Nucleon Isobars Research Articles

Eta physics at threshold

The production of eta and eta-prime mesons in elementary nucleon-nucleon collisions has been investigated at the synchrotrons CELSIUS, COSY and SATURNE. The magnitude and energy dependence of the total cross section as well as the occupation distribution of the phase space serve as observables for investigating the mechanisms underlying the production processes and the interaction of mesons with nucleons. The precise data on the eta and eta-prime creation via the pp --> pp eta(eta-prime) reactions allowed to settle the general features of the eta and eta-prime meson production and revealed the sensitivity of the mentioned observables to the nucleon-nucleon-meson final state interaction. The particular production properties, like for example the determination of the dominating exchange processes which lead to the excitation of the S_11 nucleon isobar in the case of eta creation, must be established by confrontation with other observables. The present status of this investigation with an emphasis on the results of the COSY-11 collaboration is briefly presented. The available data are interpreted in view of the production mechanism and the meson-nucleon interaction.

A covariant OBE model for η production in NN collisions

A relativistic covariant one boson exchange model, previously applied to describe elastic nucleon-nucleon scattering, is extended to study η production in NN collisions. The transition amplitudes for the elementary BN → ηN processes with B being the meson exchanged ( B = π, σ, η, ϱ, ω and δ) are taken to be the sum of four terms corresponding to s and u-channels with a nucleon or a nucleon isobar N ∗ (1535 MeV) in the intermediate states. Taking the relative phases of the various exchange amplitudes to be +1, the model reproduces the cross sections for the NN → Xη reactions in a consistent manner. In the limit where all η's are produced via N ∗ excitations, interference terms between the overall contribution from the exchange of pseudoscalar and scalar mesons with that of vector mesons cancel out. Consequently, much of the ambiguities in the model predictions due to unknown relative phases of different vector and pseudoscalar exchanges are strongly reduced.

PCAC and Modifications in Hadron Properties in Nuclear Medium

It is known that the Adler zero condition when imposed on pion amplitudes leads to several relations between hadron masses through the dual resonance formula of Lovelace, Shapiro and Veneziano. In particular the Lovelace quantization condition leads to the relation [Formula: see text] which is well satisfied experimentally. The Regge slope parameter α′ can be related to the gluon condensate. The latter is modified in the nuclear medium as compared to its value in the QCD vacuum. Combining the PCAC hypothesis with changes in gluon condensate leads to lowering of vector masses in the nuclear medium. It also leads to predictions regarding, nucleon isobar mass differences, pion decay constant in the medium, pion amplitudes and transverse momentum distribution of secondaries in heavy ion collisions.

Threshold production cross section of the pp → ppη reaction in a nonrelativistic OBE model

Abstract The pp → ppη reaction at energies near threshold is studied in a nonrelativistic one-boson exchange model, where the S11 (1535 MeV) nucleon isobar is excited through the exchange of π, η and ϱ mesons. Providing NN and ηN final state interactions are taken into account, our predictions reproduce the energy dependence as well as the scale of the total production cross section near threshold, and thus enforce the validity of the S11 mechanism and perturbative approaches applied to η production in hadronic collisions.

Rapid cooling of neutron stars

The thermal evolution of neutron stars has long been regarded as the source of information about the possible physical states of dense matter. Until recently, the general view was that, if matter consisted of nucleons, cooling would be relatively slow, while if matter were in an exotic state (Bose condensates or quark matter), cooling would be faster — so fast, in fact, that thermal emission from the star's surface would be unobservable. Recently, this view has been called into question following the demonstration that ordinary matter can cool by the so-called direct Urca process even more rapidly than matter in an exotic state. The conditions under which very rapid cooling would occur through energy loss via emission of neutrinos from the interior are as follows. The direct Urca process can occur in neutron stars if the proton concentration exceeds some critical value in the range 11–15%. The proton concentration, which is determined by the poorly known symmetry energy of matter above nuclear density, exceeds the critical value in many current calculations. If it occurs, the direct Urca process enhances neutrino emission and neutron star cooling rates by a large factor compared to any process considered previously. Direct Urca processes with hyperons and/or nucleon isobars can also occur in dense matter as long as the concentration of Λ hyperons exceeds a critical value that is less than 3% and is typically about 0.1%. The neutrino luminosities from the hyperon Urca processes are about 5–100 times less than the typical luminosity from the nucleon direct Urca process, but they are larger than those expected from other sources. These new direct Urca processes provide avenues for rapid cooling of neutron stars which invoke neither exotic states nor the large proton fraction required for the nucleon direct Urca process. It is thus likely that all neutron stars will cool rapidly, whether they contain the so-called exotic matter or not. If so, the ramifications are many and are briefly discussed.

