Searching for the neutral triple gauge couplings in the process $μ^+μ^-\to γν\barν$ at muon colliders

TL;DR

The paper assesses how future high-energy muon colliders can probe neutral triple gauge couplings through SMEFT with 14 dimension-8 operators in the $μ^{+}μ^{-}→γν\bar{ν}$ process. It compares annihilation and VBF topologies, evaluates the impact of beam polarization, and derives unitarity bounds to validate the EFT approach. The results show annihilation dominates over VBF at TeV scales and that polarized $(-+)$ beams notably tighten constraints, with the strongest reach for the pure-gauge operators $\mathcal{O}_{G+}$ and $\widetilde{\mathcal{O}}_{G+}$, surpassing current LHC bounds. It also analyzes CPV nTGC contributions to the electron EDM, finding collider limits generally stronger than EDM constraints, underlining the complementary nature of collider and precision measurements in constraining new physics in the gauge sector.

Abstract

We investigate the sensitivity of future high-energy muon colliders to neutral triple gauge couplings (nTGCs) through the process $μ^{+}μ^{-}\toγν\barν$ within the Standard Model Effective Field Theory (SMEFT) framework. Extending beyond previous studies, we consider a set of 14 dimension-8 operators, including both Higgs-related and pure gauge structures. By computing the cross sections and performing Monte Carlo simulations at multiple center-of-mass energies (3-30 TeV), we demonstrate that the annihilation process dominates over vector boson fusion (VBF) at TeV scales. We also explore the impact of beam polarization and show that the $(-+)$ polarization enhances sensitivity to several operators. After the study of the event selection strategies, we show that muon colliders can impose stronger expected constraints on nTGCs operators than current LHC bounds, with two of the pure gauge operators yielding the most stringent expected constraints. We also evaluate the contribution of CP-violating pure gauge operators to the electron electric dipole moment (EDM), finding that the expected constraints from muon colliders are stronger than those from EDM measurements.

Figures (10)

• Figure 1: Feynman diagrams illustrating the nTGCs contributions to the process $\mu^{+}\mu^{-}\to\gamma\nu\bar{\nu}$. The solid black dot represents the NP vertex. Diagram (a) corresponds to annihilation, and diagram (b) corresponds to VBF.
• Figure 2: The comparison of $\sigma_{\mathrm{annih}}^{\mathrm{NP}}$ with $\sigma_{\mathrm{VBF}}^{\mathrm{NP}}$ for $\mu^{+}\mu^{-}\to\gamma\nu\bar{\nu}$, as a function $\sqrt{s}$ with $C_{i}/\Lambda^{4}=1.0\ \mathrm{TeV^{-4}}$ allowed by LHC constraints. Only those operators which have both annihilation and VBF contributions are plotted.
• Figure 3: The comparison of $\sigma_{\mathrm{annih}}^{\mathrm{int}}$ with $\sigma_{\mathrm{VBF}}^{\mathrm{int}}$ for $\mu^{+}\mu^{-}\to\gamma\nu\bar{\nu}$, as a function $\sqrt{s}$ with $C_{i}/\Lambda^{4}=1.0\ \mathrm{TeV^{-4}}$ allowed by LHC constraints. Only those operators which have both annihilation and VBF interference cross sections are plotted.
• Figure 4: Typical Feynman diagrams of the SM backgrounds for the process $\mu^{+}\mu^{-}\to\gamma\nu\bar{\nu}$.
• Figure 5: The normalized distributions of $p_{T,\gamma}/E_{cm}$ and $E_{\gamma}/E_{cm}$ for the signal and the background of operator $\mathcal{O}_{\tilde{B}W}$, for the unpolarized case. The values of coefficients $C_{5}/\Lambda^{4}~(\mathrm{TeV}^{-4})$ used for $\mathcal{O}_{\tilde{B}W}$ in the MC simulation are: 0.35 at 3 TeV, 0.019 at 10 TeV, 0.0093 at 14 TeV, and 0.0021 at 30 TeV. The distributions of other nTGCs operators are similar.