Measurement of colour flow using jet-pull observables in $t\bar{t}$ events with the ATLAS experiment at $\sqrt{s} = 13$ TeV

TL;DR

This paper measures jet-pull observables in top-antitop events at 13 TeV to study colour flow, comparing data with predictions and unfolding results to stable-particle level.

Contribution

It provides the first detailed measurement of jet-pull angle and magnitude in $t\bar{t}$ events at the LHC, testing colour flow models against experimental data.

Findings

Some predictions agree with data for certain observables

None of the models fully describe all measured distributions

Results help refine colour connection modeling in QCD simulations

Abstract

Previous phenomenological studies and measurements have shown that weighted angular moments derived from jet constituents encode the colour connections between initiating partons that seed the jets. This paper presents measurements of two such distributions, the jet-pull angle and jet-pull magnitude, both of which are derived from the jet-pull angular moment. The measurement is performed in $t\bar{t}$ events with one leptonically decaying $W$ boson and one hadronically decaying $W$ boson, using $36.1$ fb$^{-1}$ of $pp$ collision data recorded by the ATLAS detector at $\sqrt{s} = 13$ TeV delivered by the Large Hadron Collider. The observables are measured for two dijet systems, corresponding to the colour-connected daughters of the $W$ boson and the two $b$-jets from the top-quark decays. To allow the comparison of the measured distributions to colour model predictions, the measured distributions are unfolded to stable-particle level, after correcting for experimental effects introduced by the detector. While good agreement can be found for some combinations of predictions and observables, none of the predictions describes the data well across all observables.