Recent developments in the relativistic nuclear field theory (RNFT) are reviewed. Based on the covariant meson-nucleon Lagrangian of quantum hadrodynamics, RNFT connects consistently the high-energy scale of heavy mesons, medium-energy range of pion and the low-energy domain of emergent collective vibrations (phonons) in a parameter-free way. Mesons and phonons build up the effective interaction in various channels, in particular, the phonon-exchange part takes care of the retardation effects, which are of great importance for the fragmentation of single-particle states, spreading of collective giant resonances and soft modes, quenching and beta-decay rates with significant consequences for astrophysics and for the theory of weak processes in nuclei. As examples, the recently discovered impact of isospin dynamics on the nuclear shell structure and the isospin-flip pairing vibrations are discussed.
Vector meson ($V = ω, ϕ$) production in near-threshold elementary nucleon-nucleon collisions $pp\to ppV$, $pn\to pnV$ and $pn\to dV$ is studied within an effective meson-nucleon theory. It is shown that a set of effective parameters can be established to describe fairly well the available experimental data of angular distributions and the energy dependence of the total cross sections without explicit implementation of the Okubo-Zweig-Iisuka rule violation. Isospin effects are considered in detail and compared with experimental data whenever available.
Pion-meson-exchange nucleon-nucleon potentials are derived for two nucleons in the intermediate states. The mesons we include are (i) pseudoscalar mesons: $π, η, η'$; (ii) vector mesons: $ρ, ω, ϕ$; (iii) scalar mesons: $a_{0}(980), \varepsilon(760), f_{0}(975)$; and (iv) the $J=0$ contribution from the Pomeron. Strong dynamical pair suppression is assumed, and at the nucleon-nucleon-meson vertices Gaussian form factors are incorporated into the relativistic two-body framework using a dispersion representation for the pion- and meson-exchange amplitudes. The Fourier transformations are performe…
Traditionally isospin violation (charge symmetry breaking and charge dependence) has been modeled by the exchange of mesons between two nucleons. Sources of isospin violation in these models include meson mass differences, meson mixing (of pseudoscalar, vector, and axial-vector mesons) and isospin violating couplings of mesons with nucleons. I will review the calculated results of this theoretical approach and the NN data on isospin violation.
Two-meson-exchange nucleon-nucleon potentials are derived where either one or both nucleons contains a pair vertex. Physically, the meson-pair vertices are meant to describe in an effective way (part of) the effects of heavy-meson exchange and meson-nucleon resonances. {}From the point of view of ``duality,'' these two kinds of contribution are roughly equivalent. The various possibilities for meson pairs coupling to the nucleon are inspired by the chiral-invariant phenomenological Lagrangians that have appeared in the literature. The coupling constants are fixed using the linear $σ$ model. We…
The properties of vector mesons in nuclear matter are discussed. I examine the constraints imposed by elementary processes on the widths of $ρ$ and $ω$ mesons in nuclear matter. Furthermore, results for the $ρ$- and $ω$-nucleon scattering amplitudes obtained by fitting meson-nucleon scattering data in a coupled-channel approach are presented.
Through a quantitative comparative study of the properties of deuteron and nucleon-nucleon interaction with chiral quark model and quark delocalization color screening model. We show that the $σ$-meson exchange used in the chiral quark model can be replaced by quark delocalization and color screening mechanism.
We employ meson-nucleon scattering data to deduce the properties of the low-mass vector mesons in nuclear matter, and present results for the $ρ$ and $ω$ in-medium spectral functions. The corresponding thermal emission rate for lepton pairs is also discussed.
The momentum distributions of pions and $ρ$-mesons in the nucleon are extracted from experiment. Perspectives of the quark microscopic theory of mesonic cloud are outlined.
