On mesonic interference phenomena in deep inelastic scattering

Possible effects of a π′ contribution to lepton deep-inelastic scattering (DIS) are analyzed. The exchange of the first radial excitation of the pion (π′(0^?)) has been advocated recently to solve the notorious problem of hard vertex form factors innucleon-nucleon scattering. The π′-meson exchange contribution is calculated in addition to the well known pionic (Sullivan) contribution. Interference between π and π′ contributions and their possible influence on the cut-off mass parameter of the πNN form factor extracted from DIS on the nucleon are examined. It is shown that the orthogonality relation between the π and π′ wave functions, in conjunction with number and momentum sum rules, put strong restrictions on the size of the interference effect. The interference effect between π and π′ is found to be negligible for the nucleon quark distributions and to modify only higher moments of the quark distributions. The interference effects of a pionic contribution with other mesons are discussed.     

Pionic field and renormalization of the axial coupling constant in nuclei

The renormalization of the axial coupling constant in nuclei is related by PCAC to the exchange effects in the pionic vertex. Using an optical potential as a model for the pion source function, it is shown that in large nuclei the renormalization arises from the short range correlations. The analogy between this problem and that of an electric dipole inside a dielectric medium is pointed out.     

Nucleon form factors in a covariant diquark-quark model

In a model where constituent quarks and diquarks interact through quark exchange the Bethe-Salpeter equation in ladder approximation for the nucleon is solved. Quark and diquark confinement is effectively parametrized by choosing appropriately modified propagators. The coupling to external currents is implemented via non-trivial vertex functions for quarks and diquarks to ensure gauge invariance at the constituent level. Nucleon matrix elements are evaluated in a generalized impulse approximation, and electromagnetic, pionic and axial form factors are calculated.     

Pions in the light-cone nucleon and electromagnetic form factors

The light-cone Fock-state decomposition of the nucleon involves pion-quark core (π′N′ andπ′Δ′) components. The size of the quark core is at present of special importance for the analysis of the onset of colour transparency in the A(e,e′p) reactions. With the light-coneNπ′N′ andNπ′Δ′ vertex functions deduced from experimental data on high-energy proton fragmentation into nucleons and isobars, we evaluate the effect of the pionic cloud on the electromagnetic form factors of nucleons. We find the quark core radius to be about 10% smaller than the nucleon's radius. Analysis of the renormalization of the total nucleon-nucleon cross section for the presence of the pionic cloud suggests a similar core radius.     

Exchange currents in electron scattering from light nuclei

Effects of meson exchange currents on elastic and inelastic electron scattering from nuclei are considered with specific applications to ³He, ⁶Li, ⁷Li, ⁹Be and ¹⁰B. The one-pion exchange approximation is employed (with “pair” and “pionic” graphs) in the non-relativistic limit, where only transverse isovector multipoles are allowed. This approach is shown to be consistent with current conservation. Form factors are calculated with and without exchange currents for a wide range of momentum transfers (up to 1 GeV/c) and for a variety of nuclear transitions to determine where exchange-current effects are large. The relevance of these calculations to experiment is discussed.     

Effects of the small components of the nucleon on pionic disintegration of deuterium near threshold

We have recalculated the nucleon pole diagram for pionic disintegration of deuterium, including the effects of the small components of the interacting nucleon by keeping all four invariants of the Blankenbecler and Cook vertex. We obtain the leading contribution to the cross section near threshold, in agreement with the Adler Dothan theorem. Following this result we reexamine the S-wave rescattering mechanism generally required to explain the threshold region data.     

Taming the pion cloud of the nucleon

We present a light-front determination of the pionic contribution to the nucleon self-energy, Σ(π), to second order in pion-baryon coupling constants that allows the pion-nucleon vertex function to be treated in a model-independent manner constrained by experiment. The pion mass μ dependence of Σ(π) is consistent with chiral perturbation theory results for small values of μ and is also linearly dependent on μ for larger values, in accord with the results of lattice QCD calculations. The derivative of Σ(π) with respect to μ(2) yields the dominant contribution to the pion content, which is consistent with the d[over ˉ]-u[over ˉ] difference observed experimentally in the violation of the Gottfried sum rule.     

Contribution of the σ meson exchange to pionic double charge exchange reaction

The contribution of the σ meson exchange to the pionic double charge exchange (DCX) reaction is investigated. A concrete calculation on the forward excitation function of the low energy DCX reaction ¹⁴C(π⁺, π⁻)¹⁴O has been performed. It shows that the contribution of the σ meson exchange can reproduce well the resonance like excitation function of low energy DCX reaction. Received: 5 August 1998     

