Using the $K^+p\to \pi^+K^+n$ reaction to determine the $\Theta^+$ quantum numbers

We study the $K^+p\to \pi^+K^+n$ reaction with some kinematics suited to the production of the $\Theta^+$ resonance recently observed and show that the measurement of cross sections and polarization observables can shed light on the spin,isospin and parity of the $\Theta^+$ state.