Pion-nucleus Interaction Research Articles

S-wave pion-nucleus interaction and weak coupling constants.

The chiral expansion of the off-shell \ensuremath{\pi}N amplitude suggests that the s-wave interaction with the nucleus of spacelike pions is appreciably repulsive. This has important consequences for the weak hadronic nuclear current, which are discussed here.

Coherent pion production in the ( [formula omitted]) reaction in nuclei

We have evaluated the cross section for coherent pion production in the 3 He + 12 C → t + 12 C + π + reaction. The Δ peak of the energy distribution is considerably shifted with respect to the peak in the p( 3 He,t)Δ ++ reaction. The coherent pions represent a sizable fraction of all pions produced and are a major ingredient in the interpretation of the shift of the Δ peak in the inclusive ( 3He,t ) reaction in nuclei. The coherent production process qualifies as production of virtual pions followed by elastic scattering of the virtual pions with the nucleus and the investigation of this channel will offer information about pion nucleus interaction complementary to the one obtained from scattering of real pions.

Exotic states of hadronic atoms

Two exotic regions of hadronic atoms/nuclei which are not accessed at all by atomic x-ray spectroscopy are discussed. The extreme inner region of pionic atom is predicted to be “halo nuclear states” of narrow widths due to the repulsive pion-nucleus interaction, and can be formed by various “pion transfer” reactions. In the extreme outer region, where the negative hadron occupies the same spatial region as the atomic electrons, new features of “halo atomic states” of large angular momenta have been indicated from recently discovered metastable states of π−, K−, and p̄ in helium media.

Low-energy pion-nucleus interaction and nuclear relativism

The s-wave pion-nucleus repulsion problem may be solved by taking into account nuclear relativism within the Walecka model.

Multiplicity and angular distribution of charged pions emitted following [formula omitted] annihilation on Fe at low energy

The mean multiplicity and the angular distribution of the charged pions produced in the n̄ annihilation on Fe in the range from 12 to 140 MeV is presented. The experimental technique and methods of analysis are described. A comparison with the p̄ results is performed, together with a discussion of some possible pion-nucleus interaction mechanisms in the above n̄ energy range.

Problems of pion-nucleus interaction

The review of new theoretical and experimental results in the field of the pion-nucleus interaction which are presented in the contributed papers at the IV International symposium “Mesons and Light Nuclei” (Bechyně, 1988) is given. The discussion covers a wide area of problems from the π-nucleon interaction to the pion interaction with light nuclei at low and intermediate energies.

Inclusive pion-nucleus double charge exchange.

Inclusive double charge exchange reaction is studied by using a model which combines a microscopic many-body calculation of reaction probabilities with a Monte Carlo simulation procedure. Comparison with present experimental results requires that the (\ensuremath{\pi},2\ensuremath{\pi}) reaction be included and a proper evaluation of this latter reaction is also done. The combined results compare favorably with experiment, giving extra support to this model of the pion-nucleus interaction which was previously shown to provide a good description of all other inclusive reactions and elastic scattering.

Deeply bound pionic states of heavy nuclei

We calculated the pionic bound-state spectra of heavy nuclei from the lowest 1s orbits using the standard pion-nucleus optical potential. The energy levels of the s-orbits (l=0) are shifted upward largely by the repulsive S-wave pion-nucleus interaction, which also reduces the fraction of π− staying inside the nucleus. Thus, even for π−−208Pb, the deeply bound states have small enough level widths so as to be considered as quasi-stable states. We propose possible methods for the creation and detection of deeply bound pionic states of heavy nuclei.

Exchange currents in pion-induced deuteron breakup

Exchange-current contributions in pion-nucleus reactions are in general small due to the nature of the effective pion-nucleus interaction. The reactionπ−d→π−np near the deuteron breakup threshold provides an exception. The isovector exchange current should give the largest contribution to this cross section for momentum transfer values between3 and 4 fm−1

Pion-nucleus single-charge exchange above the Δ resonance

Pion-nucleus single-charge exchange to the isobaric analogue state in the energy region from 300 MeV to 500 MeV is investigated utilizing a microscopic, momentum-space, isospin invariant optical potential. The optical potential utilizes fully relativistic kinematics, and the Fermi integration is performed exactly. The model used for the pion-nucleus interaction includes a new model for the resonant D-waves. Our results are compared to recent data for 0° excitation functions for the isobaric analogue state transitions in 14C and 90Zr. The observed rise in the cross section for energies above the resonance is reproduced by the theory, and, although not totally quantitative, the theoretical results indicate that the multiple scattering theory as defined and implemented here is significantly more convergent at the higher energie than at or below the resonance.

