Abstract
We present a lattice QCD computation of the b-quark mass, the B and B_s decay constants, the B-mixing bag parameters for the full four-fermion operator basis as well as determinations for \xi and f_{Bq}\sqrt{B_i^{(q)}} extrapolated to the continuum limit and to the physical pion mass. We used N_f = 2 twisted mass Wilson fermions at four values of the lattice spacing with pion masses ranging from 280 to 500 MeV. Extrapolation in the heavy quark mass from the charm to the bottom quark region has been carried out on ratios of physical quantities computed at nearby quark masses, exploiting the fact that they have an exactly known infinite mass limit. Our results are m_b(m_b, \overline{\rm{MS}})=4.29(12) GeV, f_{Bs}=228(8) MeV, f_{B}=189(8) MeV and f_{Bs}/f_B=1.206(24). Moreover with our results for the bag-parameters we find \xi=1.225(31), B_1^{(s)}/B_1^{(d)}=1.01(2), f_{Bd}\sqrt{\hat{B}_{1}^{(d)}} = 216(10) MeV and f_{Bs}\sqrt{\hat{B}_{1}^{(s)}} = 262(10) MeV. We also computed the bag parameters for the complete basis of the four-fermion operators which are required in beyond the SM theories. By using these results for the bag parameters we are able to provide a refined Unitarity Triangle analysis in the presence of New Physics, improving the bounds coming from B_{(s)}-\bar B_{(s)} mixing.
Highlights
B-meson leptonic decays BR(B → τ ντ ) = (1.14 ± 0.22)10−4 [2,3,4], which is potentially sensitive to NP effects already at tree level, turns out to be in agreement with the SM prediction BR(B → τ ντ )SM = (0.81 ± 0.07)10−4 [5, 6], and the recent measurements of the Bs0 → μ+μ− decay [7,8,9,10] have given a first, remarkable evidence of SM consistency [11, 12, 6]
In this paper we use gauge configurations with Nf = 2 dynamical quarks at four values of the lattice spacing, generated by European Twisted Mass Collaboration (ETMC), to obtain the continuum limit results for a number of physical quantities that are relevant for B-Physics
In our previous paper [17], we provided a determination of the b-quark mass and of the B-meson decay constants obtained by studying the heavy quark on the lattice with the so called ratio method, proposed in ref
Summary
Simulation details are given, where μl, μs and μh indicate the bare light, strange-like and heavy (i.e. charm-like and heavier) valence quark masses respectively. The usual drawback, i.e. increase of the gauge noise due to fluctuations of the links entering in the smeared fields, is controlled by replacing thin gauge links with APE smeared ones [49] With this technical improvement we can extract heavy-light meson masses and matrix elements at relatively small temporal separations while keeping noise-to-signal ratio under control. In figure 3(a) we show an example of an improved ground state plateau using smeared source for the quark masses (aμl, aμh) = (0.0080, 0.5246) at β = 3.80. Further details on the implementation of the method for computing pseudoscalar meson masses, decay constants and bag parameters are given in appendix A
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