Evidence for Quark Spin-flip in Pomeron Exchange
  Spin-parity analyses of the $\omega\pi$ system in the reaction $\gamma p \to (\omega\pi) p$ for photon laboratory energies from 20 to 70 GeV have shown that production of the $J^P=1^+$ $b_1(1235)$ meson dominates, with a $J^P=1^-$ background at the level of 20%. Using vector-meson dominance arguments, this background is shown to be consistent with the data on $e^+e^- \to \omega\pi$. The energy dependence of the data imply that the mechanism is a combination of Reggeon and Pomeron exchange. Assuming that the latter is relevant only for the $J^P = 1^-$ component and extrapolating to W=200 GeV, it is argued that this accounts for most of the preliminary $\omega\pi$ signal observed by the H1 Collaboration in the same reaction. A residual peak can be ascribed to the $b_1(1235)$, which requires a quark spin-flip from Pomeron exchange. Precisely the same mechanism occurs in the reaction $\pi p \to a_1(1260) p$. 
A. Donnachie