Sharpening the $A\to Z^{(*)}h $ Signature of the Type-II 2HDM at the LHC through Advanced Machine Learning

Abstract: The $A\to Z^{(*)}h$ decay signature has been highlighted as possibly being the first testable probe of the Standard Model (SM) Higgs boson discovered in 2012 ($h$) interacting with Higgs companion states, such as those existing in a 2-Higgs Doublet Model (2HDM), chiefly, a CP-odd one ($A$). The production mechanism of the latter at the Large Hadron Collider (LHC) takes place via $b\bar b$-annihilation and/or $gg$-fusion, depending on the 2HDM parameters, in turn dictated by the Yukawa structure of this Beyond the SM (BSM) scenario. Among the possible incarnations of the 2HDM, we test here the so-called Type-II, for a twofold reason. On the one hand, it intriguingly offers two very distinct parameter regions compliant with the SM-like Higgs measurements, i.e., where the so-called `SM limit' of the 2HDM can be achieved. On the other hand, in both configurations, the $AZh$ coupling is generally small, hence the signal is strongly polluted by backgrounds, so that the exploitation of Machine Learning (ML) techniques becomes extremely useful. In this paper, we show that the application of advanced ML implementations can be decisive in establishing such a signal. This is true for all distinctive kinematical configurations involving the $A\to Z^{(*)}h$ decay, i.e., below threshold ($m_A<m_Z+m_h$), at its maximum ($m_Z+m_h<m_A<2m_t$) and near the onset of $t\bar t$ pair production ($m_A \approx 2m_t$), for which we propose Benchmark Points (BPs) for future phenomenological analyses.