The Effective Vector Boson Approximation in High-Energy Muon Collisions

Abstract: Due to the inclination for forward gauge radiation, lepton colliders beyond a few TeV are effectively electroweak (EW) boson colliders, suggesting the treatment of EW bosons as constituents of high-energy leptons. In the context of a muon collider, we revisit the validity of $W$ and $Z$ parton distribution functions (PDFs) at leading order in $2\to n$ process. We systematically investigate universal and quasi-universal power-law and logarithmic corrections that arise when deriving (polarized) weak boson PDFs in the collinear limit. We go on to survey a multitude of $2\to n$ processes at $\sqrt{s}=2-30$ TeV via polarized and unpolarized EW boson fusion/scattering. To conduct this study, we report a public implementation of the Effective $W/Z$ and Weizs\"acker-Williams Approximations, which we collectively call the Effective Vector Boson Approximation, into the Monte Carlo event generator \texttt{MadGraph5_aMC@NLO}. This implementation lays the groundwork for developing matrix-element matching prescriptions involving EW parton showers and renormalized EW PDFs. To further with this agenda, we give recommendations on using $W/Z$ PDFs.