= Solution
<Initial-state radiation> of <gluons> gives the <W boson> nonzero <transverse momentum>. Its transverse <Lorentz boost> then changes the <charged lepton>[charged-lepton] $p_T$, washing out the sharp $M_W/2$ edge and permitting events above it. The recoil must be modelled with <Quantum chromodynamics> and constrained by measured <hadron>[hadronic] activity. Independently, the finite <W boson> <decay width> spreads its actual <invariant mass>, also smearing the fixed-mass endpoint. Either is a theoretical broadening mechanism; neither is a <particle detector>[detector-resolution] effect.
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