Measurement of tt spin correlation in pp collisions using the CDF II detector at the Tevatron

The t (cid:1) 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 (cid:1) t spin state in p (cid:1) p collisions at ffiffiffi s p ¼ 1 : 96 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 GeV =c 2 , we find a spin correlation coefficient (cid:1) ¼ 0 : 60 (cid:1) 0 : 50 ð stat Þ (cid:1) 0 : 16 ð syst Þ , consistent with the QCD prediction, (cid:1) (cid:2) 0 : 40 .