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

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 ats=13TeVby the CMS experiment at the CERN LHC and corresponds to an integrated luminosity of 138 fb-1. The invariant massmtt¯of the reconstructedtt¯system and variables sensitive to its spin and parity are used to discriminate against the standard modeltt¯background. Interference between pseudoscalar or scalar boson production and the standard modeltt¯continuum is included, leading to peak-dip structures in themtt¯distribution. An excess of the data above the background prediction, based on perturbative quantum chromodynamics (QCD) calculations, is observed near the kinematictt¯production threshold, while good agreement is found for highmtt¯. The data are consistent with the background prediction if the contribution from a simplified model of a color-singlet1S0[1]tt¯quasi-bound stateηt, 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 withoutηtcontribution.