Search for associated production of a single top quark and a Higgs boson in events where the Higgs boson decays to two photons at $\sqrt{s} = 8$~TeV

We study the associated production of a single top quark and a Higgs boson, in order to directly constrain the top-Higgs coupling. Due to the destructive interference in the t-channel for standard model couplings, this process is very sensitive to both the magnitude and the sign of a non-standard top-Higgs coupling. We focus on the Higgs decay to two photons, that would be further enhanced in the scenario of a negative Yukawa coupling, and on the leptonic top decay channel. The analysis has an expected 95\% confidence level upper limit on the Higgs production cross section times $BR(H \to \gamma \gamma)$ of 4.1 times the expectation~(with $C_t = -1$). The expected significance of the analysis is 1.2 standard deviations. Zero events are observed in the signal region, leading to a 95\% upper limit of 4.1 times the expectation~(with $C_t = -1$).