Mixing of B mesons and Decay Constants with the Non-Perturbatively Improved Action

Several quantities relevant to phenomenological studies of B 0 – ¯ B 0 mixing are computed on the lattice. Our main results are f B d q ˆ B B d = 206(28)(7) MeV, ξ = f B s q ˆ B B s /f B d q ˆ B B d = 1 . 16(7). We also obtain the related quantities f B s q ˆ B B s = 237(18)(8) MeV, f B d = 174(22) +7+4 − 0 − 0 MeV, f B s = 204(15) +7+3 − 0 − 0 MeV, f B s /f B d = 1 . 17(4) +0 − 1