Strangeness in the nucleon: neutrino–nucleon and polarized electron–nucleon scattering

After the EMC and subsequent experiments at CERN, SLAC and DESY on the deep inelastic scattering of polarized leptons on polarized nucleons, it is now established that the Q²=0 value of the axial strange form factor of the nucleon, a quantity which is connected with the spin of the proton and is quite relevant from the theoretical point of view, is relatively large.In this review, we consider different methods and observables that allow one to obtain information on the strange axial and vector form factors of the nucleon at different values of Q². These methods are based on the investigation of the neutral current induced effects such as the P-odd asymmetry in the scattering of polarized electrons on protons and nuclei, the elastic neutrino (antineutrino) scattering on protons and the quasi-elastic neutrino (antineutrino) scattering on nuclei. We discuss in detail the phenomenology of these processes and the existing experimental data.     

The effect of nonlinearity in relativistic nucleon–nucleon potential

A simple form for nucleon–nucleon (NN) potential is introduced as an alternative to the popular M3Y form using the relativistic mean field theory (RMFT) with the non-linear terms in σ-meson for the first time. In contrast to the M3Y form, the new interaction becomes exactly zero at a finite distance and the expressions are analogous with the M3Y terms. Further, its applicability is examined by the study of proton and cluster radioactivity by folding it with the RMFT-densities of the cluster and daughter nuclei to obtain the optical potential in the region of proton-rich nuclides just above the double magic core¹⁰⁰Sn. The results obtained were found comparable with the widely used M3Y NN interactions.     

A Monte Carlo generator of nucleon configurations in complex nuclei including nucleon–nucleon correlations

We developed a Monte Carlo event generator for production of nucleon configurations in complex nuclei consistently including effects of nucleon–nucleon (NN) correlations. Our approach is based on the Metropolis search for configurations satisfying essential constraints imposed by short- and long-range NN correlations, guided by the findings of realistic calculations of one- and two-body densities for medium-heavy nuclei. The produced event generator can be used for Monte Carlo (MC) studies of pA and AA collisions. We perform several tests of consistency of the code and comparison with previous models, in the case of high energy proton–nucleus scattering on an event-by-event basis, using nucleus configurations produced by our code and Glauber multiple scattering theory both for the uncorrelated and the correlated configurations; fluctuations of the average number of collisions are shown to be affected considerably by the introduction of NN correlations in the target nucleus. We also use the generator to estimate maximal possible gluon nuclear shadowing in a simple geometric model.     

ρ-Meson dominance and spin correlations in pion photopair production at nucleons near nucleon resonances

The pion photopair production cross section near nucleon resonances of spin 3/2 has been calculated in the-meson dominance model, including particle spin correlation; the expressions for the angular and energy distribution have been derived, as well as the invariantmass distribution for the system, together with an expression for the asymmetry coefficient arising from the linear polarization of the photon. These results are compared with measurements at-ray energies of 1, 2.8, and 4.7 GeV. At these energies, the agreement with experiment is satisfactory.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 6, pp. 103–108, June, 1976.     

Electronic stopping in ion–fullerene collisions

The electronic friction experienced by a multiply charged ion interacting with the valence electrons of a single fullerene is an important aspect of the collision dynamics. It manifests itself in a considerable loss of projectile kinetic energy transferred to the target, resulting in excitation. The latter mainly leads to direct ionization and multifragmentation and can be recognized in specific patterns of the fragmentation spectra. These fingerprints can be used to quantify electronic stopping and to identify its typical properties as known from particle–solid interactions, such as the oscillation of the electronic stopping with the projectile atomic number Z. These essentially many-body effects can therefore be studied in a well-defined system of finite size. Received: 4 April 2000 / Accepted: 6 November 2000 / Published online: 9 February 2001     

Neutron–proton analyzing power at 12 MeV and inconsistencies in parametrizations of nucleon–nucleon data

We present the most accurate and complete data set for the analyzing power Ay(θ)$A_y(\theta )$ in neutron–proton scattering. The experimental data were corrected for the effects of multiple scattering, both in the center detector and in the neutron detectors. The final data at En=12.0 MeV$E_n=12.0\text{MeV}$ deviate considerably from the predictions of nucleon–nucleon phase-shift analyses and potential models. The impact of the new data on the value of the charged pion–nucleon coupling constant is discussed in a model study.     

Charmed baryons in electron–positron collisions

Charmed baryons are classified, and the experimental data on charmed-baryon states are reviewed paying special attention to those obtained at the electron–positron colliders.     

Exclusive processes in electron–ion collisions

The exclusive processes in electron–ion (eA) interactions are an important tool to investigate the QCD dynamics at high energies as they are in general driven by the gluon content of the target which is strongly subject to parton saturation effects. In this Letter we compute the cross sections for the exclusive vector meson production as well as the deeply virtual Compton scattering (DVCS) relying on the color dipole approach and considering the numerical solution of the Balitsky–Kovchegov equation including running coupling corrections (rcBK). The production cross sections obtained with the rcBK solution and bCGC parametrization are very similar, the former being slightly larger.     

