The Efficacy of Event Isotropy as an Event Shape Observable

TL;DR

This paper introduces event isotropy as a new event shape observable that effectively distinguishes signals with different geometries, especially when traditional observables like thrust and C parameter are insufficient, enhancing new physics searches.

Contribution

The paper demonstrates that event isotropy provides unique information beyond traditional observables, improving the discrimination of signals with varying geometrical features in collider events.

Findings

Event isotropy is less correlated with thrust and C parameter.

Combining event isotropy with other observables improves signal separation.

An intuitive estimation technique helps interpret event shape distributions.

Abstract

Event isotropy $\mathcal{I}^\text{sph}$, an event shape observable that measures the distance of a final state from a spherically symmetric state, is designed for new physics signals that are far from QCD-like. Using a new technique for producing a wide variety of signals that can range from near-spherical to jetty, we compare event isotropy to other observables. We show that thrust $T$ and the $C$ parameter (and $\lambda_\text{max}$, the largest eigenvalue of the sphericity matrix) are strongly correlated and thus redundant, to a good approximation. By contrast, event isotropy adds considerable information, often serving to break degeneracies between signals that would have almost identical $T$ and $C$ distributions. Signals with broad distributions in $T$ (or $\lambda_\text{max}$) and in $\mathcal{I}^\text{sph}$ separately often have much narrower distributions, and are more easily distinguished, in the $({\mathcal{I}^\text{sph}},\lambda_\text{max})$ plane. An intuitive, semi-analytic estimation technique clarifies why this is the case and assists with the interpretation of the distributions.