Theoretical energy distributions around threshold from coherent pion production.

Detailed energy distributions have been calculated for exclusive [pi][sup 0] production in C+C collisions below and above threshold. The quantum-coherent, microscopic, many-body formalism used previously has been extended to include the [rho]-exchange as well as the [pi]-exchange interaction modified to produce nucleon isobars in the intermediate nuclear states. Analytic solutions to the [Delta]-hole and particle-hole coefficients using the degenerate, schematic-model approximation are used where collective-coherent, spin-isospin giant resonances are assumed for the nuclear excited states. The nuclear form factors that depend on these coefficients are solved analytically and become sums of degenerate [Delta]-hole and particle-hole states for given multipole angular momentum values of the excited nuclei. Also, in this approach, the overall amplitudes and differential cross sections for [pi][sup 0] production in either nucleus have been formally solved and the first numerical results of the pion, kinetic-energy distributions are presented.

Production mechanism of the X (1740) meson

New data from π -p charge-exchange at 38 GeV/ c confirm the existence of the narrow X (1740) meson, observed in its ηη decay mode. The upper limit on its width is 80 MeV. It is not observed decaying into either η'η or π 0π 0. This meson is produced in charge-exchange by a mechanism other than OPE with a nucleon isobar (probably Δ(1232)), and not with a neutron.

Rapid cooling of neutron stars by hyperons and Delta isobars

Direct Urca processes with hyperons and/or nucleon isobars can occur in dense matter as long as the concentration of Lambda hyperons exceeds a critical value that is less than 3 percent and is typically about 0.1 percent. The neutrino luminosities from the hyperon Urca processes are about 5-100 times less than the typical luminosity from the nucleon direct Urca process, if the latter process is not forbidden, but they are larger than those expected from other sources. These direct Urca processes provide avenues for rapid cooling of neutron stars which invoke neither exotic states nor the large proton fraction (of order 0.11-0.15) required for the nucleon direct Urca process.

The pp → ppη reaction near threshold

The differential and total cross sections for pp → pp η are evaluated close to threshold in a model where the S 11 nucleon isobar is excited through the exchange of ρ, π and η mesons. The preliminary data are not incompatible with these estimates.

Quadrupole moments of hadrons

The chiral bag model gives an expression for quark wavefunctions, taking into account quark color and pion interaction. We the calculate the quadrupole moments of nonstrange hadrons. The quadrupole moment of the nucleon isobar is found to beQ(Δ)=−6.3·10−28eΔ (cm)2. Predictions of the chiral bag model disagree with the nonrelativistic quark model.

Isospin and Isobar Components in Light Nuclei

Isospin mixing in light nuclei is examined through isospin-forbidden particle decay channels. From data on ($\stackrel{\ensuremath{\rightarrow}}{\mathrm{p}}$,$p$) isospin-forbidden resonances, isospin-mixing amplitudes are extracted. They show definitively a periodicity of 8 in $A$ for $8l~Al~40$. These results are combined with neutron- and alpha-decay data in an isospin analysis that estimates isovector and isotensor amplitudes and nucleon isobar contributions to isospin mixing.

Contributions of internal nucleon structure to the nuclear charge and current operators

The relativistic pair current and contributions of nucleon isobars are evaluated in the constituent-quark model. Isovector charge and isoscalar current contributions change substantially compared to previous calculations. The magnetic form factor of the deuteron and the charge form factors of3He and3H are modifed significantly and the previously predicted large differences in the form factors of3He and3H are considerably reduced.

The three-nucleon problems with weak NN interaction

We consider the three-nucleon system in which each two-nucleon pair interacts both strongly and weakly. The strong part is assumed to be separable but an addition of small non-separable term is possible. The weak two-nucleon interaction (which is absolutely non-separable) is assumed to be given in terms of its partial wave components in momentum space. These elements have been calculated by Lassey and McKellar starting from recent models of non-relativistic (strangeness conserving non-leotonic) NN weak interaction. An expression for the weak part of the elastic nucleon-deuteron scattering amplitude is derived within the framework of two potential scattering theory. The weak three-body driving terms are explicitly evaluated. They reflect weak form factors responsible for the weak dissociation of two nucleon isobars (including parity impurity of the deuteron wave function) as well as weak NN scattering. The expressions appearing below are numerically tractable and can be used to elaborate on previous calculations.

