= Solution
At a hadron collider, select $W\to\ell\nu$ events and reconstruct the <transverse mass>
$$
m_T=\sqrt{2p_T^\ell p_T^\nu(1-\cos\Delta\phi)},
$$
using <missing transverse momentum> for the <neutrino>. Fit the shape, especially the region above the nominal <Jacobian determinant>[Jacobian] edge, with finite-width resonance templates convolved with detector resolution, recoil modelling and backgrounds. The <Breit-Wigner resonance cross section>[Breit-Wigner] <decay width>[width] controls this off-shell tail. Calibrated <momentum> and missing-momentum resolutions must be separated from the physical <decay width>[width]. This is a lineshape measurement; the <mean lifetime>[lifetime] is far too short for a direct flight-distance measurement.
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