  In high-energy leptonic collisions well above the electroweak scale, the
collinear splitting mechanism of the electroweak gauge bosons becomes the
dominant phenomena via the initial state radiation and the final state
showering. We point out that at future high-energy lepton colliders, such as a
multi-TeV muon collider, the electroweak parton distribution functions (EW
PDFs) should be adopted as the proper description for partonic collisions of
the initial states. The leptons and electroweak gauge bosons are the EW
partons, that evolve according to the unbroken Standard Model (SM) gauge group
and that effectively resum potentially large collinear logarithms. We present a
formalism for the EW PDFs at the Next-to-Leading-Log (NLL) accuracy. We
calculate semi-inclusive cross sections for some important SM processes at a
future multi-TeV muon collider. We conclude that it is appropriate to adopt the
EW PDF formalism for future high-energy lepton colliders.
