Excited Nucleon Research Articles

Two-photon physics

It is reviewed how Compton scattering sum rules relate low-energy nucleon structure quantities to the nucleon excitation spectrum. In particular, the GDH sum rule and recently proposed extensions of it will be discussed. These extensions are sometimes more calculationally robust, which may be an advantage when estimating the chiral extrapolations of lattice QCD results, such as for anomalous magnetic moments. Subsequently, new developments in our description of the nucleon excitation spectrum will be discussed, in particular a recently developed chiral effective field theory framework for the Δ(1232)-resonance region. Within this framework, we discuss results on N and Δ masses, the γNΔ transition and the Δ magnetic dipole moment.

Alternative Experimental Evidence for Chiral Restoration in Excited Baryons

It has been suggested that chiral symmetry is approximately restored in excited hadrons at zero temperature and density (effective symmetry restoration). Using very general chiral symmetry arguments, it is shown that those excited nucleons that are assumed from the spectroscopic patterns to be in approximate chiral multiplets must only weakly decay into the Npi channel, (fN*Npi}/fNNpi)(2) << 1. However, those baryons that have no chiral partner must decay strongly with a decay constant comparable with fNNpi. It turns out that for all those well-established excited nucleons which can be classified into chiral doublets the ratio is (fN*Npi}/fNNpi)(2) approximately 0.1 or much smaller for the high-spin states. In contrast, the only well-established excited nucleon for which the chiral partner cannot be identified from the spectroscopic data, N(1520), has a decay constant into the Npi channel that is comparable with fNNpi.

Q2-evolution of nucleon-to-resonance transition form factors in a QCD-inspired vector-meson-dominance model

We adopt the vector-meson-dominance approach to investigate ${Q}^{2}$-evolution of $NR$-transition form factors ($N$ denotes nucleon and $R$ an excited resonance) in the first and second resonance regions. The developed model is based upon conventional $\ensuremath{\gamma}NR$-interaction Lagrangians, introducing three form factors for spin-$3/2$ resonances and two form factors for spin-$1/2$ nucleon excitations. Lagrangian form factors are expressed as dispersionlike expansions with four or five poles corresponding to the lowest excitations of the mesons $\ensuremath{\rho}(770)$ and $\ensuremath{\omega}(782)$. Correct high-${Q}^{2}$ form-factor behavior predicted by perturbative QCD is due to phenomenological logarithmic renormalization of electromagnetic coupling constants and linear superconvergence relations between the parameters of the meson spectrum. The model is found to be in good agreement with all the experimental data on ${Q}^{2}$-dependence of the transitions $N\ensuremath{\Delta}(1232)$, $NN(1440)$, $NN(1520)$, $NN(1535)$. We present fit results and model predictions for high-energy experiments proposed by JLab. Besides, we make special emphasis on the transition to perturbative domain of $N\ensuremath{\Delta}(1232)$ form factors.

Perturbative role in the inelastic electron scattering from 29Si

Inelastic electron scattering form factors away from the closed sd -shell are investigated. This investigation covers the low-lying states in 29Si , which is considered according to the many-particle configuration mixing shell model as the 16O core, plus thirteen nucleons distributed over the entire sd -shell orbits. The investigation concentrates on the perturbative role of the core, which is called core polarization effects, on the inelastic electron scattering form factors. Core polarization effects are taken into consideration through the excitation of nucleons from 1s and 1p core orbits, and also from the 2s -1d valence orbits into higher shells, with 2ℏω excitations. Core polarization matrix elements are calculated with the M3Y effective interaction. For the sd -shell model space, a new Hamiltonian, based on a renormalized G -matrix, USDB, is used. All calculations are performed without adjusting any parameters.

Baryons in AdS/QCD

Abstract We construct a holographic model for baryons in the context of AdS/QCD and study the spin- 1 2 nucleon spectra and its couplings to mesons, taking full account of the effects from the chiral symmetry breaking. A pair of 5D spinors is introduced to represent both left and right chiralities. Our model contains two adjustable parameters, the infrared cutoff and the Yukawa coupling of bulk spinors to bulk scalars, corresponding to the order parameter of chiral symmetry. Taking the lowest-lying nucleon mass as an input, we calculate the mass spectrum of excited nucleons and the nucleon couplings to pions. The excited nucleons show a parity-doubling pattern with smaller pion–nucleon couplings.

