusive cross sections and the corresponding theoretical expectations in the low transverse momentum (p T ) regime. Most data sets display steeper p T dependence than the next-to-leading-order ͑NLO͒ perturbative QCD ͑PQCD͒ calculations with standard choices of scales and parton distribution functions in this region. A simple implementation of higher-order soft-gluon corrections to the NLO PQCD predictions significantly improves the agreement between data and theory. This interesting feature motivated us to investigate the DO " and CDF measurements of inclusive photon cross section at ͱs ϭ1.8 TeV from the run 1b and also at ͱsϭ630 GeV. We use the latest updated parton distribution function CTEQ6M in the NLO QCD calculations for direct photon cross section to describe the data. The conventional theoretical uncertainties originating from scale dependence and gluon distributions have been illustrated. We estimate the impact of additional soft-gluon radiation on the direct photon production using PYTHIA ͑LO PQCD͒, which adds transverse momentum k T to initial-state partons through a Gaussian smearing. The impact of k T effects on the discrepancy in the low-p T region is explored using a phenomenological model, wherein we merge the NLO calculations with k T correction factors. We show that this approach provides a much more acceptable description of the Fermilab Tevatron data.

Study of noncollinear parton dynamics in the prompt photon photoproduction at HERA

“…The ZEUS fit to the data gave a k T value of about 1.7 GeV [3]. A similar situation is observed also at Tevatron energies: in order to describe the measured transverse momentum distributions of the photon the Gaussian-like k T spectrum with an average value of k T $ 3 GeV was introduced [9,10]. Of course, such large par-tonic k T must have a significant perturbative QCD component.…”

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.…”

“…Our approach is based on the k t -factorization scheme, which, unlike many early calculations [13,16], provides solid theoretical grounds for adequately taking into account the effects of initial parton momentum. The central part of our consideration is the off-shell gluon-gluon fusion subprocess g * + g * → γ + qq.…”

“…In the NLO QCD approximation, the observed discrepancy can be reduced [13,17] by attributting some additional intrinsic transverse momentum k T to the incoming partons, which is usually assumed to have a Gaussian-like distribution. The average value of this k T increases from k T ∼ 1 GeV to more than k T ∼ 3 GeV [13,16] as the √ s increases from UA6 to Tevatron energies 2 . From our point of view, a more adequate solution was found [22][23][24][25] in the framework of the k T -factorization approach [26,27].…”

GR@PPA 2.9: Radiation matching for simulating photon production processes in hadron collisions

“…Despite such a prospect, this process has been a mystery for a long time. Theories, even the next-to-leading order (NLO) calculations in Quantum Chromodynamics (QCD), could not reproduce measurement results, and intensive studies and debates were made to solve the problem (see, for example, [3,4] and the references therein). As higher energy collision data became available [5,6,7,8], it has been revealed that the photon production in high transverse momentum (p T ) regions ( 50 GeV) can be reasonably described by the NLO calculation [9], although discussions are yet to be conclusive for lower p T production.…”