Lepton flavor violation in Higgs boson decays at the HL-LHC

TL;DR

The paper investigates charged lepton flavor violation in Higgs decays, focusing on $h\to e\mu$ and $h\to \tau\mu$ within the Froggatt-Nielsen Singlet Model (FNSM) at the HL-LHC. It combines a detailed theoretical framework—complex singlet extension with a flavor symmetry, CP-conserving scalar potential, and FN-suppressed Yukawas leading to LFV couplings—with a comprehensive collider analysis that uses Monte Carlo simulations and Boosted Decision Trees to optimize signal discrimination against SM backgrounds. The main results project HL-LHC sensitivities: $\text{BR}(h\to e\mu) \lesssim 4.3\times 10^{-6}$ and $\text{BR}(h\to \tau\mu) \lesssim 5.2\times 10^{-5}$ at 3000 fb$^{-1}$, with >5σ discovery potential for $\sigma(gg\to h\to e\mu) = 7\times10^{-4}$ pb at about 2400 fb$^{-1}$ and ≈4σ for $h\to \tau\mu$ at 3000 fb$^{-1}$; the $h\to \tau e$ channel is predicted to be negligible. These findings underscore the HL-LHC’s capability to probe fundamental lepton flavor structure and fermion mass generation mechanisms in BSM scenarios.

Abstract

We investigate the prospects for discovering lepton flavor-violating Higgs decays in proton-proton collisions, focusing on $h \to \ell_i\ell_j$ ($\ell_i\neq \ell_j, \ell=e,\,μ,\,τ$). The analysis is carried out within a Froggatt-Nielsen model, which provides a natural mechanism for such processes. By scanning the phenomenologically viable parameter space, we identify scenarios that yield observable signals at the High-Luminosity LHC (HL-LHC). For an integrated luminosity of $2400~\text{fb}^{-1}$, we project that $h \to eμ$ could reach the $5σ$ discovery threshold, while $h \to τμ$ may achieve about $4σ$. We also obtain $2σ$ exclusion limits of $\mathcal{BR}(h \to eμ) \lesssim 4.3 \times 10^{-6}$ and $\mathcal{BR}(h \to τμ) \lesssim 5.2 \times 10^{-5}$. These findings underscore the HL-LHC's unique sensitivity to probe new physics through rare Higgs decays.

Figures (5)

• Figure 1: (a) $\tilde{Z}_{e\mu}-v_s$ plane, and (b) $\tilde{Z}_{\tau\mu}-v_s$ plane. Different colored points correspond to those allowed by upper limits on $\mathcal{BR}(\mu\to e\gamma)$ (cherry), $\mathcal{BR}(\tau\to \mu\gamma)$ (purple), $\mathcal{BR}(h\to \tau\mu)$ (orange), $\mathcal{BR}(\tau\to 3\mu)$ (yellow).
• Figure 4: Kinematical distributions of the most discriminant observables to separate the signal from the background. (a) Transverse momentum of the electron $p_T^e$, and (b) transverse momentum of the muon $p_T^{\mu}$.
• Figure 5: Signal significance as a function of the integrated luminosity and different cross sections for the $h \to e\mu$ channel, with a cut on XGB of 0.995.
• Figure 6: Kinematical distributions of the most discriminant observables to separate the signal from the background. (a) Transverse momentum of the muon $p_T^\mu$, and (b) transverse momentum of the charged pion $p_T^{\pi^\pm}$.
• Figure 7: Signal significance as a function of the integrated luminosity and different cross sections for the $h \to \tau\mu$ channel, with a cut on XGB of 0.95