The production of $η'$ mesons in the reactions $pp\to ppη'$ and $pn\to pnη' $ at threshold-near energies is analyzed within a covariant effective meson-nucleon theory. The description of cross section and angular distributions of the available data in this kinematical region in the $pp$ channel is accomplished by including meson currents and nucleon currents with the resonances $S_{11}(1650)$, $P_{11}(1710)$ and $P_{13}(1720)$. Predictions for the $pn$ channel are given. The di-electron production from subsequent $η'$ Dalitz decay $η' \to γγ^* \toγe^+e^-$ is also calculated and numerical resul…
Using an effective hadronic Lagrangian with physical hadron masses and coupling constants determined either empirically or from SU(4) flavor symmetry, we study the production cross sections of charm mesons from pion and rho meson interactions with nucleons. With a cutoff parameter of 1 GeV at interaction vertices as usually used in studying the cross sections for $J/ψ$ absorption and charm meson scattering by hadrons, we find that the cross sections for charm meson production have values of a few tenth of mb and are dominated by the s channel nucleon pole diagram. Relevance of these reactions …
Developed earlier concept of quasielastic knock out of pions from nucleons by high-energy electrons is propounded as a tool for checking microscopical model ($^3P_0$ - fluctuation) for decay of N to different channels $π+B$ and Preparata model of nucleus structure.
We establish the model-independent spin structure of the matrix elements for the near-threshold scalar meson production in $pp-$ and $np$-collisions, when the final particles are emitted in S-state. Polarization phenomena are derived in a general form. The properties of the $t-$channel dynamics, which is based on different meson exchanges, are studied in terms of the $s-$channel parametrization of the matrix element. The prediction of a 'realistic' model, based on $π+σ$-exchanges are also presented.
In a recent paper [Phys.\ Rev.\ C{\bf 49}, 2142 (1994)], Haberzettl presented cluster N-body equations for arbitrarily large systems of nucleons and mesons. Application to the three-nucleon system is claimed to yield a new kind of three-nucleon force. We demonstrate that these three-nucleon equations contain double counting.
The meson photoproductions off nucleons in the chiral quark model are described. The role of the S-wave resonances in the second resonance region is discussed, and it is particularly important for the Kaon, $η$ and $η'$ photoproductions.
The Schwinger-Dyson equations for the nucleon and meson propagators are solved self-consistently in an approximation that goes beyond the Hartree-Fock approximation. The traditional approach consists in solving the nucleon Schwinger-Dyson equation with bare meson propagators and bare meson-nucleon vertices; the corrections to the meson propagators are calculated using the bare nucleon propagator and bare nucleon-meson vertices. It is known that such an approximation scheme produces the appearance of ghost poles in the propagators. In this paper the coupled system of Schwinger-Dyson equations f…
The quark-meson coupling model for a nucleon is considered. The model describes a nucleon as an MIT bag, in which quarks are coupled to scalar and vector mesons. A set of coupled equations for the quark and the meson fields are obtained and are solved in a self-consistent way. It is shown that the mass of a nucleon as a dressed MIT bag interacting with sigma- and omega-meson fields significantly differs from the mass of a free MIT bag. A few sets of model parameters are obtained so that the mass of a dressed MIT bag becomes the nucleon mass. The results of our calculations imply that the self-…
Nucleon and meson effective masses in the nonlinear Relativistic Mean - Field Theory (RMF) introducing a nonlinear omega - rho and sigma coupling motivated by the Quark Meson Coupling model (QMC) is explored. It is shown that, in contrast to the usual Walecka model, not only the effective nucleon mass m_{eff,N} but also the effective sigma, rho meson masses (m_{eff, sigma}, m_{eff, rho}) and the effective omega meson mass m_{eff, omega} are nucleon density dependent.
A model is developed for the hadronic and electromagnetic interaction in the two-nucleon system above pion threshold in the framework of meson, nucleon and $Δ$ degrees of freedom. It is based on time-ordered perturbation theory and includes full meson retardation in potentials and exchange currents as well as loop contributions to the nucleonic one-body current. Results for $NN$ scattering and deuteron photodisintegration are presented.
The weak decay as well as the strong nucleon coupling constants of the isoscalar axial vector meson $f_1 (1285)$ are estimated within the octet quark model. It is shown that the empirical value for the coupling of $\overline{s}s$ quarkonium to nucleons can be understood by attributing the $f_1(1285)$-nucleon coupling to the $a_0πN$ triangle diagram.
Thermal photon emission rates due to meson-nucleon interactions have been evaluated. An exhaustive set of reactions involving p(\bar p), n(\bar n), rho, omega, a_1, pi and eta is seen to provide a sizeable contribution to the emission rate from hot hadronic matter. Contributions from baryonic resonances are found to be negligibly small.
