Spin-Spin Correlations of Top Quark Pairs at Hadron Colliders

TL;DR

This work analyzes spin-spin correlations of top-quark pairs produced at hadron colliders using a spin-density-matrix formalism in leading-order QCD. It derives Born-level structure functions for q qbar and gg initial states and defines decay-product angular observables in semihadronic ttbar decays that are sensitive to these correlations. The study predicts few-percent level signals that could be observed at the Tevatron and LHC, with detailed considerations of reconstruction, spin analyzers, and statistical significance. It also outlines the limitations of LO results and the need for NLO spin-density matrices and non-factorizable effects for more precise predictions, arguing that the measurement of top-spin correlations is feasible and informative for SM tests and beyond.

Abstract

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.