Measurement of ttbar Helicity Fractions and Spin Correlation in ppbar Collisions at sqrt{s} = 1.96 TeV.

In the production of top-antitop quark pairs during proton-antiproton collisions, the spins of the top and antitop are correlated. This correlation is quantifed by the spin correlation coefficient kappa or the fraction of top quarks produced with opposite helicity FOH, which are determined by the QCD interaction mechanism that produces top-antitop pairs. A deviation of the correlation from the predicted value could be an indication of new production mechanisms. We describe a measurement of the top-antitop spin correlation using the lepton plus jets decay channel, where the decay proceeds via ttbar to (W+ b)(W- bbar) to (q q' b)(l nu bbar) or (l nu b)(q q' bbar), in data corresponding to 4.3 fb-1 of integrated luminosity collected with the CDF detector. In the helicity basis, we find an opposite helicity fraction FOH = 0.80 +/- 0.25 (stat) +/- 0.08 (syst) and a spin correlation coefficient kappa = 0.60 +/- 0.50 (stat) +/- 0.16 (syst), which are in good agreement with the theoretical predictions FOH = 0.70 and kappa = 0.40.Ph.D.PhysicsUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/78824/1/dmietl_1.pd

Measurement of the D+D^+- Meson Production Cross Section at Low Transverse Momentum in ppˉp\bar{p} Collisions at s=1.96\sqrt{s}=1.96 TeV

International audienceWe report on a measurement of the D+-meson production cross section as a function of transverse momentum (pT) in proton-antiproton (pp¯) collisions at 1.96 TeV center-of-mass energy, using the full data set collected by the Collider Detector at Fermilab in Tevatron Run II and corresponding to 10  fb-1 of integrated luminosity. We use D+→K-π+π+ decays fully reconstructed in the central rapidity region |y|<1 with transverse momentum down to 1.5  GeV/c, a range previously unexplored in pp¯ collisions. Inelastic pp¯-scattering events are selected online using minimally biasing requirements followed by an optimized offline selection. The K-π+π+ mass distribution is used to identify the D+ signal, and the D+ transverse impact-parameter distribution is used to separate prompt production, occurring directly in the hard-scattering process, from secondary production from b-hadron decays. We obtain a prompt D+ signal of 2950 candidates corresponding to a total cross section σ(D+,1.5<pT<14.5  GeV/c,|y|<1)=71.9±6.8(stat)±9.3(syst)  μb. While the measured cross sections are consistent with theoretical estimates in each pT bin, the shape of the observed pT spectrum is softer than the expectation from quantum chromodynamics. The results are unique in pp¯ collisions and can improve the shape and uncertainties of future predictions