Measurement of event-shape observables in Z→ℓ+ℓ- events in pp collisions at s=7  TeV with the ATLAS detector at the LHC

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 1.1fb-1 of proton–proton collisions recorded by the ATLAS detector at the LHC at a centre-of-mass energy 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 F -parameter, which are in particular sensitive to properties of the underlying event at small values of the Z -boson transverse momentum. The measured observables are compared with predictions from P ythia  8, S herpa , and H erwig  7. 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.