arXiv subjects
Publications and source records attributed to Vladimir Pascalutsa.
We describe the pion electroproduction processes in the $Δ$(1232)-resonance region within the framework of chiral effective-field theory. By studying the observables of pion electroproduction in a next-to-leading order calculation we are able to make predictions and draw conclusions on the properties of the $N\to Δ$ electromagnetic form factors.
We establish relations which express the generalized parton distributions (GPDs) describing the $N\to Δ$ transition in terms of the nucleon GPDs. These relations are based on the known large-$N_c$ relation between the $N\to Δ$ electric quadrupole moment and the neutron charge radius, and a newly derived large-$N_c$ relation between the electric quadrupole ($E2$) and Coulomb quadrupole ($C2$) transitions. Namely, in the large-$N_c$ limit we find $C2=E2$. The resulting relations among the nucleon and $N\toΔ$ GPDs provide predictions for the $N\toΔ$ electromagnetic form factors which are found to be in very good agreement with experiment for moderate momentum transfers.
We review the description of the lowest-energy nucleon excitation -- the $Δ$(1232)-resonance. Much of the recent experimental effort has been focused on the precision measurements of the nucleon-to-$Δ$ transition by means of electromagnetic probes. We confront the results of these measuremnts with the state-of-the-art calculations based on chiral effective-field theories (EFT), lattice QCD, large-$N_c$ relations, perturbative QCD, and QCD-inspired models. We also discuss the link of the nucleon-to-$Δ$ form factors to generalized parton distributions (GPDs). Some of the theoretical approaches are reviewed in detail, in particular, recent dynamical and unitary-isobar models of pion electroproduction, which are extensively used in the interpretation of experiments. Also, the novel extension of chiral EFTs to the energy domain of the $Δ$-resonance is described in detail. The two-photon exchange effects in the electroexcitation of the $\De$-resonance are addressed here as well.
We develop an extension of chiral perturbation theory to the $Δ$(1232)-resonance energy region and apply it to investigate the pion electroproduction off the nucleon ($e^- N \to e^- Nπ$). We present a complete calculation of this process, in the $Δ$-resonance region, up to next-to-leading order in the {\it $δ$-expansion}. At this order, the only free parameters are the three low-energy constants corresponding to the magnetic (M1), electric (E2), and Coulomb (C2) $γN \to Δ$ transition strength. After fitting these parameters to a few well-known data, our calculation provides a prediction for observables and multipole amplitudes of pion electroproduction. These results compare favorably with the phenomenological multipole solutions and recent experimental results from MIT-Bates and MAMI. Our prediction for the pion-mass dependence of the $γNΔ$ form factors offers an explanation for the discrepancy between the recent lattice-QCD results and the experimental value for the "C2/M1 ratio" at low $Q^2$.
We study the chiral behavior of the nucleon and $Δ$-isobar masses within a manifestly covariant chiral effective-field theory, consistent with the analyticity principle. We compute the $πN$ and $πΔ$ one-loop contributions to the mass and field-renormalization constant, and find that they can be described in terms of universal relativistic loop functions, multiplied by appropriate spin, isospin and coupling constants. We show that these relativistic one-loop corrections, when properly renormalized, obey the chiral power-counting and vanish in the chiral limit. The results including only the $πN$-loop corrections compare favorably with the lattice QCD data for the pion-mass dependence of the nucleon and $Δ$ masses, while inclusion of the $πΔ$ loops tends to spoil this agreement.
We perform a relativistic chiral effective-field theory calculation of the pion electroproduction off the nucleon ($e^- N \to e^- N π$) in the Delta(1232)-resonance region. After fixing the three low-energy constants, corresponding to the magnetic (M1), electric (E2), and Coulomb (C2) $γNΔ$ couplings, our calculation provides a prediction for the momentum-transfer and pion-mass dependence of the $γNΔ$ form factors. The prediction for the pion-mass dependence resolves the discrepancy between the recent lattice QCD results and the experimental value for the "C2/M1 ratio" at low $Q^2$.
We evaluate the two-photon exchange contribution to the $e N \to e Δ(1232) \to e πN$ process at large momentum transfer with the aim of a precision study of the ratios of electric quadrupole (E2) and Coulomb quadrupole (C2) to the magnetic dipole (M1) $γ^* N Δ$ transitions. We relate the two-photon exchange amplitude to the $N \to Δ$ generalized parton distributions and obtain a quantitative estimate of the two-photon effects. The two-photon exchange corrections to the C2/M1 ratio depend strongly on whether this quantity is obtained from an interference cross section or from the Rosenbluth-type cross sections, in similarity with the elastic, $e N \to e N$, process.
We develop a Lorenz- and gauge-invariant dynamical model for pion electroproduction in the resonance region. The model is based on solving of the Salpeter (instantaneous) equation for the pion-nucleon interaction with a hadron-exchange potential. We find that the one-particle-exchange kernel of the Salpeter equation for pion electroproduction develops an unphysical singularity for a finite value of $Q^{2}$. We analyse two methods of dealing with this problem. Results of our model are compared with recent single-polarization data for pion electroproduction.
