****************  Joint CTC/NTHU HEP Seminar ****************

Speaker: Prof. Kaoru Hagiwara  (KEK)
Title: Scattering amplitudes without gauge cancellation.
Start Date/Time: 2022-11-1 / 2:00 pm (Taipei time)
End Date/Time: 2022-11-1 / 3:30 pm
Host : Prof. Kingman Cheung (NTHU)

Online Zoom link: https://cern.zoom.us/j/68742084566?pwd=dHRxaTB3RThGc2dMaDA1dDhJOHdXQT09

Abstract:

In recent works we propose a new represenntation of the gauge boson propagators, which may be called `Feynman Diagram' (FD) gauge for both unbroken (the photon and the gluons) and broken (the weak bosons) gauge theories.  When all the gauge boson propagators are expressed in the FD gauge, the helicity amplitudes corresponding to individual Feynman diagram are expressed as products of the invariant Feynman's propagator factors times the `physical' splitting amplitudes at each vertex.
The absolute value square of the splitting amplitudes give the splitting functions of the parton shower in QED and QCD, while the corresponding quantity can be used for multiple weak boson emission processes at very high energies.  Although the parton showers assume collinear emissions of partons, the FD gauge amplitudes are valid at all angles, keeping the azimuthal angle information at all vertices as their complex phases.
Consequently, the scattering amplitudes calculated in the FD gauge reproduce all physical interference among interfering Feynman amplitudes, while no room remains for `unphysical' gauge cancellation.

The talk is based on two papers which are available on the web:

arXiv:2003.03003[hep-ph]
arXiv:2203.10440[hep-ph]

The first paper discusses QED and QCD, while the second paper studies the EW theory of the SM.  In these papers, we stress the advantage of adopting the FD gauge in numerical computation programs, such as MadGraph, since subtle gauge theory cancellation among diagrams often
spoils numerical accuracy of the program

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