Discrete Ambiguities In Extracting Weak Phases from B decays
  Phases of elements of the Cabibbo-Kobayashi-Maskawa (CKM) matrix, as obtained using decays of $B$ mesons to $\pi^+ \pi^-$, $\pi^\pm K^\mp$, and $\pi^+ K^0$ or $\pi^- \overline{K}^0$, are shown to have a class of discrete ambiguities. In most cases these can be eliminated using other information on CKM phases. 
Amol S. Dighe (1), Jonathan L. Rosner (1 and 2) ((1) EFI and U. of   Chicago, (2) CERN)