Perpendicular ev mass from W decay

We point out properties of the “perpendicularev mass”, defined in terms of transverse momentap t byM T 2 (ev)=2|p eT | |p vT |−2p eT ·p vT , that make it particularly well suited toW mass and width determinations. We give an analytic expression for its distribution inW production and subsequentW→ev decay a\(\bar pp\) colliders, accurate to order 〈p WT 2 /M W 2 〉≈1%. A maximum likelihood fit of this formula to the five UA1 events givesMW=80.3 −3 +6 GeV.