We perform a relativistic chiral effective field theory calculation of the radiative pion photoproduction ($γp \to π^0 p γ'$) in the $Δ$-resonance region, to next-to-leading order in the ``delta-expansion''. This work is aimed at a model-independent extraction of the $Δ^+$ magnetic moment from new precise measurements of this reaction. It also predicts the chiral behavior of $Δ$'s magnetic moment, which can be used to extrapolate the recent lattice QCD results to the physical point.
Considering the magnetic moment and polarizabilities of the nucleon we emphasize the need for relativistic chiral EFT calculations. Our relativistic calculations are done via the forward-Compton-scattering sum rules, thus ensuring the correct analytic properties. The results obtained in this way are equivalent to the usual loop calculations, provided no heavy-baryon expansion or any other manipulations which lead to a different analytic structure (e.g., infrared regularization) are made. The Baldin sum rule can directly be applied to calculate the sum of nucleon polarizabilities. In contrast, the GDH sum rule is practically unsuitable for calculating the magnetic moments. The breakthrough is achieved by taking the derivatives of the sum rule with respect to the anomalous magnetic moment. As an example, we apply the derivative of the GDH sum rule to the calculation of the magnetic moment in QED and reproduce the famous Schwinger's correction from a tree-level cross-section calcualation. As far as the nucleon properties are concerned, we focus on two issues: 1) chiral behavior of the nucleon magnetic moment and 2) reconciliation of the chiral loop and $Δ$-resonance contributions to the nucleon magnetic polarizability.
We derive a sum rule which establishes a linear relation between a particle's anomalous magnetic moment and a quantity connected to the photoabsorption cross-section. This quantity cannot be measured directly. However, it can be computed within a given theory. As an example, we demonstrate validity of the sum rule in QED at tree level--the renowned Schwinger's correction to the anomalous magnetic moment is readily reproduced. In the case of the strong interactions, we also consider the calculation of the nucleon magnetic moment within chiral theories.
We present a relativistic dynamical model of pion photoproduction on the nucleon in the resonance region. It offers several advances over the existing approaches. The model is obtained by extending our $πN$-scattering description to the electromagnetic channels. The resulting photopion amplitude is thus unitary in the $πN$, $\ga N$ channel space, Watson's theorem is exactly satisfied. At this stage we have included the pion, nucleon, $\De(1232)$-resonance degrees of freedom. The $ρ$ and $ω$ meson exchanges are also included, but play a minor role in the considered energy domain (up to $\sqrt{s}=1.5$ GeV). In this energy range the model provides a good description of all the important multipoles. We have allowed for only two free parameters -- the photocouplings of the $Δ$-resonance. These couplings are adjusted to reproduce the strength of corresponding resonant-multipoles $M_{1+}$ and $E_{1+}$ at the resonance position.
Considering two-body integral equations we show how they can be dimensionally reduced by integrating exactly over the azimuthal angle of the intermediate momentum. Numerical solution of the resulting equation is feasible without employing a partial-wave expansion. We illustrate this procedure for the Bethe-Salpeter equation for pion-nucleon scattering and give explicit details for the one-nucleon-exchange term in the potential. Finally, we show how this method can be applied to pion photoproduction from the nucleon with $πN$ rescattering being treated so as to maintain unitarity to first order in the electromagnetic coupling. The procedure for removing the azimuthal angle dependence becomes increasingly complex as the spin of the particles involved increases.
Model-independent effects of $Δ$(1232) excitation on nucleon polarizabilities are computed in a Lorentz-invariant fashion. We find a large effect of relative order $(M_Δ- M)/M$ in some of the spin polarizabilities, with the backward spin polarizability receiving the largest contribution. Similar subleading effects are found to be important in the fourth-order spin-independent polarizabilities $α_{Eν}$, $\al_{E2}$, $\be_{Mν}$, and $\be_{M 2}$. Combining our results with those for the model-independent effects of pion loops we obtain predictions for spin and fourth-order polarizabilities which compare favorably with the results of a recent dispersion-relation analysis of data.
We formulate a new power-counting scheme for a chiral effective field theory of nucleons, pions, and Deltas. This extends chiral perturbation theory into the Delta-resonance region. We calculate nucleon Compton scattering up to next-to-leading order in this theory. The resultant description of existing $γ$p cross section data is very good for photon energies up to about 300 MeV. We also find reasonable numbers for the spin-independent polarizabilities $α_p$ and $β_p$.
I discuss the problem of consistent interactions of higher-spin fields and its relevance to resonance physics.
The nearest-neighbor mass-spacing distribution of the meson and baryon spectrum (up to 2.5 GeV) is described by the Wigner surmise corresponding to the statistics of the Gaussian orthogonal ensemble of random matrix theory. This can be viewed as a manifestation of quantum chaos in hadrons.