Abstract
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.
Highlights
Hadronic final states produced in hard collisions can be characterized by a number of variables that describe the distribution of outgoing particles in the event
Theoretical fixed order results are obtained from the Monte-Carlo integration program nlojet++ [13], while the resummed results arise from the Computer Automated Expert SemiAnalytical Resummer (CAESAR) [14, 15]
The silicon tracker is surrounded by the central outer tracker (COT), an open-cell drift chamber providing up to 96 measurements of a charged particle track over the radial region from 40 to 137 cm
Summary
Hadronic final states produced in hard collisions can be characterized by a number of variables that describe the distribution of outgoing particles in the event. Event shape observables considered in this analysis have the property that large values of the variable coincide with the emission of one or more hard partons at large angles relative to the parent parton In this regime, the distribution is well described by a traditional perturbative expansion in powers of the strong coupling, αs. Theoretical fixed order results are obtained from the Monte-Carlo integration program nlojet++ [13], while the resummed results arise from the Computer Automated Expert SemiAnalytical Resummer (CAESAR) [14, 15] These theoretical calculations include initial and final-state radiation, but do not include multiple parton interactions or beam remnant models. For sufficiently large values of the maximum accessible rapidity, the contribution from the excluded kinematic region is expected to be small and the full global predictions for the studied variables should remain valid for ln(y) ≤ kηmax , where ηmax is the maximum detector pseudorapidity coverage and k is a constant dependent on the variable y [7]
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