Nucleons In Nucleus Research Articles

K+-nucleus scattering and the "swelling" of nucleons in nuclei.

We investigate the utility of the K/sup +/ meson as a probe of effective-size modifications (''swelling'') of nucleons in the nuclear medium. The K/sup +/-nucleus elastic and total cross sections are found to be sensitive to the density-dependent effective masses of the vector mesons (rho,..omega..) whose exchange is responsible for the dominant part of the low-energy K/sup +/-nucleon interaction. This density dependence is estimated in several ways and used to construct a K/sup +/-nucleus optical potential. Such a model is consistent with the K/sup +/+/sup 12/C data at 800 MeVc, and provides a physical picture of the ''swelling'' mechanism in terms of the medium modifications of the meson cloud of the nucleon.

Quark exchange effects in nuclei

The Pauli principle requires that the quark substructure of nucleons in nuclei must be accounted for in the nuclear wave function. This becomes important when one wants to calculate consistently (weak and electromagnetic) observables for which the corresponding properties for the nucleons are determined by the quark substructure, specifically the quark wave functions. This leads to exchange contributions that constitute deviations from the simple impulse approximation description. For instance, in the nuclear form factors it destroys the factorization in a body form factor and the nucleon form factor. Examples of the effects on the elastic form factors of the A ⩽ 4 systems are shown.

Quark models of nuclear isobaric energy differences

We study the energy differences between mirror nuclei in both nonrelativistic (NR) and relativistic quark models based on the one-gluon exchange interaction. Both six-quark (6q) and resonating-group (RG) methods are used to treat the effects of overlapping nucleons in nuclei. The 6q method is simple and useful, but it can give only qualitative results because of a lack of precision and the neglect of non-(1s)6 configurations. It is used here to compare bag and potential models of quarks and to study the effects of kinetic energy, quark-quark interaction and nucleon size. Six-quark results show that the usual Breit-Fermi NR reduction has serious limitations in treating quark masses and nucleon sizes. In contrast, the RG method is much more complete, but it is also much harder to use. It is used here with simple NR potential models to study the effects of complicated 6q configurations, nucleon size and nucleon-nucleon short-range correlations. We find that NR potential models tend to give Nolan-Schiffer anomalies larger than experimental results, while bag models give too small results, especially if the bag radius is as small as 0.6 fm.

Modifications of nucleons in nuclei in quasi-elastic electron-nucleus scattering

In inelastic electron scattering two scaling regions are observed in which the scattering is dominated by quasi-elastic scattering. For large momentum transfers, √ Q 2 > 2 GeV/ c, the scattering process is dominated by quasi-elastic scattering off quarks, whereas for √ Q 2 ≈ 0.5 GeV/ c the dominant contribution is quasi-elastic scattering off nucleons. This corresponds nicely to our first order picture of the nucleus consisting of nucleons, which in turn are composed of quarks. In the nucleon-scaling region, possible modifications of nucleon properties show up through a study of the Q 2 dependence and the relative strength of the transverse and longitudinal cross sections. Results of both inclusive (e, e′) and exclusive (e, e′p) experiments in the quasi-elastic scattering region indeed show a behavior that could indicate modifications of intrinsic properties of individual nucleons in the nucleus, although the question remains if one has correctly disentangled the effects of the (long range) interactions between nucleons and those connected to the internal structure of nucleons. Even so, a simple (one-parameter) size rescaling for nucleons appears to be inconsistent with the data and also with some known conventional nuclear physics observables. Therefore the inclusion of two-nucleon correlations appears necessary in order to be able to understand the data. Such correlations can for instance be due to the effect of the Pauli principle on the quark level.

Conventional nuclear European Muon Collaboration effect in deep inelastic lepton-nucleus scattering.

The influence of the conventional nuclear structure on the European Muon Collaboration effect is critically reviewed. The ratio of deep inelastic lepton nucleon to lepton nucleus cross sections is shown to be sensitively dependent on the proper kinematics of the Fermi motion and the careful treatment of the binding effect of nucleons in nuclei. We systematically consider the example $^{40}\mathrm{Ca}$ at -${Q}^{2}$=20 (GeV/c${)}^{2}$.

Compatibility of the nuclear shell and nucleon bag models.

