Asearch 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 138fb−1. The invariant mass mt ̄ t of the reconstructed t ̄ tsystem 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 ̄ tcontinuum 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 ̄ tproduction 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]
0 t ̄ tquasi-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
365–1000GeV, relative widths between 0.5% and 25%, and two background scenarios with or
without ηt contribution
