First observations of B_s^0→ D^+D^-, D_s^+D^- and D^0D^0 decays - collaboration, L; Aaij, R; Beteta, C; Adeva, B; Adinolfi, M; Adrover, C; Affolder, A; Ajaltouni, Z; Albrecht, J; Alessio, F; Alexander, M; Ali, S; et al. (Phys.Rev. D, 2013)