Double polarization asymmetry as a possible filter for $\Theta^+$'s parity

We present an analysis of the Beam-Target double polarization asymmetry for the photoproduction of $\Theta^+$ in $\gamma n\to \Theta^+ K^-$. We show that this quantity can serve as a filter for the determination of the $\Theta^+$'s spin-parity assignment near threshold. It is highly selective between $1/2^+$ and $1/2^-$ configurations due to dynamical reasons.