= Solution
For the selected <muon> channel, finite curvature resolution in the magnetic tracking system broadens the reconstructed <transverse momentum> spectrum. A high-$p_T$ track has small <sagitta>, so spatial measurement errors, alignment errors and magnetic-field calibration errors become important; at high <momentum> an approximately fixed uncertainty in $1/p_T$ gives $\delta p_T\propto p_T^2$. <Multiple Coulomb scattering> contributes at lower <momenta>. If the <transverse mass> is fitted, finite hadronic-recoil resolution additionally smears the inferred <missing transverse momentum>. These are experimental effects, distinct from the physical recoil and width in <1/b/ii>.
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