Unitarity cutting rules for the nucleus excitation and topological cross sections in hard production off nuclei from nonlineark⊥-factorization

At the partonic level, a typical final state in small-x deep inelastic scattering off nuclei and hard proton-nucleus collisions can be characterized by the multiplicity of color-excited nucleons. Within the Reggeon field theory, each color-excited nucleon is associated with the unitarity cut of the Pomeron exchanged between the projectile and nucleus. In this communication we derive the unitarity rules for the multiplicity of excited nucleons, alias cut pomerons, alias topological cross sections, for typical hard dijet production processes. We demonstrate how the coupled-channel non-Abelian intranuclear evolution of color dipoles, inherent to pQCD, gives rise to the Reggeon field theory diagrams for final states in terms of the uncut, and two kinds of cut, Pomerons. Upon the proper identification of the uncut and cut-Pomeron exchanges, the topological cross sections for dijet production follow in a straightforward way from the earlier derived nonlinear k{sub perpendicular} factorization quadratures for the inclusive dijet cross sections. The concept of a coherent (collective) nuclear glue proves extremely useful for the formulation of the Reggeon field theory vertices of multi-Pomeron--cut and uncut--couplings to particles and between themselves. A departure of our unitarity cutting rules from the ones suggested by the pre-QCD Abramovsky-Kancheli-Gribov rules, stems from the coupled-channelmore » features of non-Abelian intranuclear pQCD. We propose a multiplicity resummation as a tool for the isolation of topological cross sections for single-jet production.« less

Two-photon physics

It is reviewed how Compton scattering sum rules relate low-energy nucleon structure quantities to the nucleon excitation spectrum. In particular, the GDH sum rule and recently proposed extensions of it will be discussed. These extensions are sometimes more calculationally robust, which may be an advantage when estimating the chiral extrapolations of lattice QCD results, such as for anomalous magnetic moments. Subsequently, new developments in our description of the nucleon excitation spectrum will be discussed, in particular a recently developed chiral effective field theory framework for the $\Delta(1232)$-resonance region. Within this framework, we discuss results on $N$ and $\Delta$ masses, the $\gamma N \Delta$ transition and the $\Delta$ magnetic dipole moment.

Nucleon-sigmacoupling constant in QCD sum rules

The external-field QCD sum rules method is used to evaluate the coupling constant of the light isoscalar-scalar meson (``\ensuremath{\sigma}'' or \ensuremath{\varepsilon}) to the nucleon. The contributions that come from the excited nucleon states and the response of the continuum threshold to the external field are calculated. The obtained value of the coupling constant is compatible with the large value required in one-boson exchange potential models of the two-nucleon interaction.

Spectral analysis of excited nucleons in lattice QCD with maximum entropy method

We study the mass spectra of excited baryons with the use of the lattice QCD simulations. We focus our attention on the problem of the level ordering between the positive-parity excited state N′(1440) (the Roper resonance) and the negative-parity excited state N∗(1535). Nearly perfect parity projection is accomplished by combining the quark propagators with periodic and anti-periodic boundary conditions in the temporal direction. Then we extract the spectral functions from the lattice data by utilizing the maximum entropy method. We observe that the masses of the N′ and N∗ states are close for wide range of the quark masses (Mπ=0.61–1.22 GeV) and for the physical point after the extrapolation in terms of the quark mass. The latter aspect is in contrast to the phenomenological prediction of the phenomenological quark models. The role of the Wilson doublers in the baryonic spectral functions is also studied.

Solution of the Skyrme–Hartree–Fock–Bogolyubov equations in the Cartesian deformed harmonic-oscillator basis. (V) HFODD(v2.08k)

Solution of the Skyrme–Hartree–Fock–Bogolyubov equations in the Cartesian deformed harmonic-oscillator basis. (V) HFODD(v2.08k)

Experimental study of nuclear structure ofMo91at high spin

The high spin structure of $^{91}\mathrm{Mo}$ with neutron and proton number close to magicity has been investigated using the $^{63}\mathrm{Cu}(^{31}\mathrm{P},\mathit{xnyp})$ reaction at an incident beam energy of $125\phantom{\rule{0.3em}{0ex}}\text{MeV}$. The level at ${E}_{x}=3811\phantom{\rule{0.3em}{0ex}}\text{keV}$ has been assigned a negative parity based on our polarization measurements. In view of this change, the observed level structure for $^{91}\mathrm{Mo}$ so far does not have any negative parity sequence. More than 30 new transitions were observed and placed in the decay scheme, which has been extended up to spin of $J\ensuremath{\sim}39∕2\ensuremath{\hbar}$ and an excitation energy of ${E}_{x}\ensuremath{\sim}10\phantom{\rule{0.3em}{0ex}}\text{MeV}$. The energy spectrum bears signature of core breaking viz. presence of high energy $({E}_{\ensuremath{\gamma}}\ensuremath{\sim}2\phantom{\rule{0.3em}{0ex}}\text{MeV})$ $\ensuremath{\gamma}$ rays originating due to excitation of nucleons across the shell gaps. The proposed level scheme is compared with large basis spherical shell model calculations. Excitation of protons across $Z=38$ shell dominates the level structure up to moderate spins i.e., $J\ensuremath{\sim}27∕2\ensuremath{\hbar}$. Configurations arising due to excitation of a single $\ensuremath{\nu}{g}_{9∕2}$ across $N=50$ shell gap dominate the higher angular momentum $J\ensuremath{\geqslant}35∕2\ensuremath{\hbar}$ states.

