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Register a new userCovariant spectator quark model description of the $gamma^ast Lambda to Sigma^0$ transition
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We study the $gamma^ast Lambda to Sigma^0$ transition form factors by applying the covariant spectator quark model. Using the parametrization for the baryon core wave functions as well as for the pion cloud dressing obtained in a previous work, we calculate the dependence on the momentum transfer squared, $Q^2$, of the electromagnetic transition form factors. The magnetic form factor is dominated by the valence quark contributions. The final result for the transition magnetic moment, a combination of the quark core and pion cloud effects, turns out to give a value very close to the data. The pion cloud contribution, although small, pulls the final result towards the experimental value The final result, $mu_{LambdaSigma^0}= -1.486 mu_N$, is about one and a half standard deviations from the central value in PDG, $mu_{LambdaSigma^0}= -1.61 pm 0.08 mu_N$. Thus, a modest improvement in the statistics of the experiment would permit the confirmation or rejection of the present result. It is also predicted that small but nonzero values for the electric form factor in the finite $Q^2$ region, as a consequence of the pion cloud dressing.
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We study the effect of the meson cloud dressing in the octet baryon to decuplet baryon electromagnetic transitions. Combining the valence quark contributions from the covariant spectator quark model with those of the meson cloud estimated based on the flavor SU(3) cloudy bag model, we calculate the transition magnetic form factors at $Q^2=0$ ($Q^2=-q^2$ and $q$ the four-momentum transfer), and also the decuplet baryon electromagnetic decay widths. The result for the $gamma^ast Lambda to Sigma^{ast 0}$ decay width is in complete agreement with the data, while that for the $gamma^ast Sigma^+ to Sigma^{ast +}$ is underestimated the data by 1.4 standard deviations. This achievement may be regarded as a significant advance in the present theoretical situation.
Established results for the quark propagator in Landau gauge QCD, together with a detailed comparison to lattice data, are used to formulate a Poincare covariant Faddeev approach to the nucleon. The resultant three-quark amplitudes describe the quark core of the nucleon. The nucleons mass and its electromagnetic form factors are calculated as functions of the current quark mass. The corresponding results together with charge radii and magnetic moments are discussed in connection with the contributions from various ingredients in a consistent calculation of nucleon properties, as well as the role of the pion cloud in such an approach.
We analyze the mixing between $Sigma^0$ and $Lambda^0$ based on the baryon masses. We distinguish the contributions from QCD and QED in the baryon mass splittings. We find that the mixing angle between $Sigma^0$ and $Lambda^0$ is $(2.07pm 0.03)times 10^{-2} $, which leads to the decay branching fraction and up-down asymmetry of $Lambda_c^+ to Sigma^0 e^+ u_e$ to be ${cal B}(Lambda_c^+ to Sigma^0 e^+ u_e)=(1.5pm 0.2)times 10^{-5}$ and $alpha(Lambda_c^+ to Sigma^0 e^+ u_e)=-0.86pm 0.04$, respectively. Moreover, we obtain that $Delta {cal B}equiv {cal B}(Lambda_c^+to Sigma^0 pi^+) - {cal B}(Lambda_c^+to Sigma^+pi^0)=(3.8pm 0.5)times 10^{-4}$ and $Delta alpha equivalpha(Lambda_c^+to Sigma^0 pi^+) -alpha(Lambda_c^+to Sigma^+pi^0)=(-1.6pm 0.7)times10^{-2}$, which should vanish without the mixing.
We analyse the angular distributions of the Sigma^0 --> Lambda^0 + gamma decay rate in the laboratory and in the rest frame of the Sigma^0 - hyperon in the dependence on baryon polarizations. We calculate the dynamical polarization vector of the Lambda^0 - hyperon. Within the Effective quark model with chiral U(3) x U(3) symmetry (PRC 59, 451 (1999)) we calculate the transition magnetic moment mu_(Sigma^0 Lambda^0). The theoretical value mu_(Sigma^0 Lambda^0) = - 1.62, measured in nuclear magnetons, agrees well with the experimental data |mu^(exp)_(Sigma^0 Lambda^0)| = (1.61 +/-0.08) and the theoretical result, predicted within the naive quark model mu_(Sigma^0 Lambda^0) = (sqrt{3}/4)(mu_(Sigma^-) - mu_(Sigma^+)) = (- 1.57 +/- 0.01).
In a relativistic quark model we study the structure of the $N(1710)$ resonance, and the $gamma^ast N to N(1710)$ reaction focusing on the high momentum transfer region, where the valence quark degrees of freedom are expected to be dominant. The $N(1710)$ resonance, a state with spin 1/2 and positive parity ($J^P = frac{1}{2}^+$), can possibly be interpreted as the second radial excitation of the nucleon, after the Roper, $N(1440)$. We calculate the $gamma^ast N to N(1710)$ helicity amplitudes, and predict that they are almost identical to those of the $gamma^ast N to N(1440)$ reaction in the high momentum transfer region. Thus, future measurement of the helicity amplitudes for the $gamma^ast N to N(1710)$ reaction can give a significant hint on the internal structure of the $N(1710)$ state.
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