Free Nucleons Research Articles

Neutrino neutral current interactions in nuclear matter

Detailed knowledge of neutrino transport properties in matter is crucial for an understanding of the evolution of supernovae and of neutron star cooling. We investigate screening of neutrino scattering from a dense degenerate gas of electrons, protons and neutrons. We take into account correlations induced by the Coulomb interactions of the electrons of the electrons and protons, and the strong interactions of the protons and neutrons. Nuclear matter is described by the σω model of quantum hadrodynamics. Results are presented for typical astrophysical scenarios. The differential cross section is strongly reduced at large energy transfer, where electrons dominate, and slightly reduced for small energy transfer, where nucleons dominate. At large densities, the nucleon effective mass is considerably lower than the free mass, and the region dominated by nucleons extends to larger energy transfer than for free nucleons.

Convenient parametrization for deep inelastic structure functions of the deuteron.

The spin, as well as spin-averaged, twist-two structure functions of the deuteron are calculated in a version of the convolution model that incorporates relativistic and binding energy corrections. A simple parametrization of these structure functions is given in terms of a few deuteron wave-function parameters and the free nucleon structure functions. This allows for an easy comparison of structure functions calculated using different deuteron models.

Soliton bag model and the EMC effect

Based on the Friedberg-Lee soliton bag model with consideration of the Fermi motion (including the effect of nucleon’s binding) we study mainA-dependence of the ratio of the structure function for bound and free nucleons in terms of a model in which bound nucleons have a slightly larger size than free nucleons. With our improved model, the theoretical curves are in good agreement with the experimental data. An empirical formula has been given about the effective radius of the quarks’ confinement region of a bound nucleonR A with atomic weightA and we also have discussed the influence of the masses of quarks upon the EMC effect.

Nonquasifree production of Δ isobars in the C( 3He, t) reaction at 4.4–18.3 GeV/c

The ( 3He, t) charge-exchange reaction on carbon nuclei and protons, accompanied by the excitation of Δ isobars in a target, has been studied at beam momenta from 4.40 to 18.3 GeV/ c. The analysis is performed to explain why the Δ-peak in the inclusive A( 3He, t) and A( p, n) measurements is shifted towards lower energy transfers (in comparison with the reactions on free nucleons) but in the exclusive ones it is shifted towards higher transfers. An attempt is undertaken to determine the characteristics of Δ excitation in nuclei by a ( 3He, t) reaction that is free of the admixture of quasifree Δ production.

Pion-induced elastic and inelastic scattering above the 3,3 resonance.

Results of Glauber model calculations of elastic and inelastic scattering from {sup 12}C, {sup 18}O, {sup 40}Ca, and {sup 48}Ca of pions having energy of 300 to 1200 MeV are presented. Experimental {pi}{ital N} phase shifts including {ital l}=0 through 5 are used in the calculation. The {ital f} wave is important above 300 MeV and the {ital g} and {ital h} wave above 900 MeV. Effects of spin flip are included. The model reasonably reproduces the 800 MeV/{ital c} data of Marlow although the inelastic scattering of the 2{sup +} of {sup 12}C requires an enhancement of the quadrupole component consistent with the effective charge. The previously observed anomaly in {sup 12}C in which a much larger oscillator parameter was required in pion scattering than in electron scattering disappears at higher energy. The {sigma}({pi}{sup {minus}}{ital n}) to {sigma}({pi}{sup +}{ital n}) ratio is found to be appreciably different from expectations based on pion scattering from free nucleons.

Inelastic [formula omitted] production in deep inelastic scattering from hydrogen and deuterium and the gluon distribution of free nucleons

We present results on inelastic J/ψ production from muon interactions with hydrogen and deuterium at an incident muon energy of 280 GeV. The measured cross section ratio per nucleon for muon-induced J ψ production in deuterium and hydrogen was found to be R(D 2/H 2) = 1.01±0.15. The colour singlet model is shown to provide a good description of the observed differential cross section apart from a normalisation factor. The comparison between the observed cross section and the colour singlet model prediction allows the extraction of the gluon structure function G( x) of the nucleon. The momentum fraction x of the nucleon carried by the gluon is measured in the range of x = [0.02, 0.30]. The normalised gluon distribution of free nucleons thus found can be parametrised as xG(x) = 1 2 (η+1) (1−x) η, withη=5+-9( sta.) .

