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

TL;DR

This paper recasts existing LHC measurements to set limits on the process $A o ZH$ with $H o t\bar{t}$ in the low-mass region, finding constraints and a mild preference for a potential new physics signal around 450-460 GeV.

Contribution

It provides the first constraints on $A\to ZH$ with $H\to t\bar{t}$ in the low-mass region by recasting $t\bar{t}Z$ measurements, exploring off-shell top quark effects.

Findings

Limits on $\sigma(A\to ZH)\times \text{Br}(H\to t\bar{t})$ range from 0.12 pb to 0.62 pb.

A mild excess with up to 2.5$\sigma$ significance around $m_A \approx 450-460$ GeV.

The observed potential signal can be explained in a top-philic 2HDM with $\mu_t \gtrsim 0.16$.

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 $H\to tt^{(*)}$ 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 $\sigma(A\to ZH)\times {\rm Br} (H\to t\bar t)$ 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 $\sigma(A \to ZH) \times {\rm Br}(H \to t\bar t) \approx 0.3$ pb. This cross section can be accommodated within a top-philic 2HDM for a top-Yukawa coupling of the second Higgs doublet of $\mu_t \gtrsim 0.16$.