We report values of R = sigma(e(+)e(-)-->hadrons)/sigma(e(+)e(-)-->mu(+)mu(-)) for 85 center-of-mass energies between 2 and 5 GeV measured with the upgraded Beijing Spectrometer at the Beijing Electron-Positron Collider.

No abstract

The axial-vector form factors and axial-vector constants of the baryon decuplet are investigated within a pion mean-field approach, which is also known as the chiral quark-soliton model. Given an axial-vector current with a specified flavor, there are four different form factors of a decuplet baryon. When we consider the singlet, triplet, and octet axial-vector currents, we have twelve different form factors for each member of the baryon decuplet. We compute all these axial-vector form factors of the baryon decuplet, taking into account the rotational 1=N c corrections and effects of flavor SU(3) symmetry breaking. We find that, for a given flavor, two of the form factors for a decuplet baryon are only independent within the present approach. We first examine properties of the axial-vector form factors of the Δ þ isobar and Ω − hyperon. We also compare the results of the triplet axial-vector form factors of Δ þ with those from lattice QCD and those of the present work for the axial-vector constants of the baryon decuplet with the lattice data. All the results for other members of the baryon decuplet are then presented. The results of the axial charges are compared with those of other works. The axial masses and axial radii are also discussed.

No abstract

We investigate η photoproduction off the neutron target, i.e., γn → ηn, employing an effective Lagrangian method combining with a Regge approach. As a background, we consider nucleon exchange in the s-channel diagram and ρ-and ω-meson Regge trajectories in the t channel. The role of nucleon resonances given in the Review of Particle Data Group in the range of W ≈ 1500 − 2100 MeV and the narrow nucleon resonance N (1685, 1/2 + ) is extensively studied. The numerical results of the total and differential cross sections, double polarization observable E, and helicity-dependent cross sections σ 1/2 , σ 3/2 are found to be in qualitative agreement with the recent A2 experimental data. The predictions of the beam asymmetry are also given.

We investigate the effects of the kaon cloud on the electromagnetic and axial-vector form factors of the Ω− baryon within the framework of the chiral quark-soliton model. We first derive the profile function of the chiral soliton in such a way that the kaon Yukawa tail is properly produced self-consistently. Then, we compute the electromagnetic form factors of the Ω− baryon. The results for the electromagnetic form factors are compared with the lattice data. We find that the results with the kaon tail employed are in better agreement with the lattice data, compared with those obtained with the pion tail. We also study the axial-vector form factors of the Ω− baryon, examining the effects of the kaon cloud.

We present results of a recent work on the reaction mechanism of ηn photoproduction off the neutron in the range of √ s ≈ 1.5 − 1.9 GeV and discuss the role of various nucleon resonances listed in the Particle Data Group (PDG). We make use of an effective Lagrangian approach combining with a Regge method. The total and helicity-dependent cross sections σ 1/2 , σ 3/2 are computed and the numerical results are in good agreement with the A2 experimental data.

No abstract