Searching for $t\to ch$ with Multi-Leptons

The results of a multi-lepton search conducted by the CMS collaboration with 5 fb${}^{-1}$ of data collected from 7 TeV $pp$ collisions are used to place the first bound on the rare flavor-changing decay of the top quark to a Higgs boson and charm quark. Combining results from a number of exclusive three- and four-lepton search channels yields an estimated upper limit of ${\rm Br}(t \to ch) < 2.7%$ for a Higgs boson mass of 125 GeV. The sensitivity of future dedicated searches for $t\to ch$ could be improved by adding exclusive same sign di-lepton channels, as well as by sub-dividing channels based on $b$-quark tagging and partial kinematic top quark and Higgs boson tagging. This bound may be interpreted more widely within a range of new physics processes that yield final states with a $W$-boson in association with a Higgs boson. For such processes with kinematics that are similar to top–anti-top production and decay, the estimated limit on cross section times branching ratio corresponds to roughly $\sigma \cdot \mathrm{B}\mathrm{r}(pp\to WhX)<9$ pb.