Progress and problems in understanding the pion-nucleus interaction

The state of the theoretical understanding of the pion nucleus interaction is discussed. Quantitative understanding of one-nucleon processes has been achieved. Basic questions concerning the two-nucleon processes of double charge exchange, pion absorption and the phenomenon of low-energy repulsion remain open.

Pion condensation in symmetric nuclear matter

Using a model which is based essentially on the chiral SU(2)×SU(2) symmetry of the pion-nucleon interaction, we examine the possibility of pion condensation in symmetric nucleon matter. We find that the pion condensation is not likely to occur in symmetric nuclear matter for any finite value of the nuclear density. Consequently, no critical opalescence phenomenom is expected to be seen in the pion-nucleus interaction.

Chiral symmetry and pion propagation in nuclear matter

In the framework of a chiral-invariant effective Lagrangian theory, the authors have discussed the propagation of a pion through an infinite nuclear medium with special emphasis on the role of the Delta (1232). In particular, the full spin-3/2 projector operator and the most general form of the pi N Delta interaction have been used in order to evaluate the Delta contribution to the pion self-energy. On the basis of an investigation into this problem, they confirm certain a priori assumptions regarding the effects of Delta (1232), which are seen very often in the literature. However, some of the results are at variance with the usual forms for the calculation of the Delta contribution to the pion-nucleus interaction, reflecting the fact that the off-mass-shell effects of the Delta (1232) are appreciable.

Nuclear bound states of the eta 0 meson and pion double-charge-exchange reactions.

We predict a narrow resonance structure (\ensuremath{\Gamma}\ensuremath{\simeq}10 MeV) in the excitaton function of (${\ensuremath{\pi}}^{+}$,${\ensuremath{\pi}}^{\mathrm{\ensuremath{-}}}$) double-charge-exchange reactions at a pion kinetic energy of \ensuremath{\sim}419 MeV. This structure is due to the interference between the usual ${\ensuremath{\pi}}^{+}$\ensuremath{\rightarrow}${\ensuremath{\pi}}^{0}$\ensuremath{\rightarrow}${\ensuremath{\pi}}^{\mathrm{\ensuremath{-}}}$ double-charge-exchange amplitudes, which have a smooth energy dependence, and a resonant ${\ensuremath{\pi}}^{+}$\ensuremath{\rightarrow}${\ensuremath{\eta}}^{0}$\ensuremath{\rightarrow}${\ensuremath{\pi}}^{\mathrm{\ensuremath{-}}}$ high-energy double-charge-exchange amplitude associated with the formation of a strongly bound \ensuremath{\eta}-nucleus system. The latter process leads to a novel feature of the pion-nucleus interaction, the mesonic compound nucleus resonance.

The isoscalar pion-nucleus interaction from pionic atoms

The analysis of pionic atom data of 1s and 2p levels for N = Z spin-zero nuclei, in terms of an improved optical potential is presented. We find a strong (unexplained) s-wave repulsion and evidence for finite-range effects. The need for a relativistic treatment of the pion—nucleus interaction is discussed.

Strong Interaction Studies via Meson-Nucleus Reactions

The role of meson-nucleus reaction studies in the investigation of strong interactions is surveyed. In the first part, the pion-nucleus interaction in the region of the 3-3 resonance is discussed. Success and limitations of the Ll-h formalism which is based on the impulse approximation are examined. A consistent picture of a large variety of one-nucleon process­ es has been obtained and will be presented as well as failures in the basic understanding of two-body processes. In the second part, the various possibilities for specific strong inter­ action studies with particularly selected hadronic systems are demonstrated. The cases of K+ -nucleus interactions to examine quantitatively the basic impulse approximation, of K--nucleus resonance interactions for the study of orbitally excited baryons and the cases of S*- and 77'-nucleus interactions to investigate hadronic interactions in the presence of exotic multiquark or gluonic components are presented. Quarks and gluons constitute the microscopic degrees of freedom in strong interaction physics. Quarks are the constituents of hadronic matter, gluons are the carriers of the strong force. Nucleons and mesons constitute the effective degrees of freedom in terms of which nuclear structure and nuclear reactions are traditionally described. The relation between the microscopic and the effective degrees of free­ dom has not been established in nuclear physics. Neither is the success of the traditional description of nuclear phenomena properly understood, nor are its limita­ tions defined in precise terms. The central concept of the traditional description of nuclear reactions is that of the impulse-approximation. In the impulse approximation, the hadron-nucleus inter­ action is calculated in terms of the experimentally determined corresponding hadron­ nucleon interaction on the one hand and in principle well defined many .. body correc­ tions on the other. By construction, the impulse approximation becomes exact in the low density limit irrespective of the underlying hadron-nucleon dynamics. This is clearly one of the reasons for the success of such an approach in terms of effective degrees of freedom and phenomenological interactions. Within this framework, the microscopic degrees of freedom can show up only in the medium modification to the free hadron-nucleon interaction, i.e., only at the level where standard many-body corrections appear. To assess the validity of this effective description of nuclear reactions is possible only if this description has been carried through beyond the

