Search for a light charged Higgs boson in the $H^{\pm }\to cs$ channel in proton-proton collisions at $\sqrt{s}=13\mathrm{TeV}$

A search is conducted for a low-mass charged Higgs boson produced in a top quark decay and subsequently decaying into a charm and a strange quark. The data sample was recorded in proton-proton collisions at $\sqrt{s}=13\mathrm{TeV}$ by the CMS experiment at the LHC and corresponds to an integrated luminosity of $35.9{\mathrm{fb}}^{-1}$. The search is performed in the process of top quark pair production, where one top quark decays to a bottom quark and a charged Higgs boson and the other to a bottom quark and a $W$ boson. With the $W$ boson decaying to a charged lepton (electron or muon) and a neutrino, the final state comprises an isolated lepton, missing transverse momentum, and at least four jets, of which two are tagged as $b$ jets. To enhance the search sensitivity, one of the jets originating from the charged Higgs boson is required to satisfy a charm tagging selection. No significant excess beyond standard model predictions is found in the dijet invariant mass distribution. An upper limit in the range 1.68%–0.25% is set on the branching fraction of the top quark decay to the charged Higgs boson and bottom quark for a charged Higgs boson mass between 80 and 160 GeV.

Figure 1

Sample diagrams of $t\overline{t}$ production via gluon-gluon scattering. The left plot shows the signal process in which the $t\overline{t}$ pair decay products include a charged Higgs boson. The right plot shows the SM decay of a $t\overline{t}$ pair in the semileptonic decay channel.

Figure 2

Distributions of $m_{\mathrm{jj}}$, prior to the fit to data, of the two highest $p_{\mathrm{T}}$ non-$b$ jets for the $\mathrm{muon}+\mathrm{jets}$ channel (left column) and the $\mathrm{electron}+\mathrm{jets}$ channel (right column). The two distributions in the upper row are obtained using reconstructed jets. The distributions in the lower row are calculated using jets after the kinematic fit. The uncertainty band (showing the absolute uncertainty in the upper panels and the relative uncertainty in the lower panels) includes both statistical and systematic components. The signal events are scaled by twice the maximum observed upper limit on $\mathcal{B}(t\to H^{+}b)$ obtained at 8 TeV [23].

Figure 3

Distributions of $m_{\mathrm{jj}}$, after a background-only fit to the data, in the exclusive charm tagging categories for the $\mathrm{muon}+\mathrm{jets}$ (left column) and $\mathrm{electron}+\mathrm{jets}$ (right column) channels. The upper row shows the exclusive loose category, the middle row shows the exclusive medium category, and the lower row shows the exclusive tight category. The expected signal significance (prior to the fit) can be observed to vary across the different categories. The uncertainty band (showing the absolute uncertainty in the upper panels and the relative uncertainty in the lower panels) includes both statistical and systematic components after the background-only fit. The signal distributions are scaled by twice the maximum observed upper limit on $\mathcal{B}(t\to H^{+}b)$ obtained at 8 TeV [23].

Figure 4

The expected and observed upper limit in % on $\mathcal{B}(t\to H^{+}b)$ as a function of $m_{H^{+}}$ using $m_{\mathrm{jj}}$ after the individual charm tagging categories have been combined, for the $\mathrm{muon}+\mathrm{jets}$ (upper left) and $\mathrm{electron}+\mathrm{jets}$ (upper right) channels and their combination (lower).