Spin-spin correlations of top quark pairs at hadron colliders

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