For the theoretical treatment of a realistic many-body system it is necessary to introduce a model with a manageable number of degrees of freedom as a zero order approximation. In most cases the choice of such a model is suggested by the experimental observation of some qualitative properties of the system. The hope is that the more detailed properties will be given by calculable corrections to the zero order model. Once a model has been formulated, the theory has the minimum task of showing its self-consistency: the essential assumptions of the model should not be clearly invalidated by the corrections. The present paper studies this self-consistency problem for the case of the nuclear independent-particle model with bag-like nucleons. A good part of the successful calculations performed in nuclear physics during the last thirty years has been based on the assumption that the independent-particle model is a good zero order approximation to the nucleus. The essence of this model is the hypothesis that any particular nucleon behaves in the nucleus as a particle of a Fermi sea, moving in a smooth potential and having a mean free path which is large compared to the nuclear dimension. Historically, the experimentally observed validity of the nuclear shell model 1 was quite surprising, beca use the large nucleon-nucleon cross sections had earlier lead to the belief that the nucleons in a nucleus would be similar to the molecules in a liquid drop, having a very small mean free path. In respect to the experimental situation, it should be stressed that the validity of the shell model is a much stronger statement than the existence of low energy single-particle-like excitations, which are not uncommon in Fermi systems. 2 In fact, the single-particle behavior of the nucleons appears to be almost literally true.3 This behavior is, for example, evident in quasifree scattering4 and in the often quite nontrivial quantum numbers of the loosely bound nucleons in heavy nuclei. 5 The compatibility of the large size of the low energy cross sections with the shell model was recognized by Weisskopf.6 He remarked that in the independent-pair model the Pauli exclusion principie drastically reduces the effects of the nucleon-nucleon interaction because the low-lying single-nucleon states are already occupied. This argument was quantitatively confirmed by means of the Bethe-Goldstone equation, 7 which allows us to show that

Polarization transfer in (p, n) reactions

Polarization-transfer studies for (p, n) reactions with normally polarized (N-type) beams have been pursued at the Indiana University Cyclotron Facility, Bloomington, IN, since 1983. A variety of targets having values of A ranging from 2 to 90 have been studied at 0° for bombarding energies between 80 and 200 MeV. Transitions to well-resolved final states in the lighter nuclei have yielded data that lend plausibility to distorted-wave impulse approximation interpretations of intermediate-energy nucleon–nucleus reactions. Measurements on heavier nuclei have revealed interesting nuclear-structure effects at high excitation energies.

Isovector excitation of stretched states in nuclei: effects of meson exchange currents, unbound wave functions, and knockout exchange amplitudes on the extraction of particle–hole strengths

It is well known that the strength for excitations of [Formula: see text] high spin, stretched states observed via inelastic scattering, is generally much smaller than that predicted by spherical shell-model calculations. In addition, results obtained from electromagnetic and hadronic studies have discrepancies at the 20% level. For us to gain a better understanding of reduced magnetic strength in electron scattering and hopefully close the gap between experiment and theory, calculations of the electron-scattering form factors have been performed including the effects due to meson exchange currents in the transition amplitude and the effects due to unbound wave functions for the valence nucleon. The effect of the meson exchange-current contributions is to uniformly enhance the form factors near the first maximum, resulting in a 16 to 20% further reduction of the stretched particle–hole strength. The effect due to the radial wave functions deduced from Woods–Saxon potentials in which the nucleon is not bound is to reduce the form factors, thereby resulting in an increase in the spectroscopic strength. As regards the comparison of results obtained with electromagnetic and hadronic probes, the implied sensitivity to higher order current and spin–current transition densities associated with the nonlocality due to the tensor knockout exchange amplitudes in nucleon–nucleus scattering is considered explicitly. It is found that the simplest correspondence between electron and nucleon–nucleus scattering is preserved for isovector excitations but not for isoscalar excitations under the usual assumptions for the tensor interaction. It is clear that precise comparisons between experiment and theory (or between probes) cannot be made unless these and related effects are consistently included.

Recent theoretical developments in nuclear charge-exchange reactions

The (p, n) and (n, p) charge-exchange reactions are a suitable tool in nuclear-structure studies. At intermediate energies, the interaction with the target resembles that between free nucleons, which makes the probes mostly susceptible to the spin dynamics of the nucleus. We review the current theoretical picture of nucleon–nucleus charge-exchange reactions. A brief introduction to the theory of direct reactions in the impulse approximation is given. We then discuss the random-phase approximation (RPA) linear response theory, which is widely used in the calculation of cross sections and related observables. To understand correctly the shape of the continuum cross sections, one needs a more sophisticated theory. We discuss suitable extensions of the RPA that incorporate spreading effects as well as escape effects. Charge-exchange reactions of the (n, p) type have important implications for the evolution of massive stars. The relevance for Type-II supernovae is discussed.

Energy distribution of pions in the nucleon–nucleus interaction at 70 GeV

Production of pions through the intermediate state called fireballs has been considered to study the energy distribution of pions in the fireball-rest system. Nikfi-R-type photoemulsion plates exposed to the 70-GeV proton beam of the Surpukhov Accelerator, Dubna, USSR, are used for this purpose. A graphical representation of the distribution of energies among the pions is shown. It is observed that this distribution closely resembles Planck's distribution of photons. Furthermore, this study reveals that the Maxwell–Boltzmann distribution deviates greatly from experimental observations. The average energy of the pions is found to be (0.770 ± 0.03) GeV.