The relativistic stripping model of (p, π+) reactions in the Δ resonance region

The relativistic stripping model is presented for the (p, π +) reaction in the Δ-resonance region. The Dirac impulse approximation and a Dirac-Hartree calculation are used for the proton nucleus interaction and neutron bound state wave function respectively. The Δ-hole model is employed for the pion-nucleus interaction. This model is applied to the reaction 16O(p, π +) 17O(g.s.).

A decay in nuclei

We briefly present a many body method used to study the Λ decay in a nucleus which provides simultaneously the mesonic and nonmesonic Λ width. The virtual pions in the ΛN → NN transition amplitude as well as the real pions from the Λ→N π decay are allowed to interact with the nuclear medium. In the case of the nonmesonic decay the effect of this renormalization is moderate but makes the results more stable under changes of unknown correlation parameters. In the case of the mesonic decay the proper care of the pion nucleus interaction results in considerable enhancements of the mesonic Λ width rendering this magnitude a useful tool for learning about properties of the pion nucleus interaction not shown by the pion nuclear scattering experiments.

Quantum-field aspects of pion interaction with lightest nuclei

Recent development in the quantum field theory approach to the pion-deuteron and pion-aHe (3 H)-interaction processes is reviewed. Some general features of the Bethe-Salpeter and Low-type equations for the processes under study derived in the last few years in the literature are discussed. At present the fundamental importance of studying the pion-deuteron or more generally the pion -- two-nucleon interaction is well understood. The nNN system is often considered as a "theoretical laboratory" for investigating different aspects of the pion-nucleus interaction. In the first stage of investigation of pion-deuteron scattering, carried out predominantly in the energy range 0--300 MeV, the multiple scattering theory has been used, where the many-channel character of the problem is neglected, particularly, the deuteron disintegration channel and the channel of pion absorption by a deuteron are neglected. The necessity of the account of such channels and relativity of pions in the energy region under consideration leads to the use of Faddeev integral equations or their generalization in the case where the relativistic kinematics of pions and pion absorption or emission by a two-nucleon system are taken into account. This type of the theory [1, 2] for the coupled nNN and NN-systems formulated in the framework of the general coupled channel theory for the pion-nucleous interaction [3] by using the projection operator method [4] is expounded (with some generalization) in [5], where other references to the subject can be found. As regards the problem of pion interaction with the simplest (after deuteron) nuclei SHe and 3H from the theoretical point of view it is primarily interesting as a tool for verifying an understanding of the dynamics of the pion-nucleus interaction resulting from an investigation of the pion two-nucleon interaction. But it is clear that study of the zc 3He(3H)-interaction per se is also interesting not only for understanding the dynamics of these processes but to get additional information on the three-nucleon forces. In [6] a successful attempt was made to generalize the multichannel theory of nNN and NN systems [1, 2] to the case of pion interaction with the 3Henucleus. It is worth-while to note that in spite of a sufficient generality of these considerations the antiparticle degree of freedom is neglected and the crossing symmetry of the problem is not taken into account. The importance of allowing for this symmetry was demonstrated in [7] taking ~-12C-scattering at low energies as an example. As to the nN-scattering, the importance of allowing for crossing-symmetry is well known from the Chew-Low theory [8]. The two aforementioned effects are of quantum-field origin and therefore can be taken into account in the framework of the quantum field theory approach to the problem. Some general results obtained using this approach are discussed below. A consistent quantum-field theoretical approach was used in [2] to derive a set of equations for the reactions

The role of pion-nucleus interaction in the γ4He → π0 4He reaction

The role of pion-nucleus interaction in the γ4He → π0 4He reaction