Soft-Drop Thrust

TL;DR

Applying soft drop to event shapes in electron-positron collisions extends the region with minimal non-perturbative effects, aiding precise strong coupling measurements, but introduces complex perturbative considerations depending on the observable and jet parameters.

Contribution

This work adapts the soft drop technique from jet physics to event shape analysis in electron-positron annihilation, exploring its impact on non-perturbative corrections and perturbative calculability.

Findings

Soft drop significantly reduces non-perturbative effects in thrust distribution.

Jet mass with small radii shows promising behavior in the intermediate spectrum region.

Perturbative calculations are sensitive to the choice of event shape and jet parameters.

Abstract

Soft drop, a technique originally developed in the context of jet physics in proton-proton collisions in order to reduce the contamination from non-perturbative effects, is applied to event shapes in electron-positron annihilation. In particular, we study the thrust distribution at the $Z$ pole and show that the region where non-perturbative corrections due to the hadronisation process are small is considerably extended if soft drop is applied. Therefore, we argue that the use of soft drop to reduce hadronisation effects is potentially of great benefit in the context of strong coupling determination using event shapes, which would be otherwise characterised by a strong correlation between $\alpha_s$ and non-perturbative parameters. However, reduced sensitivity to hadronisation corrections is only one of the aspects that need to be considered. In this context, we show that perturbative calculability, especially away from the soft and collinear region of the event-shape spectrum, has a nontrivial interplay with the soft-drop observable of choice. To this purpose, besides thrust, we investigate the behaviour of the hemisphere mass as well as the jet mass. We find that the latter shows the most promising behaviour in the intermediate region of the spectrum, especially if small jet radii are considered.