Exact expressions and improved approximations for interaction rates of neutrinos with free nucleons in a high-temperature, high-density gas

view Abstract Citations (24) References (16) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Exact Expressions and Improved Approximations for Interaction Rates of Neutrinos with Free Nucleons in a High-Temperature, High-Density Gas Schinder, Paul J. Abstract The exact expressions needed in the neutrino transport equations for scattering of all three flavors of neutrinos and antineutrinos off free protons and neutrons, and for electron neutrino absorption on neutrons and electron antineutrino absorption on protons, are derived under the assumption that nucleons are noninteracting particles. The standard approximations even with corrections for degeneracy, are found to be poor fits to the exact results. Improved approximations are constructed which are adequate for nondegenerate nucleons for neutrino energies from 1 to 160 MeV and temperatures from 1 to 50 MeV. Publication: The Astrophysical Journal Supplement Series Pub Date: September 1990 DOI: 10.1086/191498 Bibcode: 1990ApJS...74..249S Keywords: Interstellar Gas; Neutrinos; Neutron Stars; Stellar Interiors; Absorption Spectra; Antineutrinos; Electron Scattering; Emission Spectra; Particle Interactions; Supernovae; Astrophysics; DENSE MATTER; ELEMENTARY PARTICLES; NEUTRINOS; STARS: INTERIORS; STARS: SUPERNOVAE full text sources ADS |

Structure functions: Moments andA dependence

We discuss a phenomenological model, based on the convolution formalism and on the hypothesis of a change of confinement scale in nuclei, to describe the EMC effect. Particularly, we study the behaviour of the moments of the free nucleon structure function and we suggest a phenomenological law for the dependence of the bound nucleon mean squared radius on the atomic numberA. Finally, we investigate theQ2 dependence of the nucleon mean squared radius with itsA dependence.

Deep inelastic lepton-nucleus scattering from the light-cone quantum field theory

The authors show that for deep inelastic lepton-nucleus scattering, the conditions which validate the impulse approximation are hardly satisfied when using ordinary instant form dynamics in the rest frame of the nucleus, whereas they are well satisfied when using instant form dynamics in the infinite-momentum frame, or using light-front form dynamics in an ordinary frame. Therefore a reliable theoretical treatment of deep inelastic lepton-nucleus scattering should be performed in the time-ordered perturbation theory in the infinite-momentum frame, or its equivalent, the light-cone perturbation theory in an ordinary frame. To this end, the authors extend the light-cone quantum field theory to the baryon-meson field to establish a relativistic composite model of nuclei. They then apply the impulse approximation to deep inelastic lepton-nucleus scattering in this model. As a first approximation, they consider only the nucleon degrees of freedom and neglect the Z-graph contributions. It is found that the results seem able to explain the EMC effect in trend but overestimate the ratio of the differential cross section for a bound nucleon to that for a free nucleon by about 8%. Some comparisons of the model with several relevant models are also presented.

Explanation of the apparent charge dependence of the pion-nucleon coupling.

A recent, careful analysis of the world data on pp scattering concluded that the pp${\ensuremath{\pi}}^{0}$ coupling constant was 3%--4% less than expected from charge independence and the known pn${\ensuremath{\pi}}^{+}$ coupling constant. We propose a much simpler interpretation of the analysis of the pp data which involves no unreasonable level of charge dependence. Instead one must recognize that the pion-nucleon form factor needed in a boson-exchange model of the nucleon-nucleon force differs significantly from that relevant to a free nucleon.

Polarizations and cross sections of Lambda hyperons produced at backward angles in the reaction pi +12C--> Lambda +X at 4 GeV/c.

