 W-decay Jacobian peak

For an on-shell W with zero transverse recoil and massless leptons, $p_T=(M_W/2)\sin \theta$. Mapping both angular roots to one transverse momentum produces an integrable inverse-square-root edge. For a normalized polar distribution $3(1-\cos \theta {)}^2/8$, the density in $x=2p_T/M_W$ is $3x(2-x^2)/(4\sqrt{1-x^2})$. Finite width, transverse recoil and detector resolution smear the edge.