B-Bbar mixing in unquenched lattice QCD

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Abstract
We present an unquenched lattice calculation for the B^0-B^0bar transition amplitude. The calculation, carried out at an inverse lattice spacing 1/a = 2.22(4) GeV, incorporates two flavors of dynamical quarks described by the fully O(a)-improved Wilson fermion action, and heavy quark is described by NRQCD. We discuss uncertainties due to the chiral extrapolation for light quark, which could be sizable for the leptonic decay constant of the B_d meson because of pion loop effects, contrary to the B_s meson and the B parameters are expected to be stable. We obtain f_Bd = 191(10)(^{+12}_{-22}) MeV, f_Bs/f_Bd = 1.13(3)(^{+13}_{-2}), B_Bd(m_b) = 0.836(27)(^{+56}_{-62}), B_Bs/B_Bd = 1.017(16)(^{+56}_{-17}), xi = 1.14(3)(^{+13}_{-2}), where the first error is statistical, and the second is systematic, adding in quadrature a model estimate of uncertainty due to chiral extrapolation, and those of finite lattice spacing, heavy quark expansion and perturbative operator matching.

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