Polarization effects in the inclusive process e +e −→ ΔX in the non-perturbative parton model

The scaling behaviour of all ten structure functions describing the inclusive process e +e −→ ΔX in the general case, when there is no summation over nucleon isobar polarizations, is found. The investigation is carried out on the basis of the non-perturbatice parton model. In the Bjorken scaling limit, the expressions for the angular distribution of the Δ→N π decay products, and the longitudinal polarization vector of a nucleon in this decay, as well as for the isobar polarization vector, are obtained.

Electromagnetic effects in precision analyses of the low-energy neutron-proton interaction data

Detailed calculations in the frame of a potential model fitted precisely to the low-energy neutron-proton (deuteron and scattering) experimental data show that the corrections introduced into measurable quantities by electromagnetic interactions are relatively as important as the hadronic contributions from nucleon isobars. The unknown behaviour of the nucleon electromagnetic form factors at high momentum transfers affects only the details of the comparison, at least when constraints provided by current quark-parton models are imposed. But the ignorance of the explicit value of the nucleon hadronic mass is found to generate, in attempts at phenomenological extraction from the data of hadronic values for the np physical quantities, uncertainties as large as the contributions from the isobar channels themselves.

Spectrometers for measurements of recoils and scattered particles from 5 GeV pion-helium interactions

An apparatus, used to search for nucleon isobars, x, in π 4 He → πt x or π 4 He → π 3 He x reactions, is described. The nuclear fragment and the scattered fast pion were measured in a recoil spectrometer and a fast-particle spectrometer, respectively. The recoil spectrometer has multiwire proportional chambers (MWPC) for the measurement of direction and semiconductor detectors and scintillation counters for the measurement of energy. The identification of the recoil is accomplished by means of the E, ΔE method. The target is gaseous to allow the measurement of low-energy recoils and it can hold pressures up to 150 atm. The spectrometer for the scattered pion consists of a magnet and MWPC's.

Nucleon isobars in nuclear structure

The example quoted previously was that of the thermal radiative capture n+p + d+y. In the Chemtob-Rho (1) approach used by Prof. Blin-Stoyle the exchange current correction was evaluated by first deriving a two-nucleon operator. This operator was then sandwiched between two-nucleon wavefunctions to give the final correction. However? a closer examination of the result showed that the A contribution to the two-body operator was important. This term can be illustrated as in Fig. l(a) and contributes about 2.4 % to the total matrix element. The pair and pionic terms shown in Figs. l(b) and l(c) contribute a further 3.8 % and

Formation and Disintegration of High-Density Nuclear Matter in Heavy-Ion Collisions: Nonrelativistic Approximation

The formation of high-density nuclear matter which may be expected to be attained in high-energy heavy-ion collisions and the subsequent disintegration of dense matter are investigated by means of the hydrodynamics. Head-on collisions of identical nuclei are considered in the nonrelativistic approximation. The compressed density cannot exceed 4 times of the normal one so long as the freedom of only nucleons is considered, and can become higher than 4 times when other freedoms such as the productions of mesons and also nucleon isobars are additionally taken into account. The angular distributions for ejected particles predominate both forwards and backwards at low collision energies, corresponding to the formation of nuclear density less than 2 times of the normal density and become isotropic at the point of 2 times of the normal one. As the collision energy increases further, lateral ejection is intensified gradually.

Shock waves in colliding nuclei

We consider the circumstances under which nuclear matter at high densities can be produced in heavy ion collisions. We argue that lab energies of a few hundred MeV per nucleon will be suitable : the matter velocity will exceed the speed of isentropic compression waves, while the nuclear matter has sufficient stopping power to generate a shock front. From the hydrodynamic conservation laws we show that there is a maximum attainable compression ratio ν ∞, determined by the thermal properties of the high-density matter. In an independent-fermion model, ν ∞ = 4, but it can be much larger if the phase of the system changes, for example by excitation of nucleon isobars, production of π-mesons, or the scalar-field condensation conjectured by Lee and Wick. We discuss the propagation of the shock front and subsequent decompression of the dense, hot matter.

Isobar mixing and Coulomb displacement energies in nuclei

It is pointed out that because of the electromagnetic mass splitting of members of a nucleon isobar multiplet (such as the Δ 33), admixtures of this isobar in nuclei may lead to substantial corrections in nuclear Coulomb displacement energies.