Contrasting Pseudoscalar Higgs and Toponium States at the LHC and Beyond

We discuss ways to discriminate at hadron colliders between a quasi-bound toponium state and a pseudoscalar Higgs boson A, as predicted in many extensions of the Standard Model. We apply the discussion to the excess of t tbar threshold events recently observed at the LHC by the CMS collaboration \cite{CMS}, which could in principle be due to either possibility. Working in an effective theory in which only an additional pseudoscalar A boson is present in the spectrum, with a mass above the 2 m_t threshold and a significant coupling to top quarks, we discuss the interference between A production in the dominant gluon-fusion process gg \to A with subsequent A\to t tbar decays, and the QCD continuum background, gg\to t tbar. While this interference is absent in the case of toponium, it is essential for evaluating A production. It is difficult to resolve the peak/dip structure that it generates because of the experimental smearing of the t tbar invariant mass spectrum. However, by comparing the total A production rates for different integration domains of the t tbar invariant mass or, eventually, at different center of mass energies, one may be able to observe its effects. We then discuss additional mechanisms for A production in pp collisions, including loop-induced production in association with the lighter h boson, gg \to hA, and production in association with top-quark pairs, gg/q\bar q \to t tbar. A These mechanisms have small cross sections at the LHC, and their observation will necessitate higher luminosities or collider energies.