Abstract
Distributions sensitive to the underlying event in QCD jet events have been measured with the ATLAS detector at the LHC, based on \(37~\text {pb}^{-1}\) of proton–proton collision data collected at a centre-of-mass energy of 7 \(\text {TeV}\). Charged-particle mean \(p_{\mathrm {T}}\) and densities of all-particle \(E_{\mathrm {T}}\) and charged-particle multiplicity and \(p_{\mathrm {T}}\) 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 to 800 \(\text {GeV}\). The correlation of charged-particle mean \(p_{\mathrm {T}}\) with charged-particle multiplicity is also studied, and the \(E_{\mathrm {T}}\) 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.
Highlights
The low-momentum quantum chromodynamics (QCD) processes which dominate the underlying event” (UE) are the main type of interaction in proton–proton collisions
This paper reports a measurement of UE observables in inclusive jet and exclusive dijet events, recorded by the ATLAS detector [10] at the LHC using proton–proton collisions at a centre-of-mass energy of 7 TeV, from the LHC 2010 run
ATLAS measurements sensitive to the underlying event in 37 pb−1 of 7 TeV proton–proton collisions at the LHC have been presented, using observables constructed with respect to QCD jets with pT up to 800 GeV
Summary
The low-momentum QCD processes which dominate the UE are the main type of interaction in proton–proton collisions. The behaviour of such soft interactions cannot be calculated reliably with perturbative QCD methods, due to the divergence of the QCD coupling at low scales; it is typically modelled in a phenomenological manner by Monte Carlo (MC) event generator programs. Predictivity of UE modelling is important since the UE forms an irreducible background of particle activity in all hard-scale processes studied at the LHC. This paper reports a measurement of UE observables in inclusive jet and exclusive dijet events, recorded by the ATLAS detector [10] at the LHC using proton–proton collisions at a centre-of-mass energy of 7 TeV, from the LHC 2010 run.
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