Measurement of the inclusive $\mathrm{t\bar{t}}$ production cross section and search for additional scalars in $\mathrm{t\bar{t}}$ final states at CMS

Two key measurements and two phenomenological studies of top quark pair ($\mathrm{t\bar{t}}$) production with the CMS experiment at the CERN Large Hadron Collider are presented. They include the first measurement of the $\mathrm{t\bar{t}}$ cross section at the new energy frontier, a detailed analysis of the $\mathrm{t\bar{t}}$ threshold culminating in the first observation of $\mathrm{t\bar{t}}$ bound state effects, and searches for new physics in $\mathrm{t\bar{t}}$ final states, such as heavy scalars, pseudoscalars or Axion-Like Particles.The inclusive $\mathrm{t\bar{t}}$ production cross section $\sigma_{\mathrm{t\bar{t}}}$ is measured for the first time at the world-record energy of $\sqrt{s} = 13.6\,\mathrm{TeV}$, using $1.21\,\mathrm{fb}^{-1}$ of early LHC Run 3 data. By combining the dilepton and lepton+jets ($\ell\mathrm{+jets}$) decay channels and constraining the lepton and b tagging efficiencies \textit{in situ}, a precision of $3.4\%$, comparable to previous $\sigma_{\mathrm{t\bar{t}}}$ measurements, is achieved. This constitutes the first measurement of proton-proton scattering at $\sqrt{s} = 13.6\,\mathrm{TeV}$ worldwide.Furthermore, a search for heavy spin-0 states decaying to $\mathrm{t\bar{t}}$ using $138\,\mathrm{fb}^{-1}$ of LHC Run 2 data at $\sqrt{s} = 13\,\mathrm{TeV}$ in the dilepton channels is presented. The invariant $\mathrm{t\bar{t}}$ mass ($m_{\mathrm{t\bar{t}}}$) is combined with spin correlation observables to gain sensitivity to the spin and $\mathcal{CP}$ structure of possible new intermediate states. The analysis is supported by a detailed modeling study of off-shell $\mathrm{t\bar{t}}$ production and of the interference between $\mathrm{t\bar{t}}$ and tW production.An excess of events over the $\mathrm{t\bar{t}}$ continuum background is observed at low values of $m_{\mathrm{t\bar{t}}}$, with spin correlations consistent with a pseudoscalar state. It is interpreted in terms of a pseudoscalar $\mathrm{t\bar{t}}$ bound state $\eta_{\mathrm{t}}$, and its cross section is measured to be $\sigma(\eta_{\mathrm{t}}) = 8.7 \pm 1.1 \, \mathrm{pb}$ using a simplified model inspired by non-relativistic quantum chromodynamics. The excess is statistically significant at more than five standard deviations, constituting the first observation of $\mathrm{t\bar{t}}$ bound state effects.Other interpretations of the observed excess are similarly possible. In particular, scenarios with generic pseudoscalar or scalar bosons are explored, and exclusion regions on their coupling to the top quark are derived both for the dilepton channels alone as well as in a combination with a separate analysis of the $\ell\mathrm{+jets}$ channels. In addition, Axion-Like Particles (ALPs) decaying to $\mathrm{t\bar{t}}$ are considered in the case of vanishing tree-level ALP-gluon couplings, while the more generic case is investigated phenomenologically in simulation.