A look into the "hedgehog" events in pp collisions using a new event shape -- flatenicity

TL;DR

This paper investigates 'hedgehog' events in proton-proton collisions, characterized by uniform low-momentum tracks, using a new event shape variable called flatenicity, and compares experimental observations with PYTHIA 8 simulations.

Contribution

It introduces the flatenicity event shape to identify and analyze extended, jet-less high-multiplicity events in pp collisions, linking them to multiple parton scatterings.

Findings

Existence of jet-less, extended events in simulations

These events originate from multiple parton scatterings

The new event shape effectively triggers on such events

Abstract

The UA1 and CDF collaborations have reported the presence of events with a very extended structure of low momentum tracks filling in a uniform way the $\eta-\varphi$ phase space. However, these events have not caused interest with respect to the details of the interaction that originate them. We have undertaken to study whether such events are predicted by present event generators like PYTHIA 8. We report the existence of events without any discernible jet structure in high-multiplicity pp collisions. In the simulations those events originate from a moderate number of parton-parton scatterings within the same pp collision. A new event shape, $\rho$ (flatenicity), is proposed to allow for easily triggering on such events.