Isospin breaking in the pion–nucleon scattering lengths

We analyze isospin breaking through quark mass differences and virtual photons in the pion–nucleon scattering lengths in all physical channels in the framework of covariant baryon chiral perturbation theory.     

Heavy-flavour spectra in high-energy nucleus–nucleus collisions

The propagation of the heavy quarks produced in relativistic nucleus–nucleus collisions at RHIC and LHC is studied within the framework of Langevin dynamics in the background of an expanding deconfined medium described by ideal and viscous hydrodynamics. The transport coefficients entering into the relativistic Langevin equation are evaluated by matching the hard-thermal-loop result for soft collisions with a perturbative QCD calculation for hard scatterings. The heavy-quark spectra thus obtained are employed to compute the differential cross sections, the nuclear modification factors R _AA and the elliptic-flow coefficients v ₂ of electrons from heavy-flavor decay.     

Weinbergʼs approach to nucleon–nucleon scattering revisited

We propose a new, renormalizable approach to nucleon–nucleon scattering in chiral effective field theory based on the manifestly Lorentz invariant form of the effective Lagrangian without employing the non-relativistic expansion. For the pion-less case and for the formulation based on perturbative pions, the new approach reproduces the known results obtained by Kaplan, Savage and Wise. Contrary to the standard formulation utilizing the non-relativistic expansion, the non-perturbatively resummed one-pion exchange potential can be renormalized by absorbing all ultraviolet divergences into the leading S-wave contact interactions. We explain in detail the differences to the non-relativistic formulation and present numerical results for two-nucleon phase shifts at leading order in the low-momentum expansion.     

Accurate nucleon–nucleon potential based upon chiral perturbation theory

We present an accurate nucleon–nucleon (NN) potential based upon chiral effective Lagrangians. The model includes one- and two-pion exchange contributions up to chiral order three. We show that a quantitative fit of the NN D-wave phase shifts requires contact terms (which represent the short range force) of order four. Within this framework, the NN phase shifts below 300 MeV lab. energy and the properties of the deuteron are reproduced with high-precision. This chiral NN potential represents a reliable starting point for testing the chiral effective field theory approach in exact few-nucleon and microscopic nuclear many-body calculations. An important implication of the present work is that the chiral 2π exchange at order four is of crucial interest for future chiral NN potential development.     

Approximation properties of the Paris potential of nucleon–nucleon interaction

The phenomenological Paris potential of nucleon–nucleon interaction is analyzed in terms of the independence of the coefficients of various components. Analysis of the determinant of the Gram matrix reveals the extremely high dependences of the coefficients, demanding a revision of the system of basic functions.     

Charge independence breaking and charge symmetry breaking in the nucleon–nucleon interaction from effective field theory

We discuss charge symmetry and charge independence breaking in an effective field theory approach for few-nucleon systems. We systematically introduce strong isospin-violating and electromagnetic operators in the theory. The charge dependence observed in the nucleon–nucleon scattering lengths is due to one-pion exchange and one electromagnetic four-nucleon contact term. This gives a parameter free expression for the charge dependence of the corresponding effective ranges, which is in agreement with the rather small and uncertain empirical determinations. We also compare the low energy phase shifts of the nn and the np system. We present a classification scheme for corrections to the leading order results and show that power counting explains previously made phenomenological observations.     

Equilibrium and non-equilibrium effects in nucleus–nucleus collisions

Local thermal and chemical equilibration is studied for central A+A collisions at 10.7–160 AGeV in the Ultrarelativistic Quantum Molecular Dynamics model (UrQMD). The UrQMD model exhibits strong deviations from local equilibrium at the high density hadron–string phase formed during the early stage of the collision. Equilibration of the hadron–resonance matter is established in the central cell of volume V=125 fm³ at later stages, t≥10 fm/c, of the resulting quasi-isentropic expansion. The thermodynamical functions in the cell and their time evolution are presented. Deviations of the UrQMD quasi-equilibrium state from the statistical mechanics equilibrium are found. They increase with energy per baryon and lead to a strong enhancement of the pion number density as compared to statistical mechanics estimates at SPS energies.     

Excitation of strontium atom in electron–atom collisions

The excitation of ³D levels of strontium atom by slow monoenergetic electrons has been studied experimentally. Thirty six excitation cross-sections were measured at 30-eV electron energy. Optical excitation functions for most of the transitions were recorded in the 0–200-eV electron-energy range. The excitation cross-section as a function of the principal quantum number has been found to correspond to a power law for all ³D series.     

Nucleon–nucleon scattering in the light of supersymmetric quantum mechanics

By exploiting supersymmetry-inspired factorization method together with a judiciously chosen deuteron ground-state wave function, approximate higher partial wave nucleon–nucleon potentials are generated. In this context, a minor modification is also introduced to the generated potentials. The n–p scattering phase shifts are computed and analysed via the phase function method.