The Catchment Area of Jets

TL;DR

This paper introduces and analyzes the concepts of passive and active jet areas to quantify jet susceptibility to pileup and underlying event radiation, providing theoretical foundations and empirical validation for their use in jet contamination subtraction.

Contribution

It establishes a theoretical framework for jet areas, compares different algorithms, and validates the concepts with Monte Carlo simulations.

Findings

Passive areas match geometric expectations at leading order.

Active areas differ from geometric expectations, especially for cone algorithms.

Monte Carlo simulations agree with analytical predictions.

Abstract

The area of a jet is a measure of its susceptibility to radiation, like pileup or underlying event (UE), that on average, in the jet's neighbourhood, is uniform in rapidity and azimuth. In this article we establish a theoretical grounding for the discussion of jet areas, introducing two main definitions, passive and active areas, which respectively characterise the sensitivity to pointlike or diffuse pileup and UE radiation. We investigate the properties of jet areas for three standard jet algorithms, k_t, Cambridge/Aachen and SISCone. Passive areas for single-particle jets are equal to the naive geometrical expectation \pi R^2, but acquire an anomalous dimension at higher orders in the coupling, calculated here at leading order. The more physically relevant active areas differ from \pi R^2 even for single-particle jets, substantially so in the case of the cone algorithms like SISCone with a Tevatron Run-II split--merge procedure. We compare our results with direct measures of areas in parton-shower Monte Carlo simulations and find good agreement with the main features of the analytical predictions. We furthermore justify the use of jet areas to subtract the contamination from pileup.