Measurement of the underlying event in jet events from 7 TeV proton-proton collisions with the ATLAS detector

TL;DR

This paper measures underlying event activity in 7 TeV proton-proton collisions using the ATLAS detector, providing detailed data to improve Monte Carlo models of QCD processes.

Contribution

It presents the first separate measurements of underlying event observables for inclusive jet and exclusive dijet events at the LHC.

Findings

Standard Monte Carlo models generally reproduce underlying event features.

Data constraints on color reconnection and beam-remnant effects.

Need for further Monte Carlo model tuning.

Abstract

Distributions sensitive to the underlying event in QCD jet events have been measured with the ATLAS detector at the LHC, based on 37/pb of proton-proton collision data collected at a centre-of-mass energy of 7 TeV. Charged-particle mean $p_T$ and densities of all-particle $E_T$ and charged-particle multiplicity and $p_T$ have been measured in regions azimuthally transverse to the hardest jet in each event. These are presented both as one-dimensional distributions and with their mean values as functions of the leading-jet transverse momentum from 20 GeV to 800 GeV. The correlation of charged-particle mean $p_T$ with charged-particle multiplicity is also studied, and the $E_T$ densities include the forward rapidity region; these features provide extra data constraints for Monte Carlo modelling of colour reconnection and beam-remnant effects respectively. For the first time, underlying event observables have been computed separately for inclusive jet and exclusive dijet event selections, allowing more detailed study of the interplay of multiple partonic scattering and QCD radiation contributions to the underlying event. Comparisons to the predictions of different Monte Carlo models show a need for further model tuning, but the standard approach is found to generally reproduce the features of the underlying event in both types of event selection.