The pion–nucleon scattering lengths from pionic hydrogen and deuterium

This is the final publication of the ETH Zurich–Neuchatel–PSI collaboration on the pionic hydrogen and deuterium precision X-ray experiments. We describe the recent hydrogen 3p–1s measurement, report on the determination of the Doppler effect correction to the transition line width, analyze the deuterium shift measurement and discuss implications of the combined hydrogen and deuterium results. From the pionic hydrogen 3p–1s transition experiments we obtain the strong-interaction energy level shift eV and the total decay width eV of the state. Taking into account the electromagnetic corrections we find the hadronic s-wave scattering amplitude for elastic scattering and for single charge exchange, respectively. We then combine the pionic hydrogen results with the 1s level shift measurement on pionic deuterium and test isospin symmetry of the strong interaction: our data are still compatible with isospin symmetry. The isoscalar and isovector scattering lengths (within the framework of isospin symmetry) are found to be and , respectively. Using the GMO sum rule, we obtain from a new value of the coupling constant () from which follows the Goldberger–Treiman discrepancy . The new values of and imply an increase of the nucleon sigma term by at least 9 MeV. Received: 20 April 2001 / Revised version: 6 July 2001 / Published online: 24 August 2001     

The pion-nucleus many-body problem

Low energy pion-nucleus elastic scattering is discussed within the framework of many-body theory. In these considerations exchange of virtual ρ ad π mesons, as well as the π meson, are found to play an important role, producing effects like the Ericson-Ericson Lorentz-Lorenz effect. By inclusion of interactions in channels other than the pionic one, the pion-nucleus problem is brought into the context of other nuclear many-body problems, and various relations to the nucleon problem made use of .Some of the necessary parameters here are drawn from pionic atom work. With these and our deduced interactions the theoretically predicted cross-sections are found to be in good agreement with the experimental work.     

Irreducible pionic effects in nucleon-deuteron scattering below 20 MeV

The consequences of a recently introduced irreducible pionic effect in low-energy nucleon-deuteron scattering are analyzed. Differential cross-sections, nucleon (vector) and deuteron (vector and tensor) analyzing powers, and four different polarization transfer coefficients are considered. This 3NF-like effect is generated by the pion exchange diagram in presence of a two-nucleon correlation and is partially cancelled by meson retardation contributions. Indications are provided that such type of effects are capable to selectively increase the vector (nucleon and deuteron) analyzing powers, while in the considered energy range they are almost negligible on the differential cross-sections. These indications, observed with different realistic nucleon-nucleon interactions, provide additional evidences that such 3NF-like effects have indeed the potential for solving the puzzle of the vector analyzing powers. Smaller but non-negligible effects are observed for the other spin observables. In some cases, we find that the modifications introduced by such pionic effects on these spin observables (other than the vector analyzing powers) are significant and interesting and could be observed by experiments. Received: 21 November 2001 / Accepted: 18 March 2002     

Virtual and real pions in nuclei

The role of pionic degrees of freedom in photoreactions on nuclei is investigated. It is shown that the enhancement of the nuclear photoabsorption dipole cross section, given by exchange effects due to virtual pions, is accompanied by a parallel, almost compensating, hindrance with respect to the free case in the photoproduction of real pions above threshold.     

Structure of the pairing interaction in the two-dimensional Hubbard model

Dynamic cluster Monte Carlo calculations for the doped two-dimensional Hubbard model are used to study the irreducible particle-particle vertex responsible for dx2-y2 pairing in this model. This vertex increases with increasing momentum transfer and decreases when the energy transfer exceeds a scale associated with the Q=(pi, pi) spin susceptibility. Using an exact decomposition of this vertex into a fully irreducible two-fermion vertex and charge and magnetic exchange channels, the dominant part of the effective pairing interaction is found to come from the magnetic, spin S=1 exchange channel.     

First Precision Spectroscopy of Pionic Atoms at RI Beam Factory

We conducted an inclusive missing-mass spectroscopy of ¹²²Sn (d, ³He) reaction near the π¯ emission threshold at an incident energy of T _d = 250 MeV/nucleon. The experiment sets its goals to the research and development of the high precision spectroscopy of pionic atoms at the RI beam factory (RIBF) of RIKEN, which precedes a new project, pionic atom factory project (piAF), to observe pionic atoms systematically. Here, we report the preliminary results of the pilot experiment.     

Pionic transitions as tests of the connection between current and constituent quarks

A proposed connection between current and constituent quarks is discussed and tested through comparison with the magnitudes and signs of amplitudes for pionic transitions between hadrons.     

A model for vector-vector-tensor meson coupling constants

By the joint assumptions of vector-meson dominance for the electromagnetic current and tensor-meson dominance for the energy-momentum tensor a model for the vector-vector-tensor meson vertex is developed, including also the coupling of two photons to a tensor meson and the coupling of one photon to the vector-tensor meson system. In its simplest possible form the model has no undetermined parameters. Tentatively the vertex is applied to compute a tensor-meson exchange contribution to vector-meson scattering from an unpolarized target, and s-channel helicity conservation is found at large energies.     

E2 nuclear resonance effect in pionic atoms

The quadrupole nuclear resonance effect in pionic atoms is considered, and the results of calculations for pionic atoms with distorted even-even nuclei are presented.     

Neutron pick-up pion transfer reactions for the formation of deeply bound pionic atoms in 208Pb

We study (n, d) and (d, ³He) reactions on ²⁰⁸Pb leading to deeply bound pionic atoms with one neutron hole being left. We develop a theoretical model to calculate cross sections of deeply bound pionic atoms and quasi-elastic pion production. The results are compared with data which have been obtained so far. We show that the theoretical model describes the data of both reactions well.