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

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 MW and mt in the W W and t t threshold regions. With an integrated luminosity of 10 (100) fb , 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 mt = 200 MeV for xed MW would constrain a 100 GeV SM Higgs mass within about 2 GeV, while MW = 6 MeV for xed mt would constrain mh to about 10 GeV.