Measurement of underlying event characteristics using charged particles in pp collisions at $\sqrt{s} = 900 GeV$ and 7 TeV with the ATLAS detector

TL;DR

This paper reports measurements of the underlying event in proton-proton collisions at 900 GeV and 7 TeV using the ATLAS detector, revealing higher activity than predicted by existing models, thus providing new insights into hadronic interactions.

Contribution

The study provides the first detailed measurements of underlying event characteristics at LHC energies, challenging existing Monte Carlo models and improving understanding of proton-proton interactions.

Findings

Higher underlying event activity observed than in pre-LHC models

Charged particle distributions characterized in three azimuthal regions

Data informs future tuning of Monte Carlo event generators

Abstract

Measurements of charged particle distributions, sensitive to the underlying event, have been performed with the ATLAS detector at the LHC. The measurements are based on data collected using a minimum-bias trigger to select proton-proton collisions at center-of-mass energies of 900 GeV and 7 TeV. The "underlying event" is defined as those aspects of a hadronic interaction attributed not to the hard scattering process, but rather to the accompanying interactions of the rest of the proton. Three regions are defined in azimuthal angle with respect to the highest-pt charged particle in the event, such that the region transverse to the dominant momentum-flow is most sensitive to the underlying event. In each of these regions, distributions of the charged particle multiplicity, pt density, and average pt are measured. The data show a higher underlying event activity than that predicted by Monte Carlo models tuned to pre-LHC data.