Four-quark operators in hadrons

Physical Review D, 1994

Physical Review D, 1999

We introduce and study the properties of the "color-straight" four-quark operators containing heavy and light quark fields. They are of the form (bΓ b b)(qΓ q q) where both brackets are color singlets. Their expectation values include the bulk of the nonfactorizable contributions to the nonleptonic decay widths of heavy hadrons. The expectation values of the color-straight operators in the heavy hadrons are related to the momentum integrals of the elastic light-quark formfactors of the respective heavy hadron. We calculate the asymptotic behavior of the light-current formfactors of heavy hadrons and show that the actual decrease is 1/(q 2) 3 2 rather than 1/q 4. The two-loop hybrid anomalous dimensions of the four-quark operators and their mixing (absent in the first loop) are obtained. Using plausible models for the elastic formfactors, we estimate the expectation values of the color-straight operators in the heavy mesons and baryons. Improved estimates will be possible in the future with new data on the radiative decays of heavy hadrons. We give the Wilson coefficients of the four-fermion operators in the 1/m b expansion of the inclusive widths and discuss the numerical predictions. Estimates of the nonfactorizable expectation values are given.

Physics Letters B, 2007

We present results of dynamical simulations of N f = 2 degenerate Wilson twisted mass quarks at maximal twist in the range of pseudo scalar masses 300 MeV m PS 550 MeV. Reaching such small masses was made possible owing to a recently developed variant of the HMC algorithm. The simulations are performed at one value of the lattice spacing a 0.1 fm. In order to have O(a) improvement and aiming at small residual O(a 2 ) cutoff effects, the theory is tuned to maximal twist by requiring the vanishing of the untwisted quark mass. Precise results for the pseudo scalar decay constant and the pseudo scalar mass are confronted with chiral perturbation theory predictions and the low energy constants F ,l 3 andl 4 are evaluated with small statistical errors.

Nuclear Physics B - Proceedings Supplements, 1997

We calculate connected and disconnected contributions to the flavour singlet scalar density amplitude of the nucleon in a full QCD lattice simulation with n f = 2 dynamical Wilson fermions at β = 5.6 on a 16 3 × 32 lattice. We find that both contributions are of similar size at the light quark mass. We arrive at the estimate σ πN = 18(5)MeV. Its smallness is directly related to the apparent decrease of u, d quark masses when unquenching QCD lattice simulations. The y parameter can be estimated from a semi-quenched analysis, in which there are no strange quarks in the sea, the result being y = 0.59(13).

2010

We present an unquenched quark model for baryons in which the effects of quarkantiquark pair creation (uū, dd and ss) are taken into account in an explicit form via a microscopic, QCD-inspired, quark-antiquark creation mechanism. It is shown that, while the inclusion of the qq pairs does not affect the baryon magnetic moments, it leads to a sizeable contribution of the orbital angular momentum to the spin of the proton.

1998

We calculate connected and disconnected contributions to the flavour singlet scalar density amplitude of the nucleon in a full QCD lattice simulation with n_f=2 dynamical Wilson fermions at β=5.6 on a 16^3 × 32 lattice. We find that both contributions are of similar size at the light quark mass. We arrive at the estimate σ_π N = 18(5)MeV. Its smallness is directly related to the apparent decrease of u, d quark masses when unquenching QCD lattice simulations. The y parameter can be estimated from a semi-quenched analysis, in which there are no strange quarks in the sea, the result being y=0.59(13).

Proceedings of The XXVI International Symposium on Lattice Field Theory — PoS(LATTICE 2008)

Physical Review D, 1999

We calculate connected and disconnected contributions to the flavour singlet scalar density amplitude of the nucleon in a full QCD lattice simulation with n f = 2 dynamical Wilson fermions at β = 5.6 on a 16 3 × 32 lattice. We find that both contributions are of similar size at the light quark mass. We arrive at the estimate σ πN = 18(5)MeV. Its smallness is directly related to the apparent decrease of u, d quark masses when unquenching QCD lattice simulations. The y parameter can be estimated from a semi-quenched analysis, in which there are no strange quarks in the sea, the result being y = 0.59(13).

arXiv: High Energy Physics - Phenomenology, 2007

In this paper, the twist-3 two parton corrections in charmless $B\to PP$ decays are shown to be factorizable under the QCD factorization approach. The factorizability of the twist-3 two parton corrections is constructed on the following findings. Under the energetic meson limit, the pseudoscalar distribution amplitude for a light pseudoscalar meson is allowed to be non-constant by the equations of motion for the quark. The non-constant pseudoscalar distribution amplitude is then used to regularize the end-point divergences in the hard spectator corrections at twist-3 order. By retaining the momentum fraction variable of the spectator quark of the $B$ meson in the propagators, the end-point divergence in the weak annihilation corrections at twist-3 order is resolved. The factorization of the $O(\alpha_s)$ corrections under the two parton approximation is shown valid up-to $O(1/m_b)$ . The hard scattering kernels of order $O(\alpha_s)$ and $O(\Lambda_{QCD}/m_b)$ are explicitly given a...

Physical Review D, 2003

We calculate expectation values of QCD operators consisting of the products of the four operators of the light quarksqΓ X qqΓ Y q, with Γ X,Y corresponding to the scalar, pseudoscalar, vector, pseudovector (axial) and tensor Lorentz structures, in the nucleon. All combinations of the light flavors are considered. For the evaluation we use elements of the Perturbative Chiral Quark Model (PCQM), approximating the contribution of the valence quarks by the contribution of the PCQM constituent quarks. The contribution of the sea quarks is treated by averaging over the QCD pions with the distribution of the pion field being determined by the PCQM. For quarks with the same flavor the expectation values are dominated by the contribution of the sea-quarks. In the scalar case the contribution of the sea quarks is dominated by the "disconnected terms" were one of the pairs of theqq operators acts on the vacuum while the other one acts on the quarks of the pion. The role of the interference terms with one of theqq pairs acting on the sea quarks and another one acting on the valence quarks increases for quarks with different flavors. The result for the scalar condensate is compared to the one obtained earlier in the framework of the Nambu -Jona-Lasinio model.