Evidence for Spin Correlation in $t\bar{t}$ Production

Abstract

We present a measurement of the ratio of events with correlated $t$ and $\bar{t}$ spins to the total number of $t\bar{t}$ events. This ratio $f$ is evaluated using a matrix-element-based approach in 729 $t\bar{t}$ candidate events with a single lepton ℓ (electron or muon) and at least four jets. The analyzed $p\bar{p}$ collisions data correspond to an integrated luminosity of $5.3 \text{ fb}^{−1}$ and were collected with the D0 detector at the Fermilab Tevatron collider operating at a center-of-mass energy $\sqrt s=1.96 \text{TeV}$. Combining this result with a recent measurement of $f$ in dileptonic final states, we find $f$ in agreement with the standard model. In addition, the combination provides evidence for the presence of spin correlation in $t\bar{t}$ events with a significance of more than 3 standard deviations.