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 αs 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.
Highlights
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
Despite the fact that infra-red and collinear (IRC) safety guarantees that non-perturbative corrections due to the hadronisation process are power-suppressed, these have a non-negligible impact on event-shape distributions in region of phase-space where many data points live
We have put forward the idea of using techniques developed in the context of jet substructure to reduce an observable sensitivity to non-perturbative physics
Summary
The soft-drop grooming technique [39] is defined for a jet with radius R using CambridgeAachen (C/A) clustering [48, 49] as: 1. Undo the last step of the clustering for the jet, J, and split it into two subjets. We note that at asymptotically small values τSD, the distribution reverts to a double-logarithmic behaviour because the value of τSD is set by the kinematics of the emission which has been groomed away and it is sensitive to the soft-collinear region of phase-space. A more detailed analysis of this type of kinematic configuration, and the resulting O zc2ut transition point, is performed in appendix A This effect can be seen in figure 1 for a fixed order computation at LO (on the left) and NLO (on the right) accuracy, i.e. with one or two emissions off the qqdipole calculated with the program EVENT2 [51, 52]. We have checked that the resulting distributions have very similar features to the ones obtained with the current definition
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