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Register a new userGluonic contribution to $Bto eta^{(prime)}$ form factors
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We calculate the flavor-singlet contribution to the $Btoeta^{(prime)}$ transition form factors from the gluonic content of the $eta^{(prime)}$ meson in the large-recoil region using the perturbative QCD approach. The formulation for the $eta$-$eta$ mixing in the quark-flavor and singlet-octet schemes is compared, and employed to determine the chiral enhancement scales associated with the two-parton twist-3 $eta^{(prime)}$ meson distribution amplitudes. It is found that the gluonic contribution is negligible in the $Btoeta$ form factors, and reaches few percents in the $Btoeta$ ones. Its impact on the accommodation of the measured $Btoeta^{(prime)} K$ branching ratios in the perturbative QCD and QCD-improved factorization approaches is elaborated.
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We report lattice results of $D_s$ meson semi-leptonic decay form factors to $eta$ and $eta$ mesons. This decay process contains disconnected fermion loops, which are challenging in lattice calculations. Our result shows that the disconnected loops give significant contributions to the form factors.
We examine the contribution of the pion cloud to the electromagnetic $N rightarrow Delta$ transition form factors within a relativistic hybrid constituent-quark model. In this model baryons consist not only of the $3q$ valence component, but contain, in addition, a $3 q pi$ non-valence component. We start with constituent quarks which are subject to a scalar, isoscalar confining force. This leads to an $SU(6)$ spin-flavor symmetric spectrum with degenerate nucleon and Delta masses. Mass splitting is caused by pions which are assumed to couple directly to the quarks. The point-form of relativistic quantum mechanics is employed to achieve a relativistically invariant description of this system. The $N rightarrow Delta$ transition current is then determined from the one-photon exchange contribution to the $Delta$ electroproduction amplitude. We will give predictions for the ratios $R_{EM}$ and $R_{SM}$ of electric to magnetic and Coulomb to magnetic form factors, which are supposed to be most sensitive to pion-cloud effects.
We study the electromagnetic structure of the nucleon within a hybrid constituent-quark model that comprises, in addition to the $3q$ valence component, also a $3q$+$pi$ non-valence component. To this aim we employ a Poincare-invariant multichannel formulation based on the point-form of relativistic quantum mechanics. With a simple 3-quark wave function for the bare nucleon, i.e. the $3q$-component, we obtain reasonable results for the nucleon form factors and predict the meson-cloud contribution to be significant only below $Q^2lesssim 0.5$,GeV$^2$ amounting to about 10% for $Q^2rightarrow 0$, in accordance with the findings of other authors.
We report on lattice results of the form factors for semi-leptonic decays of the D_s meson to eta and eta, with n_f=2+1 configurations. The calculation contains disconnected fermion loop diagrams, which are challenging to calculate on the lattice. Our result shows that the disconnected diagrams give significant contributions to the form factors.
The exact evaluation of the disconnected diagram contributions to the flavor-singlet pseudoscalar meson mass, the nucleon sigma term and the nucleon electromagnetic form factors, is carried out utilizing GPGPU technology with the NVIDIA CUDA platform. The disconnected loops are also computed using stochastic methods with several noise reduction techniques. Various dilution schemes as well as the truncated solver method are studied. We make a comparison of these stochastic techniques to the exact results and show that the number of noise vectors depends on the operator insertion in the fermionic loop.
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