Probing flavor-diagonal couplings of doubly-charged scalar at low and high energies

TL;DR

This work investigates a TeV-scale right-handed doubly-charged scalar Δ_R^{++} from a D-parity-breaking left-right symmetric model, focusing on flavor-diagonal couplings to electrons and muons. It combines low-energy probes (Møller scattering, muon g-2, muonium-antimuonium transitions) with high-energy collider channels (CEPC/FCC-ee, muon colliders, and muTRISTAN) to map sensitivity to the Yukawa couplings f_R^{ee} and f_R^{μμ} relative to m_{Δ^{++}}. The analysis finds that f_R^{ee} and f_R^{μμ} as small as ~10^{-2} are accessible for m_{Δ^{++}} in the 1–3 TeV range, with complementary constraints from both energy regimes and direct production channels. A positive observation would strongly indicate the type-I seesaw mechanism within this LRSM framework, highlighting the synergy between precision low-energy tests and future high-energy colliders in probing lepton-flavor-conserving new physics. The study provides explicit cross sections, effective operators, and projected 95% CL bounds across multiple experimental setups, guiding future searches for lepton-number- and flavor-diagonal new physics at TeV scales.

Abstract

We investigate the phenomenology of a TeV-scale doubly-charged scalar from the right-handed sector within the framework of left-right symmetric models. Focusing on its flavor-diagonal couplings to right-handed electrons and muons, we assess probes from both high-energy colliders and low-energy precision experiments. High-energy processes include Bhabha scattering at LEP and future circular electron-positron colliders (CEPC/FCC-ee), direct production at the LHC, and dedicated searches and precision measurements at proposed muon colliders and $μ$TRISTAN. Low-energy observables encompass parity-violating Møller scattering, muon anomalous magnetic moment, and muonium-antimuonium oscillations. Our combined analysis indicates that for a doubly-charged scalar in the $1$-$3$ TeV range, the flavor-diagonal Yukawa couplings to electrons and muons as small as $10^{-2}$ are accessible. Observations of such a doubly-charged scalar would potentially point toward the type-I seesaw mechanism of neutrino masses in the left-right symmetric model with $D$-parity breaking.

Figures (4)

• Figure 1: Representative Feynman diagrams for the high-energy processes with the exchange or production of $\Delta^{\pm\pm}_R$.
• Figure 2: Total cross section as a function of $m_{\Delta^{++}}$ for various processes at different colliders: $e^+e^-$ ($\sqrt{s} = 240$ GeV), $\mu^+\mu^-$ ($\sqrt{s} = 3~\text{TeV}$), $\mu^+\mu^+$ ($\sqrt{s} = 2$ TeV), and $\mu^+e^-$ ($\sqrt{s} = 346$ GeV). Analytical results are shown as solid lines; cross-checks from MadGraph5_aMC@NLO are shown as dashed lines.
• Figure 3: Current exclusion limits and future prospects of the Yukawa couplings $f_R^{ee}$ and $f_R^{\mu\mu}$ for $m_{\Delta^{++}} = 1.4~\text{TeV}$. Gray regions indicate current 95% CL exclusion from measurements of $(g-2)_\mu$Muong-2:2025xykAliberti:2025beg, Bhabha scattering at LEP ALEPH:2013dgf, and searches for $M-\overline{M}$ transition at MACS/PSI Willmann:1998gd. Projected 95% CL sensitivity include those from future experiments: Bhabha scattering at CEPC/FCC-ee CEPCStudyGroup:2018ghiFCC:2018evy, Møller scattering at MOLLER MOLLER:2014iki, $M-\overline{M}$ transition at MACE Bai:2024skk. Also shown are prospects from high-energy analogues at a 3 TeV MuC MuonCollider:2022nsaInternationalMuonCollider:2024jyv and $\mu$TRISTAN with $\mu^+\mu^+/\mu^+e^-$ beams Hamada:2022mua, along with limits from direct pair and associated production processes at the MuC. The constraint from muon $g-2$ is too weak and not shown.
• Figure 4: Same as Fig. \ref{['fig:bound']}, but for $m_{\Delta^{++}} = 3$ TeV and 10 TeV MuC.