Mesons In Nuclei Research Articles

Coherent contributions of nuclear mesons to electroproduction and the HERMES effect

We show that nuclear σ, ω, and π mesons can contribute coherently to enhance the electroproduction cross section on nuclei for longitudinal virtual photons at low Q 2 while depleting the cross section for transverse photons. We are able to describe recent HERMES inelastic lepton–nucleus scattering data at low Q 2 and small x using photon–meson and meson–nucleus couplings which are consistent with (but not determined by) existing constraints from meson decay widths, nuclear structure, deep inelastic scattering, and lepton pair production data. We find that while nuclear-coherent pion currents are not important for the present data, they could be observed at different kinematics. Our model for coherent meson electroproduction requires the assumption of mesonic currents and couplings which can be verified in separate experiments. The observation of nuclear-coherent mesons in the final state would verify our theory and allow the identification of a specific dynamical mechanism for higher-twist processes.

Pion-induced transport of π mesons in nuclei

A large body of data for pion-induced neutral pion continuum spectra spanning outgoing energies near 180 MeV shows no dip there that might be ascribed to internal strong absorption processes involving the formation of {delta}'s. This is the same observation previously made for the charged pion continuum spectra. Calculations in an intranuclear cascade model or a cascade exciton model with free-space parameters predict such a dip for both neutral and charged pions. We explore several medium modifications to the interactions of pions with internal nucleons that are able to reproduce the data for nuclei from {sup 7}Li through Bi. (c) 2000 The American Physical Society.

Mesons in nuclei in the light-front mean field approximation and deep-inelastic scattering

A light-front treatment for the scalar and vector meson momentum distribution functions is developed using a model in which the nucleus is treated a static source of radius $R.$ The limit $\stackrel{\ensuremath{\rightarrow}}{R}\ensuremath{\infty}$ corresponds to infinite nuclear matter. In this limit, the scalar mesons are shown to carry a vanishing momentum fraction of the nuclear plus momentum ${k}^{+},$ but the vector mesons carry a significant fraction, all occuring at the experimentally inaccessible value of ${k}^{+}=0.$ This confirms earlier work. A study of the R dependence for sizes of real nuclei shows that a fraction of the scalar and vector mesons could be observable.

Mesonic and binding contributions to the EMC effect in a relativistic many-body approach

We revise the conventional nuclear effects of Fermi motion, binding and pionic effects in deep inelastic lepton scattering using a relativistic formalism for an interacting Fermi sea and the local density approximation to translate results from nuclear matter to finite nuclei. In addition we also consider effects from ϱ-meson renormalization in the nucleus. The use of nucleon Green functions in terms of their spectral functions offers a precise way to account for Fermi motion and binding. On the other hand the use of many-body Feynman diagrams in a relativistic framework allows one to avoid using prescriptions given in the past to introduce relativistic corrections in a nonrelativistic formalism. We show that with realistic nucleon spectral functions and meson nucleus self-energies one can get a reasonable description of the EMC effect for x > 0.15, outside the shadowing region.

Concept of Excessive Mesons in Nuclei

The concept of excessive mesons in nuclei is discussed. Since nucleons are identical particles, it is pointed out that the physical content of a meson distribution around a bound nucleon in nuclei is different from that of a meson cloud around a free nucleon, if the effect of the principle of identity cannot be neglected. The difference is enunciated in detail.

Meson-Nucleon Interaction and Excessive Mesons in Nuclei

The effect of excessive mesons on the meson-nucleon interaction in nuclei is studied. It is shown that even if nonlinear meson interactions are considered, excessive mesons exist in the nuclear ground state (GS) and the GS expectation value of the meson-nucleon interaction Hamiltonian is exactly equal to zero if there are no excessive mesons.

K+-nucleus quasielastic scattering.

K$^+$--nucleus quasielastic cross sections measured for a laboratory kaon beam momentum of 705 MeV/$c$ are presented for 3--momentum transfers of 300 and 500 MeV/$c$. The measured differential cross sections for C, Ca and Pb at 500 MeV/$c$ are used to deduce the effective number of nucleons participating in the scattering, which are compared with estimates based on the eikonal approximation. The long mean free path expected for K$^+$ mesons in nuclei is found. Double differential cross sections for C and Ca are compared to relativistic nuclear structure calculations.

Inter-meson potentials from the constituent quark model

The interaction of mesons is modelled as nonrelativistic pointlike particles interacting via effective potentials. The effective potentials are obtained by equating the Born order T-matrix for a given process as calculated with the constituent quark model to an effective pointlike T-matrix. Comparison with nonperturbative computations shows the method to be surprisingly accurate. The results are useful in predicting final state interaction effects in hadronic production reactions and in testing for possible mesonic nuclei. In particular, it is argued that the a 0(980) and the f 0(975) are K K molecules, the f 0(1720) is a K ∗ K ∗ molecule, and that the f 1(1420) (the E) may be a K ∗ K threshold enhancement. Furthermore, short range effects due to quark exchange are not enough to create a bound ΠΠ-ωω state (the X 2(1515)) or a bound φφ state (the f 2(2010)). The model also predicts that the quark exchange portion of low energy isotensor tensor ΠΠ scattering is dominated by confinement effects and has a short range attractive core with an intermediate range repulsion. Furthermore, the predicted phase shifts are not strongly dependent on the specific form of the confinement potential.

Compton scattering by mesons in nuclei: Experiment on 208Pb

Using 58 and 73 MeV tagged photons and scattering angles from 60° to 150°, it is shown that it is possible to observe Compton scattering by “mesons in nuclei” through an incomplete cancellation of the mesonic (exchange-current) seagull amplitude by parts of the nuclear resonance amplitude related to the giant-dipole resonance of 208Pb. This phenomenon is a property of an extended nucleus and, therefore, cannot be studied on the deuteron. Predictions of the exchange from factor which determines the angular distribution of the exchange seagull amplitude are compared with experimental data.

