New Standard-Model Test for Future Colliders

We point out that for $W$-pair production from $e^{+}{e}^{-}$ or $q\overline{q}$ beams, the correlation between the decay planes of the $W\text{'}\mathrm{s}$ is numerically negligible (at tree level) in the standard model. This occurs for the production of $\mathrm{WW}$ via intermediate fermions or gauge bosons and independently for $\mathrm{WW}$ produced via Higgs-boson exchange, and is not restricted to two-body diagrams. In general, the correlation need not vanish, and so a nonzero correlation would be a clear signal of physics beyond the standard model. Definition of the $W$-decay plane should be fairly straightforward from the decay $W\to f\overline{f}$ and, hence, the correlation measurement is experimentally feasible.