Probing collective effects in hadronisation with the extremes of the underlying event

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 reconnections or other collective phenomena. We illustrate at $$\sqrt{s} = 13 \,\mathrm{{TeV}}$$ that significant discriminatory power is obtained in comparisons between MC models with varying treatments of collective effects, including Pythia 8, epos, and Dipsy.