Precision W - boson and top - quark mass determinations at a muon collider

Precise determinations of the masses of the W boson and of the top quark could stringently test the radiative structure of the Standard Model (SM) or provide evidence for new physics. We analyze the excellent prospects at a muon collider for measuring M W and m t in the W + W − and t ¯ t threshold regions. With an integrated luminosity of 10 (100) fb −1 , the W-boson mass could be measured to a precision of 20 (6) MeV, and the top-quark mass to a precision of 200 (70) MeV, provided that theoretical and experimental systematics are understood. A measurement of ∆m t = 200 MeV for fixed M W would constrain a 100 GeV SM Higgs mass within about ±2 GeV, while ∆M W = 6 MeV for fixed m t would constrain m h to about ±10 GeV.