Clues to QCD Dynamics from Flavor Dependence of Nucleon Spin-Flip Transitions

It is much harder to flip the spin of a proton by flipping a $d$-quark spin than by flipping a $u$-quark spin. This gives very strong flavor dependence which can be used in tests of QCD models. For example, the ratio of the spin-flip transitions $\frac{\sigma ({\pi }^{-}p\to {\rho }^{-}p)}{\sigma ({\pi }^{+}p\to {\rho }^{+}p)}$ is predicted to be 1 if the mechanism is flavor-independent gluon exchange, to be 16 if the mechanism is quark-antiquark annihilation and pair creation, and to be $\frac{1}{16}$ if the mechanism is quark exchange.