Probing anomalous quartic gauge couplings via vector boson scattering at the same-sign muon collider

Abstract

The measurement of quartic gauge couplings (QGCs) provides a crucial test of the non-Abelian gauge structure of the Standard Model and offers sensitivity to new physics effects. In this work, we explore the potential of the proposed multi-TeV same-sign muon collider, $\mu$TRISTAN, to probe anomalous quartic gauge couplings (aQGCs) through vector boson scattering (VBS) processes. Owing to the same-sign initial state, s-channel contributions are absent, rendering VBS as the dominant production mode and thereby significantly enhancing the sensitivity to aQGCs. Using dimension-8 Standard Model Effective Field Theory (SMEFT) operators, we classify the relevant operator sets contributing to charged and neutral QGCs, and confront them with existing bounds from the LHC. A detailed collider analysis is performed across multiple final states: $2V2\nu$, $V\gamma \ell \nu$, $2V\ell \nu$, $2\gamma 2\ell$, and $2V2\ell$ ($V=$ reconstructed $W$ and $Z$ boson), applying optimized selection strategies. We present the projected sensitivities at the $\mu$TRISTAN with center-of-mass energies 2 TeV and 6 TeV, with integrated luminosities of 1 ab$^{-1}$ and 10 ab$^{-1}$, and demonstrate significant improvements over current experimental limits from the LHC. Our results establish $\mu$TRISTAN as a powerful probe of electroweak symmetry breaking dynamics and aQGCs in a model-independent framework.