Differential cross sections for photoemission of deuterons from the nitrogen nucleus

Momentum distributions for inclusive photoemission of deuterons from nitrogen have been measured at laboratory angles of 43° and 50°, and for maximum energies of the bremsstrahlung photon beam of 725, 775 and 825 MeV. The double differential cross sections of deuterons emitted from nitrogen are obtained for photon energies of 750 and 800 MeV and for laboratory angles of 43° and 50°. The photon energy is determined by the photon subtraction method. Results are examined by a π 0 photoproduction process off a quasi-deuteron in nitrogen ( γ″d″→ π 0d) and by a two-step process. The two-step process means the following: first, a nucleon is produced by a photo-pion production process from a quasi-free nucleon in the nitrogen nucleus ( γ″N″→ πN); second, the nucleon produced at the first step picks up another nucleon in the nucleus and then a deuteron is emitted (N+A→d+anything).

Quark versus hadronic degrees of freedom in light nuclear system

We present a review of papers presented on the subject; p -p interaction, N-N interaction at medium energies and the dibaryon resonances, the reaction pp→ πd and πd elastic scattering in the framework of the NN πNN equations. Included also, are the contributions on photo- and electro-disintegration of the deuteron, pion absorption on 3He, π−N scattering, and the size of the nucleon in nuclei.

The Nucleon-Nucleon Interaction through Boson Exchange

The boson-exchange interactions coming from n, CJ and w·exchange are discussed. It is shown how the medium dependent corrections lead to a decrease in CJ, p and w. effective masses with increasing density, and how this affects the electromagnetic form factor. Much of the swelling of nucleons in nuclei can be accounted for in this way.

Constituent quark mass and nucleon properties in nuclei

It is shown that the Nolen-Schiffer anomaly, the quenching of gA and the increase in some electromagnetic properties of nucleons in nuclei can all be explained qualitatively in a constituent quark model if the quark mass is assumed to depend on its confinement size.

Nonmesonic nucleon decay in nuclei.

The nonmesonic decay of nucleons in nuclei is calculated in the hybrid quark-hadron model. The ratio of the decay rates (scrN\ensuremath{\rightarrow}${\mathrm{scrN}}^{\mathrm{*}}$${\mathrm{e}}^{+}$)/(scrN\ensuremath{\rightarrow}${\mathrm{scrN}}^{\mathrm{*}}$${\mathrm{e}}^{+}$\ensuremath{\pi}) of a nucleus scrN is estimated to be about 15%.

Lepton-nucleus deep-inelastic scattering.

We study the European Muon Collaboration effect within the conventional nuclear theory using the free-nucleon structure function. It is shown that qualitative agreement with the experiment can be obtained by taking into account the momentum (Fermi motion) and the energy distributions of nucleons in nuclei.

Change of Scale for Nucleons in Nuclei from Quasielastic Electron Scattering

Change of Scale for Nucleons in Nuclei from Quasielastic Electron Scattering

Nucleons in nuclei in the selfconsistent soliton model to QCD

In the scalar Soliton model of QCD the gluon field operators are replaced by a classical selfcoupling scalar field with solitontype interaction. The quark wave-functions and the soliton field are calculated for the free nucleon as compared to a nucleon inside nuclear matter approximated by averaged boundary conditions to simulate the surrounding nucleons. The mass of the nucleon as well as its mean effective radius are given in terms of the nuclear-matter density and the model parameters. For large quark-soliton coupling the EMC-effect is seen, and then at high densities a plasma of free quarks and free localized solitons is the lowest energy solution.

Change of confinement scale in nuclei: Predictions for structure functions confront electroproduction data.

We confront our explanation of the ``European Muon Collaboration effect,'' in which the quark confinement scale increases in going from a free nucleon to nucleus, with a new electroproduction experiment off several nuclear targets. The change of confinement size is attributed to the overlap of nucleons in nuclei which increases with nuclear density. A change of confinement scale modifies the quark and gluon distribution functions and we compute, in leading order in QCD, its effects for a series of different nuclei. New, precise electron scattering measurements at SLAC agree well with these predictions for 0.2lxl0.7, supporting the postulate that the confinement size increases with nuclear density.

Alpha-emulsion nucleus interactions at cosmic-ray energies

The rapidity and rapidity gap distributions in alphaemulsion nucleus collisions at cosmic-ray energies (TeV region) are presented here. Comparison with the results of nucleon-nucleon and nucleon nucleus interactions in the same range of primary energies are also made. Multiparticle production via cluster formation is discussed.