Measurement of event-shape observables in $Z \to \ell^{+} \ell^{-}$ events in $pp$ collisions at $\sqrt{s}=7$ TeV with the ATLAS detector at the LHC

TL;DR

This paper reports measurements of event-shape observables in Z boson events from proton-proton collisions at 7 TeV, comparing data with Monte Carlo simulations to understand underlying event properties.

Contribution

It provides the first detailed measurements of various event-shape observables in Z boson events at the LHC, testing and comparing different Monte Carlo generators against experimental data.

Findings

Sherpa shows larger deviations from data than Pythia8 and Herwig7.

All generators agree better at high Z boson transverse momentum.

Observables are more sensitive to underlying event properties at low Z transverse momentum.

Abstract

Event-shape observables measured using charged particles in inclusive $Z$-boson events are presented, using the electron and muon decay modes of the $Z$ bosons. The measurements are based on an integrated luminosity of $1.1 {\rm fb}^{-1}$ of proton--proton collisions recorded by the ATLAS detector at the LHC at a centre-of-mass energy $\sqrt{s}=7$ TeV. Charged-particle distributions, excluding the lepton--antilepton pair from the $Z$-boson decay, are measured in different ranges of transverse momentum of the $Z$ boson. Distributions include multiplicity, scalar sum of transverse momenta, beam thrust, transverse thrust, spherocity, and $\mathcal{F}$-parameter, which are in particular sensitive to properties of the underlying event at small values of the $Z$-boson transverse momentum. The Sherpa event generator shows larger deviations from the measured observables than Pythia8 and Herwig7. Typically, all three Monte Carlo generators provide predictions that are in better agreement with the data at high $Z$-boson transverse momenta than at low $Z$-boson transverse momenta and for the observables that are less sensitive to the number of charged particles in the event.