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

Abstract 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 mₜt 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 ¹S₀^1 tt quasi-bound state 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 ₜ contribution.