We compute the binding energies, radii, and densities for selected medium-mass nuclei within coupled-cluster theory and employ the "bare" chiral nucleon-nucleon interaction at order N3LO. We find rather well-converged results in model spaces consisting of 15 oscillator shells, and the doubly magic nuclei 40Ca, 48Ca, and the exotic 48Ni are underbound by about 1 MeV per nucleon within the CCSD approximation. The binding-energy difference between the mirror nuclei 48Ca and 48Ni is close to theoretical mass table evaluations. Our computation of the one-body density matrices and the corresponding …
We obtain the RPA summed one-meson exchange potential between nucleons in symmetric nuclear matter at zero temperature, from a model which includes $ρ$, $σ$, $ω$ and $π$ mesons. The behavior of rho mesons inside the medium is first discussed using different schemes to extract a finite contribution from the vacuum polarization. These schemes give qualitatively different results for the in-medium rho mass. The results are discussed in connection with the non-renormalizability of the model. We next study the modified potential as density increases. In the intermediate distance range, it is qualit…
We perform ab initio calculations for nuclei around ^{16}O, ^{40}Ca, and ^{56}Ni using realistic nucleon-nucleon forces. In particular, ^{56}Ni is computed as the heaviest nucleus in this kind of ab initio calculation. Ground-state and single-particle energies including three-body-cluster effects are obtained within the framework of the unitary-model-operator approach. It is shown that the CD-Bonn nucleon-nucleon potential gives quite good results close to the experimental values for all nuclei in the present work.
Magnetic moment and radius of the nucleon are calculated in a nonlocal extension of the chiral linear σ-model. Properties of the nonlocal model under the vector and axial transformations are considered. The conserved electromagnetic and vector currents, and partially conserved axial vector current are obtained. In the calculation of the nucleon electromagnetic vertex the π- and σ-loop diagrams are included. Contribution from vector mesons is added in the vector meson dominance model with a gauge-invariant photon-meson coupling. The nonlocality parameter associated with the πN interaction is fi…
The temperature and density dependence of the meson-nucleon vertex form factors is studied in the framework of thermofield dynamics. Results are obtained for two rather different nucleon-nucleon potentials: the usual Bonn potential and the variation with a softer $πNN$ form factor, due to Holinde and Thomas. In general, the results show only a modest degree of sensitivity to the choice of interaction.
Starting with a quark model of nucleon structure in which the valence quarks are strongly correlated within a nucleon, the light nuclei are constructed by assuming similar correlations of the quarks of neighboring nucleons. Applying the model to larger collections of nucleons reveals the emergence of the face-centered cubic (FCC) symmetry at the nuclear level. Nuclei with closed shells possess octahedral symmetry. Binding of nucleons are provided by quark loops formed by three and four nucleon correlations. Quark loops are responsible for formation of exotic (borromean) nuclei, as well. The mo…
We show that the contact couplings of neutral pseudoscalar and axial mesons to the nucleon are proportional to $Δs$, the fraction of nucleon spin carried by the strange quark sea, and thus are strongly suppressed relative to the couplings of charged mesons to the nucleon. On the other side, recent high accuracy data on $η$ photoproduction at threshold reveal the need for non--negligibile $ηNN$ vertices, while fitting $\bar N N$ phase shifts by means of effective meson exchange potentials requires a substantial presence of $f_1$ mesons there. We here advocate the idea to attribute the couplings…
The possible role played by vector mesons in inclusive deep inelastic lepton-nucleon scattering is investigated. In the context of the convolution model, we calculate self-consistently the scaling contribution to the nucleon structure function using the formalism of time-ordered perturbation theory in the infinite momentum frame. Our results indicate potentially significant effects only when the vector meson---nucleon form factor is very hard. Agreement with the experimental antiquark distributions, however, requires relatively soft form factors for the $πN$, $ρN$ and $ωN$ vertices.
At zero-density, meson-nucleon vertex corrections to vacuum polarization are studied in the $σ-ω$ model with the cutoff. It is shown that the properties of the vertex corrections to vacuum polarization are somewhat different from those described by the ordinary renormalization procedures when the cutoff is small (< 5 GeV). The low-energy effective Lagrangian is constructed in the framework of the renormalization group method. The weak tensor and derivative couplings of meson-nucleon interactions may be needed in the low-energy effective theory of mesons and nucleons.
