Quantum coherence, top transverse polarisation and the Tevatron asymmetry A_FB^ℓ

We revisit the relation between the asymmetries A_FB and A_FB^ℓ in tt^¯ production at the Tevatron, using as new physics benchmark a colour octet. We find that A_FB^ℓ receives large contributions from the interference between λ=±1/2 top helicity states, which has been ignored in some of the previous literature on the subject. The omission of these contributions results in a severe underestimation of the asymmetry, around 1/2 and 1/50 of the true value for right-handed and left-handed top couplings to the octet, respectively. Interference effects are closely related to a sizeable transverse top polarisation, as yet not considered in this context