The efficacy of event isotropy as an event shape observable

Abstract

Abstract Event isotropy $$ {\mathcal{I}}^{\mathrm{sph}} $$ I 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 [1] 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 λmax, 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 λmax) and in $$ {\mathcal{I}}^{\mathrm{sph}} $$ I sph separately often have much narrower distributions, and are more easily distinguished, in the ($$ {\mathcal{I}}^{\mathrm{sph}} $$ I sph , λmax) plane. An intuitive, semi-analytic estimation technique clarifies why this is the case and assists with the interpretation of the distributions.