Decay Distribution of High-Transverse-Momentum Rho Mesons

The exclusive process ${\pi }^{-}p\to {\rho }^{-}p$ has been measured at 90° c.m. with an incident pion momentum of 9.9 GeV/c. We present data on the angular dependence of the decay ${\rho }^{-}\to {\pi }^{-}{\pi }^0$. We observe a strong azimuthal dependence in the decay in the c.m. helicity frame of the $\rho$. Such an azimuthal dependence is not compatible with SU(6) valence-quark perturbation calculations.