Constraining $A\to ZH$ with $H\to t\bar{t}$ in the Low-Mass Region

Abstract

The decay $A\to ZH$ is a characteristic signal of two-Higgs-doublet models (2HDMs), where $A$ and $H$ lie primarily within the same $SU(2{)}_L$ multiplet, leading to a coupling of order $g_2$ to the $Z$ boson. The subsequent decay $H\to t{t}^{(\ast )}$ is particularly promising, as it gives rise to distinct final states involving multiple leptons and $b$-jets. The required splitting between $m_A$ and $m_H$ can naturally occur near the electroweak scale while being consistent with perturbative unitarity. Whereas dedicated ATLAS and CMS searches focused on the region with both top-quarks on-shell, we cover lower masses where one top quark is off-shell by recasting Standard Model $t\bar{t}Z$ measurements of ATLAS and CMS. The obtained limits on $\sigma (A\to ZH)\times \mathrm{Br}(H\to t\bar{t})$ are between $0.12$ pb and $0.62$ pb. Interestingly, we observe these stringent limits despite a preference (up to $2.5\sigma$) for a non-zero new physics signal, most pronounced around for $m_A\approx 450-460$ GeV and $m_H\approx 290$ GeV, with a best-fit value of $\sigma (A\to ZH)\times \mathrm{Br}(H\to t\bar{t})\approx 0.3$ pb. This cross section can be accommodated within a top-philic 2HDM for a top-Yukawa coupling of the second Higgs doublet of ${\mu }_t\gtrsim 0.16$.

Comments: Version to appear in Phys. Rev. D (as a Letter)