Measurement of<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mi>t</mml:mi><mml:mover accent="true"><mml:mi>t</mml:mi><mml:mo>¯</mml:mo></mml:mover></mml:math>spin correlation in<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mi>p</mml:mi><mml:mover accent="true"><mml:mi>p</mml:mi><mml:mo>¯</mml:mo></mml:mover></mml:math>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 $\sqrt{s}=1.96\text{ }\text{ }\mathrm{TeV}$ 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 $172.5\text{ }\text{ }\mathrm{GeV}/{c}^{2}$, we find a spin correlation coefficient $\ensuremath{\kappa}=0.60\ifmmode\pm\else\textpm\fi{}0.50\text{ }\text{ }(\mathrm{stat})\ifmmode\pm\else\textpm\fi{}0.16\text{ }\text{ }(\mathrm{syst})$, consistent with the QCD prediction, $\ensuremath{\kappa}\ensuremath{\approx}0.40$.