Solution of the Skyrme–Hartree–Fock–Bogolyubov equations in the Cartesian deformed harmonic-oscillator basis. (IV) HFODD (v2.08i): a new version of the program

We describe the new version (v2.07f) of the code HFODD which solves the nuclear Skyrme-Hartree-Fock or Skyrme-Hartree-Fock-Bogolyubov problem by using the Cartesian deformed harmonic-oscillator basis. In the new version, all symmetries can be broken, which allows for calculations with angular frequency and angular momentum tilted with respect to the mass distribution. The new version contains an interface to the LAPACK subroutine ZHPEVX.

Formula omitted] pairing correlation in symmetric nuclear matter with Debye screening effects

The 1S0 pairing of symmetric nuclear matter is discussed in the framework of relativistic nuclear theory with Dyson–Schwinger equations (DSEs). The in-medium nucleon and meson propagators are treated in a more self-consistent way through meson polarizations. The screening effects on mesons due to in-medium nucleon excitation are found to reduce the 1S0 pairing gap and shift remarkably the gap peak to low density region.

Shell model study on E2 effective charges in light neutron-rich nuclei

Theoretical values of quadrupole moments of neutron-rich nuclei are larger than experimental values if we use E2 effective charges of stable nuclei. We consider that the nucleon excitations to outside model space decrease with neutron excess. Using the shell model, we investigate quadrupole moments and effective charges in the light mass region, from stable nuclei to neutron-rich nuclei.

Flavor independent systematics of excited baryons and intra-band transition

Transitions among excited nucleons are studied within a non-relativistic quark model with a deformed harmonic oscillator potential. The transition amplitudes are factorized into the l-th moment and a geometrical factor. This fact leads to an analogous result to the “Alaga-rule” for baryons.

The helicity structure of the nucleon excitation spectrum

The helicity dependence of the total and partial photoproduction reaction channels has been measured for the first time in an energy range 200 MeV ≤ ν ≤ 800 MeV at MAMI using circularly polarized photons together with a longitudinally polarized proton target and a 4π detector system. The reaction γp → p π 0 is predominantly sensitive to the D 13(1520) resonance and the data are used to adjust its parameters.

Excited baryons from the FLIC fermion action

Masses of positive and negative parity excited nucleons and hyperons are calculated in quenched lattice QCD using anO( a 2) improved gluon action and a fat-link clover fermion action in which only the irrelevant operators are constructed with fat links. The results are in agreement with earlier N * simulations with improved actions, and exhibit a clear mass splitting between the nucleon and its parity partner, as well as a small mass splitting between the two low-lying J P =½ − N * states. Study of different A interpolating fields suggests a similar splitting between the lowest two½ −Λ * states, although the empirical mass suppression of theΛ * (1405) is not seen.

Excitations of the nucleon with dynamical fermions

We measure the spectrum of low-lying nucleon resonances using Bayesian fitting methods. We compare the masses obtained in the quenched approximation to those obtained with two flavours of dynamical ferinions at a matched lattice spacing. At the pion masses employed in our simulations, we find that the mass of the first positive-parity nucleon excitation is always greater than that of the parity partner of the nucleon.

Elastic magnetic electron scattering from 17O, 25Mg and 27Al

Elastic magnetic electron scattering form factors for the 5/2 + state of the stable odd- A nuclei, in the sd-shell ( 17O, 25Mg and 27Al) have been investigated in terms of the configuration mixing shell model, including core polarization effects. Core polarization effects are taken into consideration through excitations of nucleons from the 1s and 1p core orbits and also from the valence 2s1d shell orbits into higher shells, with 2ℏ ω excitations. Core polarization effects improve the agreement with the data in the region of momentum transfer where M1 and M3 multipoles are important. The higher momentum transfer data, where the M5 contribution is the dominant contribution, are less affected by core polarization. In this region, the data are well described if the valence orbital is given a size, which is smaller than that required, by the root mean square charge radius.

Lorentz covariant orbital-spin scheme for the effectiveN*NMcouplings

For excited nucleon states ${N}^{*}$ of arbitrary spin coupling to nucleon (N) and meson $(M),$ we propose a Lorentz covariant orbital-spin $(L\ensuremath{-}S)$ scheme for the effective ${N}^{*}\mathrm{NM}$ couplings. To be used for the partial wave analysis of various ${N}^{*}$ production and decay processes, it combines merits of two conventional schemes, i.e., covariant effective Lagrangian approach and multipole analysis with amplitudes expanded according to angular momentum L. As examples, explicit formulas are given for ${N}^{*}\ensuremath{\rightarrow}N\ensuremath{\pi},$ ${N}^{*}\ensuremath{\rightarrow}N\ensuremath{\omega},$ and $\stackrel{\ensuremath{\rightarrow}}{\ensuremath{\psi}}{N}^{*}\overline{N}$ processes which are under current experimental studies.