Spin-Spin Correlations of Top Quark Pairs at Hadron Colliders

Top quark pairs are produced with strongly correlated spins in the partonic reactions $q\bar{q}\to t\bar{t}$ and $gg\to t\bar{t}$. A complete description of these effects in terms of the spin density matrix of the $t\bar{t}$ system in leading order QCD is given. We further discuss the prospects to observe the spin-spin correlations at $p\bar{p}$ and $pp$ colliders by measuring suitable angular correlations among the $t$ and $\bar{t}$ decay products.