Power Counting to Better Jet Observables

TL;DR

This paper demonstrates how power counting methods from effective theory can be used to design, understand, and predict jet substructure observables for identifying boosted particles at the LHC, providing a robust analytic approach validated by Monte Carlo simulations.

Contribution

It introduces a power counting framework for jet observables that offers analytic predictions and insights, complementing and validating Monte Carlo methods.

Findings

Power counting predicts optimal observables for boosted Z discrimination.

Predictions are validated against Pythia8 and Herwig++ simulations.

The method clarifies effects of phase space cuts and pile-up contamination.

Abstract

Optimized jet substructure observables for identifying boosted topologies will play an essential role in maximizing the physics reach of the Large Hadron Collider. Ideally, the design of discriminating variables would be informed by analytic calculations in perturbative QCD. Unfortunately, explicit calculations are often not feasible due to the complexity of the observables used for discrimination, and so many validation studies rely heavily, and solely, on Monte Carlo. In this paper we show how methods based on the parametric power counting of the dynamics of QCD, familiar from effective theory analyses, can be used to design, understand, and make robust predictions for the behavior of jet substructure variables. As a concrete example, we apply power counting for discriminating boosted Z bosons from massive QCD jets using observables formed from the n-point energy correlation functions. We show that power counting alone gives a definite prediction for the observable that optimally separates the background-rich from the signal-rich regions of phase space. Power counting can also be used to understand effects of phase space cuts and the effect of contamination from pile-up, which we discuss. As these arguments rely only on the parametric scaling of QCD, the predictions from power counting must be reproduced by any Monte Carlo, which we verify using Pythia8 and Herwig++. We also use the example of quark versus gluon discrimination to demonstrate the limits of the power counting technique.