Search for heavy pseudoscalar and scalar bosons decaying to a top quark pair in proton-proton collisions at √s = 13 TeV

A search for pseudoscalar or scalar bosons decaying to a top quark pair (tt¯) in final states with one or two charged leptons is presented. The analyzed proton-proton collision data was recorded at √s = 13 TeV by the CMS experiment at the CERN LHC and corresponds to an integrated luminosity of 138 fb^-1. The invariant mass m_tt¯ of the reconstructed tt¯ system and variables sensitive to its spin and parity are used to discriminate against the standard model tt¯ background. Interference between pseudoscalar or scalar boson production and the standard model tt¯ continuum is included, leading to peak-dip structures in the mtt¯ distribution. An excess of the data above the background prediction, based on perturbative quantum chromodynamics (QCD) calculations, is observed near the kinematic tt¯ production threshold, while good agreement is found for high mtt¯. The data are consistent with the background prediction if the contribution from a simplified model of a color-singlet ¹S^[1]₀ tt¯ quasi-bound state η_t, 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 η_t contribution.