Qcd Corrections to Top Quark Spin Correlations at Hadron Colliders *

Future hadron collider experiments will allow for a detailed investigation of t ¯ t spin correlation effects. In this talk, recent progress in the theoretical description of these effects is reported. In particular, next-to-leading order results for the t ¯ t spin correlations in q ¯ q → t ¯ tX are presented, using various spin quantization axes. Top quark pair production and decay can be described by perturbative methods. This is so because the top quark width is large compared to the QCD hadronisation scale and therefore the top decays before hadronisation takes place. The full information of the top quark interactions including spin correlations between t and ¯ t can thus in principle be obtained from studying differential distributions of the decay products. In this talk, we will report on recent progress in the theoretical description of top quark spin correlation effects at hadron colliders. Consider the reactions p¯ p, pp → t ¯ tX →    2ℓ + n ≥ 2 jets + P miss T ℓ + n ≥ 4 jets + P miss T n ≥ 6 jets. (1) To leading order in the QCD coupling, two parton reactions contribute: q ¯ q, gg → t ¯ t → bW + ¯ bW − → 6 fermions. (2) The amplitudes for these two-to six-body processes , with intermediate top quarks of non-zero total width, were given first in [ 1]. The calculation of the fully differential cross sections for the reactions (1) including NLO QCD corrections is a formidable task. A substantial simplification in the computation of the radiative corrections is achieved if one performs an expansion in Γ t /m t and Γ W /m W and keeps only the terms with the highest degree of resonance. In this leading pole approximation [ 2] the radiative corrections can be classified into so-called fac-torisable and non-factorisable corrections. The non-factorisable NLO QCD corrections were calculated in [ 3]. For the factorisable corrections the square of the complete matrix element M