500 research outputs found

    Non-perturbative renormalization constants on the lattice from flavour non-singlet Ward identities

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    By imposing axial and vector Ward identities for flavour-non-singlet currents, we estimate in the quenched approximation the non-perturbative values of combinations of improvement coefficients, which appear in the expansion around the massless case of the renormalization constants of axial, pseudoscalar, vector, scalar non-singlet currents and of the renormalized mass. These coefficients are relevant for the completion of the improvement programme to O(a) of such operators. The simulations are performed with a clover Wilson action non-perturbatively improved.Comment: 9 pages, latex file + 4 eps files of figure

    On the extraction of zero momentum form factors on the lattice

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    We propose a method to expand correlation functions with respect to the spatial components of external momenta. From the coefficients of the expansion it is possible to extract Lorentz-invariant form factors at zero spatial momentum transfer avoiding model dependent extrapolations. These objects can be profitably calculated on the lattice. We have explicitly checked the validity of the proposed procedure by considering two-point correlators with insertions of the axial current, the form factors of the semileptonic decay of pseudoscalar mesons, and the hadronic vacuum polarization tensor entering, for example, the lattice calculation of the anomalous magnetic moment of the muon.Comment: 10 pages, 5 figures, published versio

    Quenched lattice calculation of the B --> D l nu decay rate

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    We calculate, in the continuum limit of quenched lattice QCD, the form factor that enters in the decay rate of the semileptonic decay B --> D l nu. Making use of the step scaling method (SSM), previously introduced to handle two scale problems in lattice QCD, and of flavour twisted boundary conditions we extract G(w) at finite momentum transfer and at the physical values of the heavy quark masses. Our results can be used in order to extract the CKM matrix element Vcb by the experimental decay rate without model dependent extrapolations.Comment: 5 pages, 4 figures, accepted for publication on Phys. Lett. B, corrected one typ

    Determination of the strong coupling gB∗Bπg_{B^* B\pi} from semi-leptonic B→πℓνB\to \pi \ell \nu decay

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    According to heavy-meson chiral perturbation theory, the vector form factor f+(q2)f_+(q^2) of exclusive semi-leptonic decay B→πℓνB\to \pi \ell \nu is closely related, at least in the soft-pion region (i.e., q2∼(mB−mπ)2q^{2} \sim (m_B-m_{\pi})^2), to the strong coupling gB∗Bπg_{B^* B\pi} or the normalized coupling g^\hat g. Combining the precisely measured q2q^2 spectrum of B→πℓνB\to \pi \ell \nu decay by the BaBar and Belle collaborations with several parametrizations of the form factor f+(q2)f_+(q^2), we can extract these couplings from the residue of the form factor at the B∗B^* pole, which relies on an extrapolation of the form factor from the semi-leptonic region to the unphysical point q2=mB∗2q^2=m_{B^*}^2. Comparing the extracted values with the other experimental and theoretical estimates, we can test these various form-factor parametrizations, which differ from each other by the amount of physical information embedded in. It is found that the extracted values based on the BK, BZ and BCL parametrizations are consistent with each other and roughly in agreement with the other theoretical and lattice estimates, while the BGL ansatz, featured by a spurious, unwanted pole at the threshold of the cut, gives a neatly larger value.Comment: 19 pages, no figure. Revise

    Lattice Determination of the B∗BπB^\ast B \pi Coupling

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    The coupling gB∗Bπg_{B^\ast B \pi} is related to the form factor at zero momentum of the axial current between B∗B^\ast and BB states. Moreover it is related to the effective coupling between heavy mesons and pions that appear the heavy meson chiral Lagrangian. This coupling has been evaluated on the lattice using static heavy quarks and light quark propagators determined by a stochastic inversion of the fermionic bilinear. We found the value g=0.42(4)(8)g=0.42(4)(8). Beside its theoretical interest, this quantity has phenomenological implications in B→π+lˉlB \to \pi + \bar l l decays.Comment: Lattice 99, 3 page

    BB Potentials in Quenched Lattice QCD

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    The potentials between two B-mesons are computed in the heavy-quark limit using quenched lattice QCD at mπ∼400 MeVm_\pi\sim 400~{\rm MeV}. Non-zero central potentials are clearly evident in all four spin-isospin channels, (I,s_l) = (0,0) , (0,1) , (1,0) , (1,1), where s_l is the total spin of the light degrees of freedom. At short distance, we find repulsion in the I≠slI\ne s_l channels and attraction in the I=s_l channels. Linear combinations of these potentials that have well-defined spin and isospin in the t-channel are found, in three of the four cases, to have substantially smaller uncertainties than the potentials defined with the s-channel (I,s_l), and allow quenching artifacts from single hairpin exchange to be isolated. The BB*\pi coupling extracted from the long-distance behavior of the finite-volume t-channel potential is found to be consistent with quenched calculations of the matrix element of the isovector axial-current. The tensor potentials in both of the s_l = 1 channels are found to be consistent with zero within calculational uncertainties.Comment: 30 page

    Determination of B*B pi coupling in unquenched QCD

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    The B∗BπB^* B\pi coupling is a fundamental parameter of chiral effective Lagrangian with heavy-light mesons and can constrain the chiral behavior of fBf_B, BBB_B and the B→πlνB\to \pi l \nu form factor in the soft pion limit. We compute the B∗BπB^* B \pi coupling with the static heavy quark and the O(a)O(a)-improved Wilson light quark. Simulations are carried out with nf=2n_f=2 unquenched 123×2412^3\times 24 lattices at β=1.80\beta=1.80 and 163×3216^3\times 32 lattices at β=1.95\beta=1.95 generated by CP-PACS collaboration. To improve the statistical accuracy, we employ the all-to-all propagator technique and the static quark action with smeared temporal link variables following the quenched study by Negishi {\it et al.}. These methods successfully work also on unquenched lattices, and determine the B∗BπB^*B\pi coupling with 1--2% statistical accuracy on each lattice spacing.Comment: 19pages,26figure
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