Search for flavor-changing neutral current interactions of the top quark and Higgs boson in proton-proton collisions at $\sqrt{s} = 13$ TeV

A search for flavor-changing neutral current interactions of the top quark (t) and Higgs boson (H) is presented. The search is based on Run 2 proton-proton collisions collected in 2016--2018 at a center-of-mass energy of 13 TeV with the CMS detector at the CERN LHC and corresponding to $138~\mathrm{fb}^{-1}$. Events containing a pair of leptons with the same-sign electric charge and at least one jet are considered. Two boosted decision trees are trained to distinguish the signal from the standard model background. No significant excess above the estimated background is observed. The results are used to constrain the branching ratio ($\mathcal{B}$) of the top quark decaying to a Higgs boson and an up (u) or charm quark (c). The observed (expected) $95\%$ confidence level upper limits are found to be $0.072\%$ ($0.059\%$) for $\mathcal{B}\left(t\to Hu\right)$ and $0.043\%$ ($0.062\%$) for $\mathcal{B}\left(t\to Hc\right)$. A combination of searches for flavor-changing neutral current interactions of top quarks and Higgs bosons in final states with a pair of photons or a pair of bottom quarks, with data collected by the CMS experiment, is performed. The resulting observed (expected) $95\%$ confidence level upper limits are $0.019\%$ ($0.027\%$) for $\mathcal{B}\left(t\to Hu\right)$ and $0.037\%$ ($0.035\%$) for $\mathcal{B}\left(t\to Hc\right)$. This result constitutes the best exclusion limits to date.