Delta Transition Form Factors Research Articles

Electromagnetic transition form factors, E2/M1 and C2/M1 ratios of the baryon decuplet

We investigate the electromagnetic transition form factors of the baryon decuplet to the baryon octet, based on the self-consistent SU(3) chiral quark-soliton model, taking into account the effects of explicit breaking of flavor SU(3) symmetry. We emphasize the Q^2 dependence of the electromagnetic Nrightarrow Delta transition form factors and the ratios of E2/M1 and C2/M1 in comparison with the experimental and empirical data. In order to compare the present results of the electromagnetic transition form factors of the Nrightarrow Delta with those from lattice QCD, we evaluate the form factors with the pion mass deviated from its physical value. The results of the E2/M1 and C2/M1 ratios are in good agreement with the lattice data. We also present the results of the electromagnetic transition form factors for the decuplet to the octet transitions.

Elastic and Transition Form Factors of the Δ(1232)

Predictions obtained with a confining, symmetry-preserving treatment of a vector-vector contact interaction at leading-order in a widely used truncation of QCD's Dyson-Schwinger equations are presented for \Delta and \Omega baryon elastic form factors and the \gamma N -> \Delta transition form factors. This simple framework produces results that are practically indistinguishable from the best otherwise available, an outcome which highlights that the key to describing many features of baryons and unifying them with the properties of mesons is a veracious expression of dynamical chiral symmetry breaking in the hadron bound-state problem. The following specific results are of particular interest. The \Delta elastic form factors are very sensitive to m_\Delta. Hence, given that the parameters which define extant simulations of lattice-regularised QCD produce \Delta-resonance masses that are very large, the form factors obtained therewith are a poor guide to properties of the \Delta(1232). Considering the \Delta-baryon's quadrupole moment, whilst all computations produce a negative value, the conflict between theoretical predictions entails that it is currently impossible to reach a sound conclusion on the nature of the \Delta-baryon's deformation in the infinite momentum frame. Results for analogous properties of the \Omega baryon are less contentious. In connection with the N->\Delta transition, the Ash-convention magnetic transition form factor falls faster than the neutron's magnetic form factor and nonzero values for the associated quadrupole ratios reveal the impact of quark orbital angular momentum within the nucleon and \Delta; and, furthermore, these quadrupole ratios do slowly approach their anticipated asymptotic limits.

Empirical Transverse Charge Densities in the Nucleon and the Nucleon-to-ΔTransition

Using only the current empirical information on the nucleon electromagnetic form factors we map out the transverse charge density in proton and neutron as viewed from a light front moving towards a transversely polarized nucleon. These charge densities are characterized by a dipole pattern, in addition to the monopole field corresponding with the unpolarized density. Furthermore, we use the latest empirical information on the N-->Delta transition form factors to map out the transition charge density which induces the N-->Delta excitation. This transition charge density in a transversely polarized N and Delta contains both monopole, dipole and quadrupole patterns, the latter corresponding with a deformation of the N and Delta charge distribution.

Light cone sum rules forγ*N→Δtransition form factors

A theoretical framework is suggested for the calculation of gamma* N -> Delta transition form factors using the light-cone sum rule approach. Leading-order sum rules are derived and compared with the existing experimental data. We find that the transition form factors in a several GeV region are dominated by the ``soft'' contributions that can be thought of as overlap integrals of the valence components of the hadron wave functions. The ``minus'' components of the quark fields contribute significantly to the result, which can be reinterpreted as large contributions of the quark orbital angular momentum

Hadron Structure from Lattice QCD

The structure of neutrons, protons, and other strongly interacting particles is now being calculated in full, unquenched lattice QCD with quark masses entering the chiral regime. This talk describes selected examples, including the nucleon axial charge, structure functions, electromagnetic form factors, the origin of the nucleon spin, the transverse structure of the nucleon, and the nucleon to Delta transition form factor.

Understanding hadron structure from lattice QCD in the SciDAC era

The structure of neutrons, protons, and other strongly interacting particles in terms of their quark and gluon constituents can be calculated from first principles by solving QCD on a discrete space-time lattice. With the advent of SciDAC software and prototype clusters and of DOE supported dedicated lattice QCD computers, it is now possible to calculate physical observables using full QCD in the regime of large lattice volumes and light quark masses that can be compared with experiment. This talk will describe selected examples, including the nucleon axial charge, structure functions, electromagnetic form factors, the origin of the nucleon spin, the transverse structure of the nucleon, and the nucleon to Delta transition form factor.

The N Delta form factors: from the real photon to pseudothreshold

We study the extrapolation of the electromagnetic N Delta transition form factors in the time-like region, from the real photon point to zero CM momentum (pseudothreshold), where the transition moments can be compared with quark model predictions. Our method consists of calculating the long-range contributions to the form factors due to the Delta -excitation by a 'quasi-real' pion photoproduced through the background. These long-range dispersive effects are of the order of 30% for the magnetic dipole and of 100% for the two electric quadrupoles. In particular, they account for the presently accepted experimental E2/M1 ratio and predict a large difference, for real photons, between the transverse and the longitudinal quadrupole moments. The imaginary parts are also investigated and found to be different from zero, through very small, only near pseudothreshold. As regards the k2 dependence, it is found that these contributions yield for the magnetic form factor a faster decrease than the dipole in the space-like region, as expected, but essentially the same for time-like photons. For the quadrupole moments, the extrapolation to pseudothreshold yields a vanishingly small value as foreseen by the naive quark model.

Pionic contribution to the N- Delta transition form factors

The gamma N Delta electric form factors gE and gC are predicted to differ for nearly real photons by an amount of the same order of magnitude of the poorly known gE. Long-range pionic dispersive contributions, responsible for this effect, only marginally influence gM. The above finding should prove essential in the comparison with future more accurate data to assess possible baryon deformations.