Torleif Erik Oskar Ericson

Pions and Nuclei

The pion has emerges as the main feature of nuclear structure beyond the traditional description in terms of neutrons and protons. It manifests itself in a number of areas which are normally only loosely interlinked, but intimately related to the pion-nucleon and pion-nuclear interactions: the nucleon-nucleon force; the nuclear many-body problem; nuclear electromagnetic and weak interactions; nuclear spinisospin interactions; pion-nucleus scattering and reactions; etc. This book is a systematic introduction to and survey of nuclear pion physics, a major sub-field of nuclear pion physics. The theoretical foundations are padagogically developed and the physical picture is illustrated with supporting experimental examples.

Optical properties of low-energy pions in nuclei

Abstract A simple nonlocal potential for low-energy pions in finite nuclei is calculated from the amplitudes for πN scattering and for π production in NN collisions. The potential includes absorption and has no free parameters. The appropriate multiple scattering equations are derived in the coordinate representation with nuclear pair correlations included. Owing to the large mass of the scatterers the pion field behaves nearly classically. It is shown that short-range pair correlations are important in the multiple scattering owing to the dominant dipole component in the πN scattering. To a good approximation this produces a Lorentz-Lorenz effect analogous to the one occurring in the scattering of electromagnetic waves in dense media. The contributions of Fermi motion to the potential are shown to be small. The isospin part of the potential is derived and is shown to have a tensor component in addition to the ordinary vector one. The former gives rise to direct double charge exchange of pions; its strength becomes important for high momentum pions. The potential is further found to have a term which gives a small but possibly observable hyperfine coupling. A comparison is made between predictions of the potential and experimental data on level shifts and widths for π mesic atoms. Satisfactory agreement is found within experimental uncertainties which is quantitative evidence for nuclear pair-absorption of pions. There is also some indication of short range anticorrelations between nucleons.

A theory of fluctuations in nuclear cross sections

A quantitative description of the statistical fluctuations in the region of overlapping resonances of nuclear cross sections is given. General nuclear reaction theory is used in order to systematically obtain a separation of direct interactions and compound nuclear contributions in the region of overlapping resonances. (R.E.U.)

The deuteron asymptotic D-state as a probe of the nucleon-nucleon force

Abstract A critical discussion is given of the various contributions to the asymptotic D/S ratio from the πNN interaction. Particular emphasis is laid on the analysis of contributions from 2π exchange, the dependence on the πNN form factor, and on the πN coupling constant, so as to assess quantitatively the model dependence of the result. From this analysis, significant restrictions are deduced on the shape of the πNN form factor and, in quark bag models, the value of the quark bag radius. The results of previous theoretical approaches for the D/S ratio and quadrupole moment are shown to obey a universal and linear relation, when expressed in reduced units so as to compensate for small differences in effective range and πN coupling constant. The quadrupole moment is also quantitatively analyzed in detail. Its value is proved to be an accurate and almost model-independent consequence of the experimental D/S ratio (apart from relativistic and meson-exchange corrections) in agreement with observations. In the comparison with experiments, a critical survey of the relevant experimental data is given. In particular, a considerably improved value for the quadrupole moment is deduced.

Determination of the pion{nucleon coupling constant and scattering lengths

c =4 =0 :0786(11). This value is intermediate between that of indirect methods and the direct determination from backward np dierential scattering cross sections. We also use the pionic atom data to deduce the coherent symmetric and antisymmetric sums of the pion{proton and pion{ neutron scattering lengths with high precision, namely (a p + a n)= 2=( 172(statistical)8 (systematic))10 4 m 1 and (a p a n)=2 = (9003(statistical)13 (systematic))10 4 m 1 . Report-no: CERN/TH 2000-166,TSL/ISV-2000-0232, LPNHE/LPTPE 2709-0011, ADP-0036/T419

Strength of pion exchange in hadronic molecules

Abstract We discuss the physics of pion exchange in forming hadronic molecules. The notion of critical mass — the minimal reduced mass required for molecule formation — is introduced. The critical mass is estimated using scaling arguments based on the deuteron. We also discuss the role of tensor forces in such molecules.

Evidence Against the Existence of a Low Mass Scalar Boson from Neutron-Nucleus Scattering

Abstract The existence of a weakly-coupled scalar boson, recently proposed to explain the apparent discrepancy in X-rays from muonic atoms is shown to be inconsistent with the angular distribution measurements in low energy neutron-nucleus scattering. Other negative evidence from various different physical situatiions is also briefly reviewed.

Charge dependence of nuclear forces

Abstract The observed charge dependence in the singlet NN scattering lengths ( a −a np = 5.5 fm ) is explained in terms of one- and two-pion exchange mechanisms as well as (γπ) exchanges.

A New Effect in Pionic Atoms

Abstract We show that a likely explanation of the anomalous energy shifts in heavy pionic atoms is the energy dependence of the s-wave πN scattering amplitudes. This leads to an energy dependence of the π -nuclear optical potential. As a result, a non-trivial Coulomb term appears which leads to a remarkably better agreement with experiments without additional free parameters. An anomaly persists for the widths.

The Deuteron Properties

Abstract The low energy deuteron parameters are discussed in view of their physical information and their constraints on the description of the deuteron in terms of more exotic subsystems. Important recent progress has recently been achieved on the understanding of the root mean square radius, the quadrupole moment, the D S parameter and the asymptotic S state amplitude. There is a strong model-independent interlinkage of various quantities, which holds independent of the detailed model. Strong evidence is found for quantitative dominance of pionic interactions, particularly for the D S ratio and quadrupole moment.