Optimizing $pp\to A\to Z^{*}h\to \ell^+\ell^- b\bar b$ Searches at the LHC in the 2HDM Type-I with Inverted Hierarchy

Abstract

In this study, we investigate the Large Hadron Collider (LHC) search for the signal process $pp\to A \to Z^{*}h \to \ell^{+} \ell^{-} b \bar{b}$ ($\ell=e,\mu$) within the framework of the 2-Higgs-Doublet Model (2HDM) Type-I, considering an Inverted Hierarchy (IH) scenario wherein the Standard Model (SM)-like Higgs boson $H$ is heavier than $h$ (i.e., $m_h < m_H=125$ GeV). We reproduce the dominant background distributions from a CMS analysis of $pp\to H \to Z A \to \ell^{+} \ell^{-} b \bar{b}$ (for a on-shell $Z$), which we do for validation purposes, so that we can explore different invariant mass selection criteria to enhance the signal significance of our target process. Specifically, we compare the CMS baseline cuts ($70$ GeV $ < m_{\ell^+\ell^-} < 110$ GeV, no $m_{b\bar b}$ restriction) with the alternative selections $20~{\rm GeV}~< m_{\ell^+\ell^-} < 50$ GeV with $m_{b\bar b} < 100$ GeV. The latter cuts are enforced to account for the off-shellness of the $Z^*$ boson and the low mass Higgs state in our case. We show that these modifications reduce drastically the dominant Drell-Yan (DY) and top-(anti)quark pair backgrounds, leading to a significant excess from the analysis of the reconstructed $m_{\ell^+\ell^-b\bar b}$ invariant mass in the illustrative ranges 145 GeV $<m_{A}<$ 150 GeV and 68 GeV $<m_{h}<$ 75 GeV. Our results demonstrate that such cuts enable effective signal discrimination, suggesting an optimized strategy in future searches for an established topology but in a new kinematical regime, which is of particular relevance to the 2HDM Type-I with IH in its mass spectrum.