Search for Associated Production of a Single Top Quark and a Higgs Boson in Leptonic Channels

A search for the production of a Higgs boson in association with a single top quark is presented, focusing on leptonic signatures provided by the $\mathrm{H}\rightarrow\mathrm{W}\mathrm{W}$ and $\mathrm{H}\rightarrow\tau\tau$ decay modes. Due to a strong interference of the two main contributing diagrams, the production cross section of this process benefits from a substantial enhancement in the case of negative values of the top-Higgs coupling, $C_{\mathrm{t}}$. The analysis exploits signatures with two same-sign and three leptons in the final state, and uses the full data sample collected with the CMS detector at the LHC at a center of mass energy of 8~TeV, which corresponds to an integrated luminosity of $19.7 \pm 0.5\,\mathrm{fb}^{-1}$. Multivariate techniques are used to discriminate the signal from the dominant backgrounds. The analysis yields a 95\% confidence level upper limit on the single top quark plus Higgs boson production cross section of 6.7 times the expectation for $C_{\mathrm{t}}$~=~-1, with an expected limit of 5.0.