Searching for $t \to c h$ with Multi-Leptons

TL;DR

This paper reports the first experimental upper limit on the rare top quark decay to a charm quark and a Higgs boson using multi-lepton data from CMS, constraining new physics processes involving Higgs and W bosons.

Contribution

It provides the first bound on the $t o c h$ decay from multi-lepton searches and suggests improvements for future dedicated searches.

Findings

Upper limit on ${ m Br}(t o ch)$ is 2.7% for 125 GeV Higgs.

Estimated limit on cross section times branching ratio is about 9 pb.

Future searches could improve sensitivity by adding same sign di-lepton channels.

Abstract

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 $t \to c h$ 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 $\sigma \cdot {\rm Br}(pp \to WhX) < 9$ pb.