In this seminar, a search for heavy pseudoscalar or scalar bosons decaying to a top quark pair (ttbar) in final states with one or two charged leptons will be presented. The results are based on 138 fb⁻¹ of proton-proton collisions at √s = 13 TeV recorded by the CMS experiment between 2016 and 2018. The invariant mass of the reconstructed ttbar system, along with variables sensitive to its spin state, are used to discriminate against the standard model ttbar background and infer properties of the spin quantum numbers. An excess is observed in the data above the background prediction, modeled using perturbative quantum chromodynamics (QCD) alone, with a significance exceeding five standard deviations.
Three possible hypotheses to interpret the excess, which is located near the ttbar production threshold, will be discussed: scalar production, pseudoscalar production, or the existence of a color-singlet pseudoscalar ttbar quasi-bound state, as predicted in a simplified model of nonrelativistic QCD. For the first two hypotheses, upper limits on the coupling of pseudoscalar or scalar bosons to top quarks will be presented in a mass range of 365–1000 GeV and relative widths of 0.5–25%. For the third scenario, first cross-section estimates will be shown, and the stability of the result against alternative background predictions will be discussed.
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