Glauber Gluons and Multiple Parton Interactions

TL;DR

This paper demonstrates that Glauber gluons cause a breakdown of the standard factorisation for certain hadronic observables like transverse energy, due to multiple parton interactions disrupting the usual cancellation mechanisms.

Contribution

It introduces the concept that MPI-sensitive observables do not obey standard factorisation at leading power because Glauber gluons and multiple parton interactions play a significant role.

Findings

Glauber gluons cause failure of CSS cancellation at two-gluon level.

MPI-sensitive observables are disrupted by multiple parton interactions.

Standard factorisation does not hold for certain observables like $E_T$, beam thrust, and transverse thrust.

Abstract

We show that for hadronic transverse energy $E_T$ in hadron-hadron collisions, the classic Collins-Soper-Sterman (CSS) argument for the cancellation of Glauber gluons breaks down at the level of two Glauber gluons exchanged between the spectators. Through an argument that relates the diagrams with these Glauber gluons to events containing additional soft scatterings, we suggest that this failure of the CSS cancellation actually corresponds to a failure of the `standard' factorisation formula with hard, soft and collinear functions to describe $E_T$ at leading power. This is because the observable receives a leading power contribution from multiple parton interaction (or spectator-spectator Glauber) processes. We also suggest that the same argument can be used to show that a whole class of observables, which we refer to as MPI sensitive observables, do not obey the standard factorisation at leading power. MPI sensitive observables are observables whose distributions in hadron-hadron collisions are disrupted strongly by the presence of multiple parton interactions (MPI) in the event. Examples of further MPI sensitive observables include the beam thrust $B^+_{a,b}$ and transverse thrust.