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 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. 
Tim Martin, Peter Skands, Sinead Farrington
hep-ph