We have measured the polarizations and the differential cross sections of the produced \ensuremath{\Lambda}'s at backward angles (70\ifmmode^\circ\else\textdegree\fi{}${\ensuremath{\theta}}_{\mathrm{lab}145\mathrm{\ifmmode^\circ\else\textdegree\fi{}}}$) using a large-aperture spectrometer. The angular range is kinematically not accessible in interactions of the incident pions with free nucleons. The calculated polarizations in terms of superposition of an elementary process incorporating Fermi motion of nucleons in the nucleus are opposite in sign to the observed ones, while a quark-parton model of \ensuremath{\Lambda} production with a Thomas precession effect reproduces both the polarizations and the cross sections well.

ANTIPROTON ANNIHILATION IN COMPLEX NUCLEI OF EMULSION AND ISOBAR MODEL OF PION ABSORPTION

Antiproton [Formula: see text], on annihilation with a nucleon in a complex nucleus of emulsion emits charged pions which are about 20% less than that observed in an antiproton annihilation on a free nucleon. Predominant reduction takes place for the pions in the energy range of 140 MeV. This reduction of charged pion multiplicity and the corresponding pion energy range for which such reduction takes place have been explained on the basis of isobar model of pion absorption. The energy and the opening angle distributions of the protons resulting from absorption of such pions were measured and were fitted with those from the Monte Carlo calculations based on the isobar model. Good agreement in each case further supports the isobar model of pion absorption. (Nuclear reaction ([Formula: see text], emulsion), pion absorption in complex nuclei, isobar model)

Antiproton-helium annihilation at 1 MeV

The p − 4He annihilation cross sections have been measured in the energy range from 0.85 to 1.55 MeV (40 + 54 MeV/c) in the PS-179 experiment at the LEAR facility of the CERN using a streamer chamber in a magnetic field. The σan (0.85 + 1.55 MeV) mean value has been found to be (1541 ± 262 ± 92) mbarn. Recently Mahalanabis et al. have obtained the behaviour of the pp annihilation cross section down to zero energy and values at 40 and 50 MeV/c are given for np annihilation. Our p4He annihilation cross section is approximately twice that on free nucleon like in the interval 200–600 MeV/c.

Nucleons in nuclei (I)

A summary and a critical discussion of the working session Nucleons in Nuclei are presented focussing the attention on present and future investigations of the free nucleon form factor, the medium modifications of nucleon properties and the extraction of nucleon momentum distributions from exclusive and inclusive (y-scaling) scattering.

A study of the electrodisintegration reaction 4He(e, e′ p) 3H with transverse longitudinal separation

Longitudinal and transverse cross sections for the reaction 4He(e, e′ p) 3H were determined for missing momenta p m up to 190 MeV/ c and momentum transfers q up to 829 MeV/ c (at p m=90 MeV/ c). “Experimental” spectral functions were obtained; the transverse one exhibits the free nucleon q-dependence at fixed pm and agrees with microscopic theories; the longitudinal spectral function is low by 30%.

Electroexcitation of the Delta (1232) in nuclei.

Inclusive electron-scattering measurements of \ensuremath{\Delta} electroexcitation in nuclei are reported. Electrons with energies of 0.96, 1.1, 1.3, and 1.5 GeV were scattered from $^{1}\mathrm{H}$, $^{4}\mathrm{He}$, C Fe, and W at 37.5\ifmmode^\circ\else\textdegree\fi{}, corresponding to ${Q}^{2}$=0.20--0.52 (GeV/c${)}^{2}$ at the \ensuremath{\Delta} peak. The centroid of the \ensuremath{\Delta}-region cross-section peak is above that for the free nucleon and it shifts to higher invariant mass as ${Q}^{2}$ increases. The A dependence in the dip region and ratios of nuclear to nucleon integrated cross sections indicate that at these ${Q}^{2}$ values there is little specifically nuclear, e.g., quasideuteron, background contribution.

Study of bound nucleons by quasiexclusive scattering with large momentum transfer.

We describe a method for determining the four-momentum distribution of nucleons in nuclei which is particularly sensitive to large momenta. It may also prove useful in probing the differences between bound and free nucleons.

