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Chiral behavior of the B(s,d)-Bbar(s,d) mixing amplitude in the Standard Model and beyond

We compute the chiral logarithmic corrections to the Bd and Bs mixing amplitudes in the Standard Model and beyond. We then investigate the impact of the inclusion of the lowest-lying scalar heavy-light states to the decay constants and bag-parameters and show that this does not modify the pion chiral logarithms, but it does produce corrections which are competitive in size with the K- and eta-meson chiral logarithms. This conclusion is highly relevant to the lattice studies since the pion chiral logarithms represent the most important effect in guiding the chiral extrapolations of the lattice data for these quantities. Also important is to stress that the pion chiral logarithmic corrections are useful in guiding those extrapolations as long as Mpi << Delta, where Delta is the mass gap between the scalar and pseudoscalar heavy-light mesons.