  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.

High Energy Physics - Experiment