Sean Solari - PHS3350 Project 2020

Crossing Relations for Double-Gluon Emission Antenna functions

It is known that charged particles emit radiation when they are accelerated up to high energies. This is observed in abundance from those particles accelerated within the Large-Hadron Collider, as well as from the by-products of collisions between these accelerated particles. As a consequence of this, a series of cascaded emissions or bremsstrahlung is observed in the detectors around these high-energy collisions. Due to the large number of particles involved in these so-called parton showers, direct matrix element calculations are not a viable option in trying to model these events. Instead, the known singularity structure of matrix elements in general is exploited and used to factor out a splitting function, thereby isolating the effect of individual bremsstrahlung and enabling an iterative approach to modelling parton showers.

We explore structure within the antenna function for double-gluon emission from a quark-antiquark pair in the initial state, before they annihilate to form a Z-boson. Within this antenna function, there is physically interesting structure related to the different configurations that the emitted gluons can take with respect to the quarks. Specifically, we explore the double-soft limit, wherein both gluons are emitted with negligible momenta, as well as the triple-collinear limit, where both gluons become collinear with the quark from which they were emitted. To validate the antenna function, we employ a uniform sampling over its domain and compare the resulting values to direct matrix element calculations. In a similar fashion, cuts are introduced into the domain of the antenna function to distinguish those points in phase-space corresponding to double-soft or triple-collinear configurations, thereby enabling a similar sampling of the derived limits.

Ultimately, having the structure of the antenna function in these singular limits is useful as they are likely candidates in trying to find a simple overestimate of the full matrix element to enable Monte Carlo simulations of these events. Current parton shower models view double-gluon emission as iterated single- emission, and so are not able to account for the interference effects of Feynman diagrams containing two consecutive gluon emissions. Exploring the double-gluon emission splitting function will therefore aid in understanding the role of these coherent emissions.

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