Measurement of jet activity produced in top-quark events with an electron, a muon and two $b$-tagged jets in the final state in $pp$ collisions at $\sqrt{s}=13$ TeV with the ATLAS detector

TL;DR

This paper measures jet activity in top-quark pair events at 13 TeV using ATLAS data, comparing results to theoretical models to understand QCD radiation effects in proton-proton collisions.

Contribution

It provides detailed measurements of additional jet activity and gap fractions in top-quark events, testing various QCD radiation models against experimental data.

Findings

Jet kinematics from top decays are well modeled.

Different theoretical models vary in predicting additional jet production.

Measurements help refine QCD radiation modeling in top-quark events.

Abstract

Measurements of jet activity in top-quark pair events produced in proton--proton collisions are presented, using 3.2 fb$^{-1}$ of $pp$ collision data at a centre-of-mass energy of 13 TeV collected by the ATLAS experiment at the Large Hadron Collider. Events are chosen by requiring an opposite-charge $e\mu$ pair and two $b$-tagged jets in the final state. The normalised differential cross-sections of top-quark pair production are presented as functions of additional-jet multiplicity and transverse momentum, $p_{\mathrm T}$. The fraction of signal events that do not contain additional jet activity in a given rapidity region, the gap fraction, is measured as a function of the $p_{\mathrm T}$ threshold for additional jets, and is also presented for different invariant mass regions of the $e\mu b\bar{b}$ system. All measurements are corrected for detector effects and presented as particle-level distributions compared to predictions with different theoretical approaches for QCD radiation. While the kinematics of the jets from top-quark decays are described well, the generators show differing levels of agreement with the measurements of observables that depend on the production of additional jets.