The quark-meson coupling model due to Guichon is formulated on the basis of the independent quark model of the nucleon proposed by Bogoliubov and is applied to a phenomenological description of symmetric nuclear matter. The model predicts, at saturation density, the compressibility $K=249$ MeV and the quark effective mass $m_q^*= 249.1$ MeV, {the effective nucleon mass being $M^*=747.3$ MeV}. The predicted nucleon mass radius is $r=0.93$ fm.
We considered the quark-meson coupling model for a nucleon. The model describes a nucleon as an MIT bag, in which quarks are coupled to the scalar and the vector mesons. A set of coupled equations for the quark and the meson fields are obtained and are solved in a self-consistent manner. We show that the mass of a dressed MIT bag interacting with $σ$- and $ω$-meson fields differs considerably from the mass of the free MIT bag. The effects of the density-dependent bag constant are investigated. The results of our calculations imply that the self-energy of the bag in the quark-meson coupling mod…
A brief overview on recent model calculations for near threshold pion production in nucleon-nucleon collisions is given. Results from our own investigations of the reactions $pp \rightarrow ppπ^0$ and $pn \rightarrow dπ^0$ are presented. Direct production, heavy meson exchange and pion rescattering are taken into account. For the latter a T-matrix obtained from a microscopic model of the $πN$ interaction is employed. We obtain a significant contribution from rescattering, but not enough to describe the data for $pp \to ppπ^0$. The missing production rate can be provided by heavy meson exchange…
We develop a model to evaluate the formation rate of the heavy mesic nuclei in the two nucleon pick-up reactions, and apply it to the $^6$Li target cases for the formation of heavy meson-$α$ bound states, as examples. The existence of the quasi-deuteron in the target nucleus is assumed in this model. It is found that the mesic nuclei formation in the recoilless kinematics is possible even for heavier mesons than nucleon in the two nucleon pick-up reactions. We find the formation rate of the meson-$α$ bound states can be around half of the elementary cross sections at the recoilless kinematics …
We present the first study of pairing in nuclei including three-nucleon forces. We perform systematic calculations of the odd-even mass staggering generated using a microscopic pairing interaction at first order in chiral low-momentum interactions. Significant repulsive contributions from the leading chiral three-nucleon forces are found. Two- and three-nucleon interactions combined account for approximately 70% of the experimental pairing gaps, which leaves room for self-energy and induced interaction effects that are expected to be overall attractive in nuclei.
We investigate a relativistic quantum field theory in the particle representation using a non-perturbative variational technique. The theory is that of two massive scalar particles, `nucleons' and `mesons', interacting via a Yukawa coupling. We calculate the general Euclidean Green function involving two external nucleons and an arbitrary number of external mesons in the quenched approximation for the nucleons. The non-perturbative renormalization and truncation is done in a consistent manner and results in the same variational functional independent of the number of external mesons. We check …
We propose a simple meson-exchange model of the photoproduction of $ρ$- and $ω$-mesons off protons near threshold ($E_γ$ less than 2 GeV). This model provides a good description of the available data and implies a large $ρ$-nucleon interaction in the scalar channel ($σ$-exchange). We use this phenomenological interaction to estimate the leading contribution to the self-energy of $ρ$-mesons in matter. The implications of our calculation for experimental studies of the $ρ$-meson mass in nuclei are discussed.
A microscopic calculation of near-threshold eta-meson production in the reaction NN -> NNeta is presented. It is assumed that the eta meson is produced via direct emission and via elementary rescattering processes MN -> etaN of various mesons M = pi, rho, etc. As a novel feature the amplitudes for the latter production mechanism are taken from a multi-channel meson-exchange model of the pi N system developed by the Juelich group which contains explicitly the channels pi N, rho N, eta N, sigma N, and pi Delta. Furthermore, effects of the NN interaction in the final as well as in the initial sta…
Within the relativistic mean-field approach, we extend the Miyazaki model, where the NN$σ$ and NN$ω$ interactions are modified to suppress the couplings between positive- and negative-energy states of a nucleon in matter. Assuming appropriate density-dependence of the meson-nucleon couplings, we study nuclear matter and finite nuclei. The model can reproduce the observed properties of $^{16}$O and $^{40}$Ca well. We also examine if the model is natural.
