QED and QCD helicity amplitudes in parton-shower gauge

Abstract

We introduce photon and gluon propagators in which the scalar polarization component is subtracted systematically by making use of the BRST invariance of the off-shell vector boson created from physical on-shell states. The propagator has the light-cone gauge form, where the spacial component of the gauge vector points along the negative of the off-shell vector boson momentum. We call the gauge as parton-shower gauge, since in collinear configurations the absolute value squared of each Feynman amplitude reproduces all the singular behaviors of the corresponding parton shower in this gauge. We introduce new HELAS codes that can be used to calculate the tree-level helicity amplitudes of arbitrary QED and QCD processes by using MadGraph. The absence of subtle gauge cancellation among Feynman amplitudes allows numerical codes to evaluate singular behaviors accurately, and helps us gaining physical insights on interference patterns.

arXiv comment10 pages, 6 figures; v2: references added; v3: some issues clarified, a reference added, version to appear in EPJC

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• UFO - The Universal FeynRules Output
• MadGraph/MadEvent v4: The New Web Generation
• MadEvent: Automatic Event Generation with MadGraph
• Parton showers with quantum interference
• ALOHA: Automatic Libraries Of Helicity Amplitudes for Feynman diagram computations
• Jet angular correlation in vector-boson fusion processes at hadron colliders
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