Probing the low mass pseudoscalar in flipped Two Higgs Doublet Model

TL;DR

This work investigates the viability of a light pseudoscalar $A$ in the flipped 2HDM (with $m_A$ in the 20–60 GeV range) and proposes a collider search in the channel $pp\to h\to AZ(Z^*)\to bb\ell^+\ell^-$. Using MCMC scans to identify phenomenologically allowed regions and representative benchmarks, the study demonstrates that the signal can be probed despite dominant $A\to bb$ decays by exploiting the leptonic $Z$ tag and the Higgs-mediated production. Cut-based analyses at the HL-LHC show strong potential, particularly around $m_A \approx 30$ GeV, with significances up to ~15σ (10% systematics); a BDT approach further enhances sensitivity, extending reach to higher $m_A$ and improving discovery prospects. The results indicate that dedicated searches in this channel could reveal or constrain flipped 2HDM scenarios in the low-mass pseudoscalar regime, with some regions already accessible in Run II.

Abstract

The phenomenology of the flipped two-Higgs-doublet model (2HDM) is relatively less explored so far, as compared to the other, commonly discussed, types. It is found that this scenario, like several others, admits of a light neutral pseudoscalar $A$ in the mass range 20 - 60 GeV, consistently with all current experimental data and theoretical constraints. However, the fact that such a pseudoscalar decays overwhelmingly into a $b\bar{b}$ pair makes its identification at the Large Hadron Collider (LHC) a challenging task. After identifying the region of the flipped 2HDM parameter space yielding a light pseudoscalar, we identify a useful search channel in the process $pp \rightarrow A Z(Z^{*}) \rightarrow b\bar{b} \ell^+ \ell^-$. A cut-based analysis, followed by one based on Boosted Decision Trees, shows that the light-$A$ scenario in flipped 2HDM should be detectable with rather high statistical significance at the high-luminosity LHC run, even after including systematic uncertainties. Furthermore, part of the parameter space, especially around $m_A = 30 - 40$ GeV is amenable to detection at the discovery level within Run-2 itself.

Figures (9)

• Figure 1: Allowed scattered points in $m_A$-$\tan \beta$ plane on the left panel with third axis $m_{H^\pm}$ (color-coded) and in $m_A$-$\cos(\beta-\alpha)$ plane on the right panel with third axis $\tan \beta$ (color-coded), once we impose all theoretical and experimental constraints discussed in the text.
• Figure 2: Feynman diagram for the signal process where the SM Higgs is produced through Gluon-Gluon fusion (ggF) and decaying further into a $Z$ boson and a pseudoscalar.
• Figure 3: The left panel of the figure shows the variation of the cross-section (pb) for the process, $p p \rightarrow AZ(Z^*)\rightarrow b\bar{b} \ell^+ \ell^-$ through the gluon-gluon fusion with third axis $\cos (\beta - \alpha)$ (color-coded). The right panel of the Figure represents the cross-section variation with the pseudoscalar mass keeping $\tan \beta$ on the third axis, which indicates the cross-section decreases with increasing $\tan \beta$ for all pseudoscalar masses.
• Figure 4: Histogram of the invariant mass of leptons ($m_{\ell \ell}$) for three different signal benchmark points at the parton level. The Y-axis represents the number of events at 3000$fb^{-1}$ luminosity. The blue and orange histograms represent the number of events at the parton level with $p_T^b > \text{10 GeV}$ and $p_T^b > \text{20 GeV}$, respectively.
• Figure 5: The left panel shows the distribution of different backgrounds along with BP2, $m_A = \text{30 GeV}$. The signal lies in the lower side of the $E_T^{\rm miss}$; this variable kills the $t\bar{t}$ background to a large extent. Whereas the four-particle invariant mass is plotted in the right panel, where the signal peaks at the SM Higgs mass and the background deviates significantly and peaks near 210 GeV.