Talk given at the "International Workshop on (e,e'p) Processes", July 2-6, 2017, Bled, Slovenia. Various many-body calculations are compared and the results provided by the Normalization Conserving Linked Cluster Expansion with realistic NN interactions are reviewed and analyzed in detail, as far as ground state energies, momentum distributions and spectral functions of few-nucleon system and complex nuclei are concerned.
The role of the nucleon recoil corrections in low--energy meson--nucleus interactions is examined. We demonstrate explicitly when calculations within the static approximation are justified and when the recoil terms need to be kept explictly in the propagators, depending on whether the S-wave two--nucleon intermediate state is Pauli blocked or not, while the meson is in flight. While the effect is studied in detail for $πd$ scattering only, a large class of other reactions is discussed for which the findings are relevant as well.
This report summarizes our understanding of antikaon-nucleon interactions and reviews the present theoretical situation in the quest for quasibound antikaon-nuclear systems.
We report variational Monte Carlo calculations of single-nucleon momentum distributions for A <= 12 nuclei and nucleon-pair and nucleon-cluster momentum distributions for A <= 8. The wave functions have been generated for a Hamiltonian containing the Argonne v18 two-nucleon and Urbana X three-nucleon potentials. The single-nucleon and nucleon-pair momentum distributions exhibit universal features attributable to the one-pion-exchange tensor interaction. The single-nucleon distributions are broken down into proton and neutron components and spin-up and spin-down components where appropriate. Th…
Relationships between mass intervals for free hadrons and in nuclei are studied in two theoretical approaches inspired by QCD: naive quark model and skyrmion model, taking one example each from mesons and baryons, that of pi-rho splitting in mesons, and nucleon-Delta splitting in baryons. Possible deconfinement effects in nuclei are examined.
We use the meson cloud model, including the kaon and the $K^*$ contributions, to estimate the electric and magnetic strange form factors of the nucleon. We compare our results with the recent measurements of the strange quark contribution to parity-violating asymmetries in the forward G0 electron-proton scattering experiment. We conclude that it is not possible to explain the data using this model.
The ground-state energy of the doubly magic nuclei 4He and 16O has been calculated within the framework of the Goldstone expansion starting from modern nucleon-nucleon potentials. A low-momentum potential V-low-k has been derived from the bare potential by integrating out its high-momentum components beyond a cutoff Lambda. We have employed a simple criterion to relate this cutoff momentum to a boundary condition for the two-nucleon model space spanned by a harmonic-oscillator basis. Convergence of the results has been obtained with a limited number of oscillator quanta.
We tested several choices of the in-medium value of the Bjorken scaling variable assuming the nucleon structure function in nucleus to be the same as that of free nucleon. The results unambiguously show that it is different.
Modifications to the nucleon electric polarizability induced by pion and sigma exchange in the q-q potentials are studied by means of sum rule techniques within a non-relativistic quark model. Contributions from meson exchange interactions are found to be small and in general reduce the quark core polarizability for a number of hybrid and one-boson-exchange q-q models. These results can be explained by the constraints that the baryonic spectrum impose on the short range behavior of the mesonic interactions.
It is shown that most of the models for analyzing meson-baryon reactions in the nucleon resonance region can be derived from a Hamiltonian formulation of the problem. An extension of the coupled-channel approach to include $ππN$ channel is briefly described and some preliminary results for the $N^*(1535)$ excitation are presented.
