Searching for a Charged Higgs Boson in Top-Quark Decays via the $WZ$ Mode

Abstract

Top-quark decays are sensitive probes of light charged Higgs bosons ($H^{\pm }$) due to the sizable $t\bar{t}$ production cross section at the LHC in conjunction with their distinct experimental signatures. While dedicated ATLAS and CMS searches considered only $H^{\pm }$ decays into $\tau \nu$, $cs$, or $cb$ for $m_{H^{\pm }}<m_t$, the $WZ$ channel remains unexplored, despite being the dominant mode in $SU(2{)}_L$ triplet models. Since, top-quark pair production with $t\to H^{\pm }b$ and $H^{\pm }\to WZ$ gives rise to $t\bar{t}Z$-like signatures, we recast existing $t\bar{t}Z$ analyses to search for signs of charged Higgs bosons and set novel limits on the product of branching fractions Br$(t\to H^{\pm }b)\times$Br$(H^{\pm }\to WZ)$. These constraints turn out to be at the sub-permille level, despite the observed $2\sigma$ preference for a non-zero value. Interpreted within the hypercharge $Y=0$ Higgs triplet model, this translates into a stringent constraint on the triplet Higgs vacuum expectation value of $v_{\Delta }\lesssim 2$ GeV, which is stronger than those from the $cs,\tau \nu$ modes and even surpasses electroweak precision constraints from the $\rho$ parameter. Moreover, the $2\sigma$ preference for a non-zero cross section further strengthens the cumulative case for a $\approx 152$ GeV boson as suggested, in particular, by di-photon excesses.

Comments: revised for improved presentation; version to appear in Phys. Rev. D (as a Letter)