A search for pseudoscalar or scalar bosons decaying to a top quark pair (t¯t) in final states with one or two charged leptons is presented. The analyzed proton–proton collision data was recorded at √s = 13TeV by the CMS experiment at the CERN LHC and corresponds to an integrated luminosity of 138 fb−1. The invariant mass mt¯t of the reconstructed t¯t system and variables sensitive to its spin and parity are used to discriminate against the standard model t¯t background. Interference between pseudoscalar or scalar boson production and the standard model t¯t continuum is included, leading to peak-dip structures in the mt¯t distribution. An excess of the data above the background prediction, based on perturbative quantum chromodynamics (QCD) calculations, is observed near the kinematic t¯t production threshold, while good agreement is found for high mt¯t. The data are consistent with the background prediction if the contribution from a simplified model of a color-singlet 1S[1]t¯t quasi-bound state ηt, inspired by nonrelativistic QCD, is added. Upper limits at 95% confidence level are set on the coupling between the pseudoscalar or scalar bosons and the top quark for boson masses in the range 365–1000 GeV, relative widths between 0.5% and 25%, and two background scenarios with or without ηt contribution.
