Charming top interactions with a neutral Higgs Boson decaying into $τ^+ τ^-$ at the LHC

TL;DR

This work addresses flavor-changing neutral Higgs (FCNH) interactions in top quark decays within a general two-Higgs-doublet model (2HDM). It analyzes the process $t \to c\phi^0$ with $\phi^0$ decaying to $\tau^+\tau^-$, followed by one tau decaying leptonically and the other hadronically, using a full Monte Carlo pipeline (MadGraph5, Pythia8, Delphes) and the collinear tau reconstruction to recover the Higgs mass from $M_{\tau\tau}$ and to exploit the charm-quark energy as a discriminant. The study finds that at the LHC energies $\sqrt{s}=13$, 13.6, and 14$\,$TeV, with high luminosity (HL-LHC) data, the FCNH signal can be discovered for $\tilde{\rho}_{tc} \gtrsim 0.1$ and $m_{\phi} \lesssim 170$ GeV, with doubled sensitivity when the heavy Higgs states are nearly degenerate ($m_H \simeq m_A$). The results emphasize that in the alignment limit FCNH couplings to $H^0$ and $A^0$ remain sizable, offering a complementary probe to $t \to c h^0$ channels and providing a practical path toward discovering new Higgs-mediated flavor physics at the HL-LHC.

Abstract

We investigate the discovery potential of flavor changing neutral Higgs (FCNH) interactions in top quark decays at the LHC. A general two Higgs doublet model (2HDM) is adopted to study the top quark decay $t \to c φ^0$, where $φ^0$ is either a CP-even heavy neutral scalar ($H^0$), or a CP-odd pseudoscalar ($A^0$), followed by the Higgs boson decaying into $τ^+τ^-$. The complete process is $pp \to t\bar{t} \to t cφ^0 \to (bjj) (cτ_{lep}τ_{had}) +X$, where $t$ or $\bar{t}$ become virtual when $m_φ > m_t -m_c$, $τ_{lep}$ and $τ_{had}$ represent the $τ$ leptonic decay ($τ^\pm \to \ell^\pm$ + MET) and the $τ$ hadronic decay ($τ^\pm \to j_τ$ + MET), respectively. We evaluate the cross section applying realistic selection criteria for the FCNH signal and the dominant physics background at the parton-level. In addition, we employ \textsc{MadGraph}, \textsc{Pythia}~8, and \textsc{Delphes} to perform a realistic event-level Monte Carlo simulation analysis. The collinear approximation is applied since the $τ$'s from Higgs boson decays are highly boosted such that $τ_{lep}$ and $τ_{had}$ momenta are approximately in the same direction as the $τ$'s, which enables the reconstruction of the Higgs boson mass. The energy of the charm quark provides an additional kinematic variable that discriminates the signal from the background. In the alignment limit, the FCNH couplings for the heavy Higgs bosons ($H^0$ and $A^0$) will not be suppressed [$λ_{tcH} \propto \sin(β-α) \approx 1$]. We study the discovery potential for the heavy neutral Higgs bosons at the LHC with collider energies of $\sqrt{s} = 13$ TeV, 13.6 TeV and 14 TeV, and find the High-Luminosity LHC offers great promise for the discovery of heavier Higgs bosons from $t \to c φ^0$ or $t^* \to c φ^0$.

Figures (11)

• Figure 1: Leading-order Feynman diagrams for the signal.
• Figure 2: Invariant mass distribution $d\sigma/dM_{jj}$ for the Higgs signal (magenta, solid) and for the dominant background from $ttj$ (blue, dashed) and $ttjj$ (red, dotted) at the LHC with $\sqrt{s} = 13$ TeV. Results are shown for (a) parton level, and (b) event level.
• Figure 3: Invariant mass distribution $d\sigma/dM_{bjj}$ for the Higgs signal (magenta, solid) and for the dominant background from $ttj$ (blue, dashed) and $ttjj$ (red, dotted) at the LHC with $\sqrt{s} = 13$ TeV. Results are shown for (a) parton level, and (b) event level.
• Figure 4: Invariant mass distributions $d\sigma/dM_{\tau\tau}$ for the Higgs signal with $m_H=$ 130 GeV (magenta, solid), 150 GeV (cyan, dot-dashed), 200 GeV (black, solid) and for the dominant background from $ttj$ (blue, dashed) and $ttjj$ (red, dotted) at $\sqrt{s} = 13$ TeV. Results are shown for (a) parton level, and (b) event level with detector simulation in $pp$ collisions.
• Figure 5: Invariant mass distributions $d\sigma/dM_{c\tau\tau}$ for the Higgs signal ($t\to c\phi^0$) with $m_H =$ 130 GeV (magenta, solid) and 150 GeV (cyan, dot-dashed) and for the dominant background from $ttj$ (blue, dashed) and $ttjj$ (red, dotted) at $\sqrt{s} = 13$ TeV. Results are shown for (a) parton level, and (b) event level with detector simulation in $pp$ collisions.