Top pair production near threshold, spin correlations, entanglement, and toponium production at the LHC

by Paolo Nason

Wednesday 3 Dec 2025, 16:00 → 17:00 Europe/Rome

0M04

Recent ATLAS and CMS studies of spin correlations and entanglement in top pair production at hadron colliders have evidenced some inadequacy in the current modeling of these processes. It has been argued that one needs to include the effect of a very short lived $t{\bar t}$ pseudoscalar bound state to reconcile the data with the theoretical prediction. More recent CMS and ATLAS publications on $t{\bar t}$ production near threshold also support this conclusion.
In this talk I review the problem, and argue that in fact standard perturbation theory is enough to correctly describe these phenomena, without the need for resummation of threshold enhance corrections to all orders or for the introduction of bound states. A simple quantum mechanical model is used to show that the leading correction due to bound state formation has the same form as other corrections that arise from the continuum just above threshold, and that the combination of two leads to a well defined expansion in perturbation theory. Furthermore, by examining critically the calculation of the process as implemented in current Monte Carlo generators, I show that these corrections, that are of the third subleading order in the strong coupling constant, are tiny, while threshold enhanced corrections of first and second subleading order (not related to bound states) give sizeable contributions.