Enhanced Three-Body Decay of the Charged Higgs Boson

If the charged Higgs boson $H^{+}$ exists with $m_{H^{+}}<m_t{+m}_b$, the conventional expectation is that it will decay dominantly into $c\overline{s}$ and ${\tau }^{+}{\nu }_{\tau }$. However, the three-body decay mode $H^{+}\to W^{+}b\overline{b}$ is also present and we show that it becomes very important in the low $\tan \beta$ region for $m_{H^{+}}\gtrsim 140\mathrm{GeV}$. We then explore its phenomenological implications for the charged-Higgs-boson search in top-quark decay.