Angular distribution of decay leptons from e+e−→W+W− at threshold

The reaction e+e−→W+W− produces a W-boson pair with a non-trivial spin correlation at threshold. This correlation leads to a characteristic angular correlation between the leptons produced in W±→ℓ±ν (angles relative to the e− beam direction): dσ/d cosθ+d cosθ−∼(1−cosθ+)(1+cosθ−)(1+cosθ−cosθ+). If only the ℓ− is observed, its angular distribution is dσ/dcosθ−∼(1+cosθ−)(3−cosθ−) , implying a forward-backward asymmetry of 3/8. An analytic result is also given for the azimuthal correlation. These results are reproduced by a Monte Carlo program, that also enables us to study the effects of the W decay width. The threshold behaviour, which stems from the dominance of ν-exchange, is contrasted with that due to γ- and Z-exchange, which is relevant for annihilation in the helicity state e−Re+L.