Quark Spin-flip in Pomeron Exchange
  It has been shown that the energy dependence of the reaction gamma p --> b1(1235) requires a contribution from pomeron exchange. This necessitates spin-flip at the quark level as the transition is from a 3S1 state to a 1P1 state. The same mechanism occurs in the reaction pi p --> a1(1260) p, which is a 1S0 to 3P1 transition. 
A Donnachie