Optimal estimation of Dimension-8 Neutral Triple Gauge Couplings at $e^+ e^-$ Colliders

TL;DR

This work assesses neutral triple gauge couplings (nTGCs) in the SMEFT using dim-8 operators in $e^+e^- \to ZZ$ at a future collider. It adopts the optimal observable technique to extract 95\% CL limits on the Wilson coefficients $C_i/\Lambda^4$ for CP-even and CP-odd nTGCs, leveraging beam polarization at $\sqrt{s}=3$ TeV and $\mathcal{L}_{int}=1000$ fb$^{-1}$. The results show that OOT significantly outperforms cut-based analyses and can improve bounds by factors up to $60$ relative to LHC ATLAS/CMS limits, while polarization enhances sensitivity by $15\%$ to $45\%$. CP-violating couplings exhibit correlations in a two-parameter space, yet the overall constraints remain stronger than current bounds, highlighting the potential of $e^+e^-$ colliders to probe BSM physics in the gauge sector.

Abstract

We investigate the measurement of non-standard ZZV (V = γ, Z) couplings through Z-boson pair production at the $e^+ e^-$ colliders. We adopt Standard Model Effective Field Theory (SMEFT) approach to study these anomalous neutral triple gauge couplings. There are one CP-conserving and three CP-violating dim-8 SMEFT operators that contribute to ZZV couplings. Using optimal observable technique, the sensitivity of these NP couplings has been estimated and then we compare it with the latest experimental limits on dim-8 couplings at CERN LHC. Effect of beam polarization and correlations among CP-violating ZZV couplings are discussed. Comparison of statistical limits of new physics couplings between optimal observable technique and contemporary cut-based analysis has also been studied in detail.

Figures (7)

• Figure 1: $Z$ boson pair production and subsequent decay to $\tt OSL$ + missing energy ($E_{\tt miss}$); left: SM, right: dim-8 SMEFT.
• Figure 2: Normalized event distribution of $\tt OSL$ + missing energy for signal and non-interfering SM background with $\sqrt{s}= 3$ TeV, $C_{ij}=1$, $\Lambda=3.2$ TeV and unpolarized beam. Left: missing energy ($E_{\tt miss}$), right: invariant dilepton mass ($m_{\ell \ell}$).
• Figure 3: Variation of $ZZ$ cross-section with the different dim-8 effective couplings at the $e^+ \, e^-$ colliders with $\sqrt s$ = 3 TeV. Left: $\{P_{e^-}:P_{e^+}=0\%:0\%\}$; Middle: $\{P_{e^-}:P_{e^+}=-80\%:0\%\}$; Right:$\{P_{e^-}:P_{e^+}=+80\%:0\%\}$.
• Figure 4: $\chi^2$ function of different dim-8 nTGCs for different choice of polarization combinations at CLIC. Polarization information is written in the inset. Top left: $C_{\tilde{B} W}/\Lambda^4$; top right: $C_{ B W}/\Lambda^4$; bottom left: $C_{W W}/\Lambda^4$; bottom right: $C_{BB}/\Lambda^4$. The CM energy is $\sqrt{s}=3$ TeV and integrated luminosity $\mathfrak{L}_{\tt int}=1000$$\rm fb^{-1}$.
• Figure 5: Comparison of the variation of the $\chi^2$ function of different NP couplings for unpolarized beam at the CLIC. Top left: $C_{\tilde{B} W}/\Lambda^4$; top right: $C_{ B W}/\Lambda^4$; bottom left: $C_{W W}/\Lambda^4$; bottom right: $C_{BB}/\Lambda^4$.