On the characterisation of the underlying event

TL;DR

This paper compares traditional and jet-area/median methods for measuring the underlying event in hadron collisions, using a toy model and Monte Carlo simulations to evaluate their effectiveness and explore new UE characteristics.

Contribution

It introduces a simple analytical toy model for the underlying event and assesses the jet-area/median approach's advantages over traditional methods.

Findings

Both methods measure average UE properties effectively.

Jet-area/median better captures UE fluctuations.

Identifies new UE features like rapidity dependence and correlations.

Abstract

The measurement of the underlying event (UE) and its separation from hard interactions in hadron-collider events is a conceptually and practically challenging task. We develop a simple, mostly analytical toy model for the UE in order to understand how different UE measurement approaches fare on the practical aspects of this problem, comparing the traditional approach used so far at Tevatron with a recently proposed "jet-area/median" approach. Both are found to perform comparably well in measuring average properties of the UE, such as the mean transverse momentum flow, but the jet-area/median approach has distinct advantages in determining its fluctuations. We then use the jet-area/median method to investigate a range of UE properties in existing Monte Carlo event-generator tunes, validating the main results of the toy-model and highlighting so-far unmeasured characteristics of the UE such as its rapidity dependence, as well as its intra- and inter-event fluctuations and correlations.