Contrasting pseudoscalar Higgs and toponium states at the LHC and beyond

This is the authors' abstract. We don't add key points for this paper.

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.