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 ( t t ¯ ) 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 t 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 m t 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 m t t ¯ . The data are consistent with the background prediction if the contribution from a simplified model of a color-singlet 1 S 0 1 t t ¯ 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.