A Fragmentation Approach to Jet Flavor

TL;DR

This paper introduces a new infrared-safe jet flavor definition based on the Winner-Take-All axis, with a perturbative evolution framework connecting ultraviolet and infrared regimes, validated against simulations.

Contribution

A novel jet flavor definition in the infrared that is safe to all orders and a perturbative evolution equation connecting it to the ultraviolet regime.

Findings

Quantitative agreement with parton shower simulations.

Fixed points in the infrared for the evolution equations.

Infrared and collinear safe observables sensitive to the new flavor definition.

Abstract

An intuitive definition of the partonic flavor of a jet in quantum chromodynamics is often only well-defined in the deep ultraviolet, where the strong force becomes a free theory and a jet consists of a single parton. However, measurements are performed in the infrared, where a jet consists of numerous particles and requires an algorithmic procedure to define their phase space boundaries. To connect these two regimes, we introduce a novel and simple partonic jet flavor definition in the infrared. We define the jet flavor to be the net flavor of the partons that lie exactly along the direction of the Winner-Take-All recombination scheme axis of the jet, which is safe to all orders under emissions of soft particles, but is not collinear safe. Collinear divergences can be absorbed into a perturbative fragmentation function that describes the evolution of the jet flavor from the ultraviolet to the infrared. The evolution equations are linear and a small modification to traditional DGLAP and we solve them to leading-logarithmic accuracy. The evolution equations exhibit fixed points in the deep infrared, we demonstrate quantitative agreement with parton shower simulations, and we present various infrared and collinear safe observables that are sensitive to this flavor definition.