Inclusive quasi-elastic electron scattering off nuclei is investigated at high momentum transfer (Q^2>1 (GeV/c)^2) and x>1 adopting a consistent treatment of nucleon-nucleon correlations in initial and final states. It is shown that in case of light as well as complex nuclei the inclusive cross section at 1.3<x<2 is dominated by the absorption of the virtual photon on a pair of correlated nucleons and by their elastic rescattering in the continuum, whereas at x>2 it is governed by the rescattering of the outgoing off-mass-shell nucleon in the complex optical potential generated by the ground s…
We have studied the low lying magnetic spectra of 12C, 16O, 40Ca, 48Ca and 208Pb nuclei within the Random Phase Approximation (RPA) theory, finding that the description of low-lying magnetic states of doubly-closed-shell nuclei imposes severe constraints on the spin and tensor terms of the nucleon-nucleon effective interaction. We have first made an investigation by using four phenomenological effective interactions and we have obtained good agreement with the experimental magnetic spectra, and, to a lesser extent, with the electron scattering responses. Then we have made self-consistent RPA c…
We extend the polaron variational treatment previously developed for the propagator to the case where one nucleon and $n$ external mesons are present. Using the particle representation of the scalar Wick-Cutkosky model this is done in lowest order of an expansion of the exact action around a retarded quadratic trial action. In particular, we evaluate the form factor for scattering of mesons from the scalar nucleon and determine the radius of the dressed particle. After analytic continuation to Minkowski space we study elastic meson-nucleon scattering both analytically and numerically near thre…
We generalize the Bogoliubov quark-meson coupling model to also include hyperons. The hyperon-$σ$-meson couplings are fixed by the model and the hyperon-$ω$-meson couplings are fitted to the hypreon potentials in symmetric nuclear matter. The present model predicts neutron stars with masses above 2$M_\odot$ and the radius of a 1.4$M_\odot$ star equal to 13.83 km.
The general gauge-invariant photoproduction formalism given by Haberzettl is applied to kaon photoproduction off the nucleon at the tree level, with form factors describing composite nucleons. Numerical results show that this gauge-invariance procedure, when compared to Ohta's, leads to a much improved description of experimental data. Predictions for the new Bonn SAPHIR data for $p(γ,K^+)Λ$ are given.
The pseudoscalar and pseudovector $ηN$ coupling constants are calculated from an effective vertex associated with the $a_0(980)πN$ triangle diagram. The predicted values are in agreement with the ones concluded from fitting $η$ photoproduction amplitudes. In this context we stress the importance of the properties of the scalar meson octet for $η$ meson physics.
The 1S0 pairing gap in isospin-symmetric nuclear matter and finite nuclei is investigated using the chiral nucleon-nucleon potential at the N3LO order in the two-body sector, and the N2LO order in the three-body sector. To include realistic nuclear forces in RHB (Relativistic Hartree Bolgoliubov) calculations we rely on a separable form of the pairing interaction adjusted to the bare nuclear force. The separable pairing force is applied to the analysis of pairing properties for several isotopic and isotonic chains of spherical nuclei.
We solve the ab initio no-core shell model (NCSM) in the complete Nmax = 8 basis for A = 7 and A = 8 nuclei with two-nucleon and three-nucleon interactions derived within chiral effective field theory (EFT). We find that including the chiral EFT three-nucleon interaction in the Hamiltonian improves overall good agreement with experimental binding energies, excitation spectra, transitions and electromagnetic moments. We predict states that exhibit sensitivity to including the chiral EFT three-nucleon interaction but are not yet known experimentally.
The Dirac structure of the nucleon self-energy in asymmetric nuclear matter cannot reliably be deduced from the momentum dependence of the single-particle energies. It is demonstrated that such attempts yield an isospin dependence with even a wrong sign. Relativistic studies of finite nuclei have been based on such studies of asymmetric nuclear matter. The effects of these isospin components on the results for finite nuclei are investigated.
The problem under consideration in this thesis has, actually, two aspects. The first one concerns the case when a nucleon is in free space that is in vacuum, while the second one studies a nucleon embedded into a nuclear environment. Both of these two aspects have been considered in the framework of chiral topological models of QCD. The whole content of the thesis may be divided into two main parts. In the first part the original Skyrme model with finite pion mass has been extended by inclusion of the light scalar - isoscalar sigma - meson. The Lagrangian has been further extended by explicit …
We present an overview of the description of K and eta meson productions in nucleon-nucleon collisions within an effective Lagrangian model where meson production proceeds via excitation, propagation and subsequent decay of intermediate baryonic resonant states. The $K$ meson contains a strange quark ($s$) or antiquark ($\bar s$) while the $η$ meson has hidden strangeness as it contains some component of the $s{\bar s}$ pair. Strange meson production is expected to provide information on the manifestation of quantum chromodynamics in the non-perturbative regime of energies larger than that of …
We study the influence of the nucleon spectral function on eta photo- and electroproduction on nuclei. Besides kinematical effects due to groundstate correlations, also a modification of the S11(1535) decay width is taken into account, which is caused by the possible decay into nucleons with mass smaller than the pole mass in the medium. Hence, resonances with masses below the free N eta threshold can contribute to eta production.
