Analysis of the $ q\bar q\to Z^* \to hA \to4τ$ process within the lepton-specific 2HDM at the LHC

TL;DR

This work investigates the feasibility of producing light Higgs states $h$ and $A$ in the lepton-specific (Type-X) 2HDM via $pp\to Z^{*}\to hA$ and their subsequent decay to $4\tau$, focusing on the signature with two same-sign leptons and two hadronic $\tau$ decays. Taking $H$ as the SM-like Higgs with $M_H=125$ GeV and exploring the large $\tan\beta$ regime, the authors perform a detailed MC analysis at $\sqrt{s}=14$ TeV and $L=300\,\mathrm{fb}^{-1}$, employing both kinematic cuts and a gradient-boosted decision tree to suppress dominant SM backgrounds (notably $ZZ$ and $t\bar t$). They define a set of nine benchmark points in the constrained parameter space, derive LO cross sections, and show that the signal can achieve significances well above discovery in many cases, including via an approximate extrapolation method across the parameter space. The study also connects the high $\tan\beta$ region to muon $g-2$ explanations and discusses how tau-tagging efficiencies would affect the reach. Overall, the paper demonstrates that the $4\tau$ final state from $q\bar q$-initiated $Z^{*}$-mediated production in Type-X 2HDM could be probed or constrained with current and near-future LHC data, providing a complementary window into extended Higgs sectors.

Abstract

We analyse light Higgs scalar and pseudoscalar associated hadro-production in the 2-Higgs Doublet Model (2HDM) Type-X (or lepton-specific) within the parameter space allowed by theoretical self-consistency requirements as well as the latest experimental constraints from the Large Hadron Collider (LHC), precision data and $B$ physics. Over the viable regions of such a scenario, the Standard Model-like Higgs boson discovered at the LHC in 2012 is the heavier CP-even state $H$. Furthermore, in the Type-X scenario, due to large $\tanβ$, the lighter Higgs scalar $h$ and the pseudoscalar $A$ mainly decay into two $τ$ leptons. Therefore, we concentrate on analysing the signal process $pp\to Z^{*} \to hA\to τ^{+}τ^{-}τ^{+}τ^{-}\to \ell ν_\ell \ell ν_\ell τ_h τ_h$ (where $\ell= e, μ$ whereas $τ_h$ represents the hadronic decay of the $τ$) and explore the feasibility of conducting such a search at the LHC with a centre-of-mass energy of $\sqrt{s}~=$ 14 TeV and a luminosity of $L~=~300~fb^{-1}$. To suppress the huge SM background, we confine ourselves to consider the fraction of signal events with two same-sign $τ$ leptons further decaying into same-sign leptons while the other two $τ$ leptons decay hadronically. We find that a combination of kinematical selection and machine learning (ML) analysis will yields significant sensitivity to this process at the end of the LHC Run 3.

Figures (11)

• Figure 1: Scatter plot of ${\rm{BR}}(h \to \tau^+\tau^-)\times {\rm{BR}}(A \to \tau^+\tau^-)$ as a function of $M_{{{h}}}$ and $M_{{{A}}}$ (left panel) and $M_{{{A}}} - M_{{{h}}}$ as a function of $\text{sin}(\beta-\alpha)$ and $\text{tan}\beta$ (right panel).
• Figure 2: Scatter plot of $\text{tan}\beta$ as a function of $M_{{{h}}}$ and $M_{{{A}}}$ when the $a_\mu$ effect is considered in addition to the constraints imposed in the previous figure.
• Figure 3: Normalised distributions in transverse momentum of the first- and second-leading jets (a) and leptons (b) for BP4 and backgrounds at the LHC with $\sqrt{s}~=~\text{14 TeV}$ and $L~=~\text{300}~\text{fb}^{-1}$.
• Figure 4: Distributions of the invariant mass of a lepton and a jet for BP4 and background processes at the LHC with $\sqrt{s}~=~\text{14 TeV}$ and an integrated luminosity of $L = \text{300}~\text{fb}^{-1}$. The reconstructed masses, obtained from the best $\chi^2$ fit, are denoted by $M_{lj}^1$ and $M_{lj}^2$, and are shown in normalized form in panels (a) and (b), respectively. Panel (c) displays the unnormalized distributions (corresponding to $L = \text{300}~\text{fb}^{-1}$) with the correlation between $M_{lj}^1$ and $M_{lj}^2$. In the bottom frame, the signal (background) is in blue(red).
• Figure 5: Normalised distributions in invariant mass of lepton (a) and jet (b) pairs for BP4 and backgrounds at the LHC with $\sqrt{s}~=~\text{14 TeV}$ and $L~=~\text{300}~\text{fb}^{-1}$.