Contrasting pseudoscalar Higgs and toponium states at the LHC and beyond

We discuss the phenomenology at hadron colliders of a pseudoscalar Higgs boson A , as predicted in many extensions of the Standard Model. In particular, we highlight the aspects that makes it different from the case of a quasi-bound toponium state. We apply the discussion to the excess of t t ¯ threshold events recently observed at the LHC by the CMS collaboration [1] , which could in principle be due to either or both possibilities, although Occam's razor favours the toponium scenario. 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 g g → A with subsequent A → t t ¯ decays, and the QCD continuum background, g g → t t ¯ . 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 t ¯ invariant mass spectrum. However, by comparing the total A production rates for different integration domains of the t t ¯ 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, g g → h A , and production in association with top-quark pairs, g g / q q ¯ → t t ¯ A . These mechanisms have small cross sections at the LHC, and their observation will necessitate higher luminosities or collider energies.