  A study of event shape observables in proton-antiproton collisions at 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 NLO accuracy (NLO+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+NLL predictions, which do not
account for the underlying event. To overcome this difficulty, we introduce a
new observable, a weighted difference of the mean values of the thrust and
thrust minor, which is less sensitive to the underlying event, allowing for a
comparison with NLO+NLL. Both pythia Tune A and the NLO+NLL calculations agree
well within the 20% theoretical uncertainty with the data for this observable,
indicating that perturbative QCD successfully describes shapes of the hadronic
final states.
