Search for a Light Charged Higgs Boson Decaying to a $W$ Boson and a $CP$-Odd Higgs Boson in Final States with $e\mu \mu$ or $\mu \mu \mu$ in Proton-Proton Collisions at $\sqrt{s}=13\mathrm{TeV}$

A search for a light charged Higgs boson ($H^{+}$) decaying to a $W$ boson and a $CP$-odd Higgs boson ($A$) in final states with $e\mu \mu$ or $\mu \mu \mu$ is performed using data from $pp$ collisions at $\sqrt{s}=13\mathrm{TeV}$, recorded by the CMS detector at the LHC and corresponding to an integrated luminosity of $35.9{\mathrm{fb}}^{-1}$. In this search, it is assumed that the $H^{+}$ boson is produced in decays of top quarks, and the $A$ boson decays to two oppositely charged muons. The presence of signals for $H^{+}$ boson masses between 100 and 160 GeV and $A$ boson masses between 15 and 75 GeV is investigated. No evidence for the production of the $H^{+}$ boson is found. Upper limits at 95% confidence level are obtained on the combined branching fraction for the decay chain, $t\to b{H}^{+}\to b{W}^{+}A\to b{W}^{+}{\mu }^{+}{\mu }^{-}$, of $1.9\times {10}^{-6}$ to $8.6\times {10}^{-6}$, depending on the masses of the $H^{+}$ and $A$ bosons. These are the first limits for these decay modes of the $H^{+}$ and $A$ bosons.

Figure 1

The $m_{\mu \mu }$ distribution of candidate muon pairs from $A$ bosons in the $e\mu \mu$ and $\mu \mu \mu$ final states. A constant bin size (1 GeV) is used in the figure except for the last bin of [80, 81.2] (GeV). The expected signal distribution for $m_{H^{+}}=130$ and $m_A=45\mathrm{GeV}$ is also shown on top of the expected backgrounds assuming $\sigma (t\overline{t})=832\mathrm{pb}$ and $\mathcal{B}(t\to b{H}^{+})\mathcal{B}(H^{+}\to W^{+}A)\mathcal{B}(A\to {\mu }^{+}{\mu }^{-})=6\times {10}^{-6}$.

Figure 2

Expected and observed upper limits at 95% C.L. on ${\mathcal{B}}_{\text{sig}}$ for the $m_A$ values defined in Table 1, with an assumption of $m_{H^{+}}=m_A+85\mathrm{GeV}$ (upper) or $m_{H^{+}}=160\mathrm{GeV}$ (lower). The green (yellow) bands indicate the regions containing 68% (95%) of the limit values expected under the background-only hypothesis.