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Register a new userDynamical diquarks in the ${boldsymbol{gamma^{(ast)} pto N(1535)tfrac{1}{2}^-}}$ transition
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The $gamma^{(ast)}+p to N(1535) tfrac{1}{2}^-$ transition is studied using a symmetry-preserving regularisation of a vector$,otimes,$vector contact interaction (SCI). The framework employs a Poincare-covariant Faddeev equation to describe the initial and final state baryons as quark+di-quark composites, wherein the diquark correlations are fully dynamical, interacting with the photon as allowed by their quantum numbers and continually engaging in breakup and recombination as required by the Faddeev kernel. The presence of such correlations owes largely to the mechanisms responsible for the emergence of hadron mass; and whereas the nucleon Faddeev amplitude is dominated by scalar and axial-vector diquark correlations, the amplitude of its parity partner, the $N(1535) tfrac{1}{2}^-$, also contains sizeable pseudoscalar and vector diquark components. It is found that the $gamma^{(ast)}+p to N(1535) tfrac{1}{2}^-$ helicity amplitudes and related Dirac and Pauli form factors are keenly sensitive to the relative strengths of these diquark components in the baryon amplitudes, indicating that such resonance electrocouplings possess great sensitivity to baryon structural details. Whilst SCI analyses have their limitations, they also have the virtue of algebraic simplicity and a proven ability to reveal insights that can be used to inform more sophisticated studies in frameworks with closer ties to quantum chromodynamics.
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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.
We study the photoproduction of the $N(1440)$ resonance in $gamma^*pto N^*$ process in quark models, where the $N(1440)$ takes different wave functions: first radial excitation of the nucleon imported from low-lying baryon mass spectrum calculations, a general radial excitation of the nucleon, and a $q^3$ state with positive parity. The comparison between the theoretical results and experimental data on the helicity amplitudes $A_{1/2}$ and $S_{1/2}$ and the analysis of the spatial wave function of the $N(1440)$ resonance reveal that the $N(1440)$ resonance is mainly the $q^3$ first radial excitation.
Results of a partial wave analysis of new high-statistics data on $gamma pto peta$ from MAMI are presented. A fit using known broad resonances and only standard background amplitudes can not describe the relatively narrow peaking structure in the cross section in the mass region of 1660-1750 MeV which follows a minimum. An improved description of the data can be reached by either assuming the existence of a narrow resonance at a mass of about 1700 MeV with small photo-coupling or by a threshold effect. In the latter case the observed structure is explained by a strong (resonant or non-resonant) $gamma ptoomega p$ coupling in the $S_{11}$ partial wave. When the beam asymmetry data, published by part of the GRAAL collaboration, are included in the fit, the solution with a narrow $P_{11}$ state is slightly preferred. In that fit, mass and width of the hypothetical resonance are determined to $Msim$1694 MeV and $Gammasim 40$ MeV, respectively, and the photo-coupling to $sqrt{{rm Br}_{eta N}} A_{1/2}^p sim 2.6cdot 10^{-3}$ GeV$^{-1/2}$. High precision measurements of the target asymmetry and $F$-observable are mandatory to establish the possible existence of such a narrow state and to provide the necessary information to define which partial wave is responsible for the structure observed in the data.
We study the electromagnetic $Omega_c gamma rightarrowOmega_c^ast$ transition in 2+1 flavor lattice QCD, which gives access to the dominant decay mode of $Omega_c^ast$ baryon. The magnetic dipole and the electric quadrupole transition form factors are computed. The magnetic dipole form factor is found to be mainly determined by the strange quark and the electric quadrupole form factor to be negligibly small, in consistency with the quark model. We also evaluate the helicity amplitudes and the decay rate.
The form-factors for the transition $ N^*(1535)to N $ induced by isovector and isoscalar axial currents within the framework of light-cone QCD sum rules by using the most general form of the interpolating current are calculated. In numerical calculations, we use two sets of values of input parameters. It is observed that the $ Q^2 $ dependence of the form-factor $ G_A $ can be described by the dipole form. Moreover, the form-factors $ G_P^{(S)} $ are found to be highly sensitive to the variations in the auxiliary parameter $ beta $.
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