Probing Collective Effects in Hadronisation with the Extremes of the Underlying Event

TL;DR

This paper introduces new observables to analyze the underlying event in hadron collisions, aiming to distinguish collective effects and soft-QCD dynamics by comparing models and data at the LHC.

Contribution

It proposes a novel set of measurements in the transverse region of jet events to probe collective phenomena and test jet universality in high-energy collisions.

Findings

Significant discrimination between MC models with different collective effect treatments.

Potential to test jet universality across different collision types.

Insights into soft-QCD dynamics like colour reconnections.

Abstract

We define a new set of observables to probe the structure of the underlying event in hadron collisions. We use the conventional definition of the `transverse region' in jet events and, for a fixed window in jet $p_\perp$, propose to measure several discriminating quantities as a function of the level of activity in the transverse region. The measurement of these observables in LHC data would reveal whether, e.g., the properties of `low-UE' events are compatible with equivalent measurements in $e^+e^-$ collisions (jet universality), and whether the scaling behaviour towards `high-UE' events exhibits properties of non-trivial soft-QCD dynamics, such as colour re-connections or other collective phenomena. We illustrate at $\sqrt{s} = 13$ TeV that significant discriminatory power is obtained in comparisons between MC models with varying treatments of collective effects, including Pythia 8, EPOS, and Dipsy.