The regularity for elliptic problems

Question

Assume $$\Omega=\{(r,\theta):0<r<1,0<\theta<\omega\},$$ here $\omega>\pi$. That means $\Omega$ is not a convex domain. We solve Laplace problem in $\Omega$. Solution $u$ will behave like $$u\sim r^{\pi/\omega}\sin(\dfrac{\pi}{\omega}\theta).$$ I know a conclusion that $u\in H^{1+\pi /\omega-\epsilon}$, here $\epsilon>0$. But I don't know how to prove it, or if there are some clear references?