Measurement of $t\overline{t}$ spin correlation in $p\overline{p}$ collisions using the CDF II detector at the Tevatron

The $t\overline{t}$ spin correlation at production is a fundamental prediction of QCD and a potentially incisive test of new physics coupled to top quarks. We measure the $t\overline{t}$ spin state in $p\overline{p}$ collisions at $\sqrt{s}=1.96\mathrm{TeV}$ using 1001 candidate events in the lepton plus jets decay channel reconstructed in the CDF II detector. In the helicity basis, for a top-quark mass of $172.5\mathrm{GeV}/c^2$, we find a spin correlation coefficient $\kappa =0.60\pm 0.50(\mathrm{stat})\pm 0.16(\mathrm{syst})$, consistent with the QCD prediction, $\kappa \approx 0.40$.

Figure 1

Distributions of the $\cos {\theta }_l\cos {\theta }_d$ and $\cos {\theta }_l\cos {\theta }_b$ variables, after detector simulation, event selection, and reconstruction, in our same-helicity and opposite-helicity simulated $t\overline{t}$ samples.

Figure 2

Distribution of the $\cos {\theta }_l\cos {\theta }_d$ and $\cos {\theta }_l\cos {\theta }_b$ variables in data compared to the sum of our background model, the same-helicity model template, and the opposite-helicity model template. The relative normalizations of the model distributions are determined by our fit result.