A study of event-shape observables in proton-antiproton collisions at
¼ 1:96 TeV is presented. The
data for this analysis were recorded by the CDF II Detector at the Tevatron Collider. The variables studied
are the transverse thrust and thrust minor, both defined in the plane perpendicular to the beam direction.
The observables are measured using energies from unclustered calorimeter cells. In addition to studies of
the differential distributions, we present the dependence of event-shape mean values on the leading-jet
transverse energy. Data are compared with PYTHIA Tune A and to resummed parton-level predictions that
were matched to fixed-order results at next-to-leading-order (NLO) accuracy (NLO þ NLL). Predictions
from PYTHIA Tune A agree fairly well with the data. However, the underlying event contributes
significantly to these observables, making it difficult to make direct comparisons to the NLO þ NLL
predictions, which do not account for the underlying event.