= Solution
The useful modes are $\boxed{W^+\to e^+\nu_e\ \text{and}\ W^+\to\mu^+\nu_\mu}$ and their charge conjugates. An <electron> gives an isolated track and localized <electromagnetic shower>; a <muon> gives an isolated penetrating track. Both allow a well-calibrated <charged lepton>[charged-lepton] <transverse momentum> and a <missing transverse momentum> measurement for the <neutrino>.
The <tau lepton> decays before detection, usually adding further <neutrinos>. Its visible <momentum> is only part of the original tau <momentum>, so the simple two-body endpoint is lost. For the direct <electron> and <muon> modes, the <transverse mass>
$$
M_T^2=2p_T^\ell p_T^\nu[1-\cos(\Delta\varphi)]
$$
also has an ideal endpoint at $M_W$ and avoids needing the unmeasured <neutrino> longitudinal <momentum>. Fits to this distribution or the <charged lepton>[charged-lepton] $p_T$ spectrum determine $M_W$.
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