Evidence of isospin effects in antiproton-nucleus annihilation

Antiproton- 3 He annihilation events at rest have been detected using a self-shunted streamer chamber. The ratio of the cross section for annihilation on neutrons and on protons has been measured (0.467 ± 0.035). It is compared with other results from annihilation on free nucleons, deuterium, 3He and 4He. The low value of the ratio seems to indicate a strong isospin dependence of the antinucleon-nucleon P-wave amplitude.

Quark-exchange contribution to the European Muon Collaboration effect in nuclear matter.

Fermi statistics require that all quarks in a nucleus be antisymmetrized, and hence that quarks belonging to different nucleons be exchanged in proportion to the degree of nucleon overlap. This leads to a shift in the distribution of quark momentum relative to that in isolated nucleons, and hence to a shift in the structure function F2(x). This paper extends previous work on the European Muon Collaboration (EMC) effect for the trinucleon system to systems with large numbers of nucleons. For large N, certain divergences were encountered and traced to contributions arising from unlinked quark clusters. After renormalization a smooth N~ ~ limit was obtained. The conclusion derived in the previously considered %=3 case is upheld: Quark exchange is very important — perhaps even dominant — in calculating the deviation away from unity of the ratio of the nuclear structure function to that of a free nucleon. I. INTRODUCTION Quark exchange contributions to the nuclear structure function F2(x) arise because quarks in different nucleons must be antisymmetrized. This was demonstrated by Jaffe' in a simple, solvable one-dimensional model. A realistic calculation with three-dimensional Fadeev nuclear wave functions for ¹=3 nuclei was subsequently undertaken by Hoodbhoy and JafFe. The results of this calculation were surprising: The exchange of quarks between the three nucleons led to a shift in the nuclear structure function which was almost of the same rnagnitude and shape as the expected European Muon Collaboration (EMC) ratio. The three nucleon system was chosen in Ref. 2 for good reasons: It is fairly dense, has reliable wave functions directly obtained from solving the Schrodinger equation with realistic nuclear forces, and a calculation appeared more tractable for smaller N. But, on the other hand, this was a compromise solution because there is no deep inelastic lepton scattering data from trinucleon systerns. Furthermore, the complexity of the three nucleon wave function somewhat obscured the simplicity of the physical ideas. And finally, the connection with heavier nuclei was far from evident. In this paper we have undertaken the task of extending the treatment of Ref. 2 by considering nuclei with an arbitrary number of nucleons N. More specifically, we seek to evaluate the quark exchange contribution to the EMC effect in nuclear matter. This involves essentially two steps: One must first calculate the shift in quark rnomentum distribution originating from quark exchanges, and then calculate the effect of this on the nuclear structure function. The latter has been fully dealt with in Ref. 2 and will not be repeated. However, the former step involves a new element; in the N~~ limit, quark exchange between nucleons leads to the occurrence of certain divergences and hence the relevant matrix elements must be appropriately renormalized. In the following paragraphs we briefly explain this point as well as summarize the essential physics underlying the calculation. Conventional nuclear physics, viewed in the second quantization formalism, takes on a very simple appearance. If at and a& denote the creation and destruction operators of an elementary nucleon, then one takes Ia, att) =5 & and all other anticommutators to be zero. The most general nuclear state with N nucleons is

The importance of nucleon substructure in nuclear ground states

Using a potential model, constrained by the hadron spectrum, for the confinement of relativistic quarks we explore the consequences of the substructure of nucleons for the binding energy and ground state wavefunction of 4He. In its simplest form, this model gives a binding energy of 19 MeV. Quark wavefunctions differ from those associated with free nucleons by less than 10%, the rms quark radius is 1.34 fm and the resulting structure differs considerably from that of an expansion beginning from the (0s) 4 shell model. Considerable contributions to the binding energy, attractive from quark delocalization and repulsive from the quark hyperfine interaction, appear unavoidable. We conclude that these effects cannot be excluded from a detailed understanding of the properties of nuclear ground states.