Flavour-Changing Decays of a 125 GeV Higgs-like Particle

Abstract

The ATLAS and CMS experiments at the LHC have reported the observation of a possible excess of events corresponding to a new particle $h$ with mass $\sim 125$ GeV that might be the long-sought Higgs boson, or something else. Decyphering the nature of this possible signal will require constraining the couplings of the $h$ and measuring them as accurately as possible. Here we analyze the indirect constraints on flavour-changing $h$ decays that are provided by limits on low-energy flavour-changing interactions. We find that indirect limits in the quark sector impose such strong constraints that flavour-changing $h$ decays to quark-antiquark pairs are unlikely to be observable at the LHC. On the other hand, the upper limits on lepton-flavour-changing decays are weaker, and the experimental signatures less challenging. In particular, we find that either $\mathcal{B}(h\to \tau \bar{\mu }+\bar{\mu }\tau )$ or $\mathcal{B}(h\to \tau \bar{e}+\bar{e}\tau )$ could be $\mathcal{O}(10)$, i.e., comparable to $\mathcal{B}(h\to {\tau }^{+}{\tau }^{-})$ and potentially observable at the LHC.

Comments: 8 pages, 1 figure. v2: a few typos and numerical errors corrected, discussion of 2-loop bounds added, extended list of references, conclusions unchanged