Measurement of Event Shapes in Proton-Antiproton Collisions at Center-of-Mass Energy 1.96 TeV
2011
A study of event shape observables in proton-
antiprotoncollisions at {radical}s = 1.96 TeV is presented. The data for this analysis were recorded by the CDF II detector at the
Tevatroncollider. The variables studied are the transverse thrust and thrust minor, both defined in the plane perpendicular to the beam direction. The
observablesare
measuredusing 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 eventcontributes 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 QCDsuccessfully describes shapes of the hadronic final states.
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