= Solution
Choose $W^+\to\mu^+\nu_\mu$. Important backgrounds include <Quantum chromodynamics> production of heavy-flavour <hadrons> with semileptonic decays, in-flight <pion> or <kaon> decays, and <hadrons> penetrating the calorimeters. These generally have accompanying jet activity; semileptonic decays can contain real <neutrinos>, while jet mismeasurement can fake <missing transverse momentum>. Heavy-flavour tracks can originate at a <displaced decay vertex>. Require a high-$p_T$ isolated <muon>, agreement between inner tracking and the external <muon> system, and substantial <missing transverse momentum> inconsistent with <particle detector>[detector] noise or a mismeasured jet.
The genuine electroweak backgrounds include $Z\to\mu^+\mu^-$ with one <muon> lost and $W\to\tau\nu\to\mu\nu\nu\nu$. Veto a second reconstructed opposite-sign <muon> and use the dimuon <invariant mass> when both are present to reject the <Z boson> channel. The tau-mediated spectrum is softer and contains extra missing <particles>; template fits and control samples estimate its residual contribution. <Detector acceptance> determines how much of the lost-muon background survives.
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