Revisiting the theory of spin correlations and entanglement in top pair production at the LHC

About

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 the process of top pair production and decay. 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. In this talk I review the problem, and argue that in fact standard perturbation theory is enough to correctly describe these phenomena, with no need to consider threshold-enhanced corrections to all orders and to deal with bound states formation. 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 right above threshold, and that the combination of the two leads to a well-defined expansion in perturbation theory. Furthermore, by examining 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) seem to be enough to understand the current discrepancies. .

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