TTP07-37 Resolving the sign ambiguity in Delta Gamma_s with B_s -> D_s K

D_s K > The analysis of tagged $B_s\to J/\psi \phi$ decays determines the CP phase $\phi_s$ in \bbms\ with a two–fold ambiguity. The solutions differ in the sign of $\cos \phi_s$ which equals the sign of the width difference $\dg_s$ among the two $B_s$ mass eigenstates. We point out that this ambiguity can be removed with the help of $B_s \to D_s K$ decays. We compare untagged and tagged strategies and find the tagged analysis more promising. The removal of the sign ambiguity in $\dg_s$ can be done with relatively low statistics and could therefore be a target for the early stage of $B_s \to D_s K$ studies.