Meson exchange currents in few-nucleon systems and radiative capture of neutrons

Three beams of the high flux reactor HFR in Petten are being used for nuclear physics research primarily aiming at a test of the validity of models describing mesons in nuclei. Mesonic exchange currents (MEC) manifest themselves in the cross section as well as in the channel spin interference of radiative capture of neutrons in H, D and 3He. Experiments with polarized neutrons and polarized target nuclei clearly indicate the existence of MEC. In the low cross section reaction 3He(n, gamma ), which partly proceeds by double-photon emission, regular internucleon contributions are much more suppressed than MEC.

From ‘mesons in nuclei’ to ‘quarks in nuclei’

The mesonic aspects of nuclear magnetic moments are discussed with the author's personal proximity and reminiscence. It is emphasized that the observed anomaly in orbitalg factors, traditionally related to the meson exchange effect, may have to be reconsidered from the viewpoint of quark structure of nuclei.

From Collective States to Quarks in Nuclei

H Arenhovel and A M Saruis (eds) 1981 Berlin: Springer vii + 414 pp price DM47 (paperback) It is appropriate that volume 137 of the Springer lecture notes series should be devoted to a conference (Bologna, 1980) on nuclear physics with real and virtual photons. The main topics are: (i) the nucleon-nucleon interaction and the many-body problem; (ii) collective phenomena; (iii) baryons and mesons in nuclei; (iv) quarks in nuclei; and (v) experimental facilities.

Quarks and mesons in nuclei

Meson and quark degrees of freedom in finite nuclei and in the nuclear matter are discussed in the light of the recent progress made in the theory of strong interactions.

Mesons and quarks in nuclei

A short review of the topic of mesons in nuclei is exposed paying particular attention to the relationship between several mesonic processes. Special emphasis is put into the microscopic pictures that can ultimately relate all these processes with the elementary coupling of mesons to the nuclear hadronic components. The importance of the short range part of the nuclear interaction opens the doors to a more basic understanding in terms of the quark components of nucleons and isobars.

Collision broadening in particle physics

The lifetime of a particle resonance, such as an N ∗, propagating through a nucleus or in the vicinity of a ‘spectator’ nucleon is reduced by collisions, and consequently the width of the resonance is increased. This phenomenon is the nuclear analogue of collision broadening, well known in atomic spectroscopy. It explains quantitatively the discrepancy between the πd total cross section in the resonance region and Glauber theory, a discrepancy previously ascribed to variation with energy of the parameter 〈 r −2〉. A simple rule is given for the energy of πd resonances relative to πN. The same arguments applied to Kd total cross sections indicate that the KN branching ratios of Y ∗ resonances should be systematically increased over previously quoted results. It is shown how the channel πd → NN is related to Glauber theory, and an anomalous result obtained by Beder for the πd scattering length is explained away. A general rule is proposed for the shape of the N ∗(1230) in a nucleus of atomic number A, namely that the product of the width and the height of the resonance should be A times that for free πN collisions. In production reactions, absorption in the final state makes the resonance width increase with the momentum transfer, − t, at which the resonance is produced; the reaction π −p → ϱ 0n is treated quantitatively as an example. The broadening of ϱ mesons photoproduced in nuclei is also mentioned. Large collision broadening offers an explanation of the observation that the total cross section of A 1 mesons in nuclei is close to that of pions, and offers fresh insight into the phenomenon of diffraction dissociation. Two modifications to Glauber theory are given for the lifetime of a resonance in the final or intermediate state. It is shown that the usual ad hoc procedure of folding Fermi motion into the amplitude is exactly equivalent to including the propagator of the resonance.

The K − nucleon interaction in K mesic atoms

A study of the K − nucleon interaction is made as it pertains to the absorption of K − mesons in nuclei. Effective interaction strenghts are derived which yield good agreement with recent X ray experiments that determine both the strong interaction energy shifts and widths.

A phenomenological approach for a π− meson nucleus potential

The observed level shifts and widths of π-mesic atoms are described in terms of an optical potential between the pion and the nucleus. A special form of this potential is proposed: herein the microscopic picture of the scattering and absorption of pions by bound nucleons served as a guideline; the parameters, however, are choosen freely to give a best fit to the experimental data. It turns out that only three parameters are necessary to describe satisfactorily the available data for all known 2p-1s and 3d-2p transitions.

Evidence of Nuclear Structures fromK−-Meson Absorption

The characteristics of the multinucleon capture of ${K}^{\ensuremath{-}}$ mesons in nuclei have been investigated in both normal and water-soaked $G5$ nuclear emulsion. It has been found that the emission of fast (⋝ 60 MeV) hyperons is more often associated with ${K}^{\ensuremath{-}}$ captures in light (C,N,O) than in heavy (Ag,Br) nuclei. Further, considering the relative absorptions of $\ensuremath{\Sigma}$ hyperons in the parent nucleus in which they are produced, we inferred that the rate of multinucleon ${K}^{\ensuremath{-}}$ absorptions is approximately the same for all nuclei of nuclear emulsion and corresponds to \ensuremath{\sim}17% of all ${K}^{\ensuremath{-}}$ absorptions at rest. From those events which can be definitely assigned to interactions in light nuclei, by virtue of energy and momentum conservation, it was found that the reaction ${K}^{\ensuremath{-}}+(n, n)\ensuremath{\rightarrow}{\ensuremath{\Sigma}}^{\ensuremath{-}}+n$ is of comparable intensity with the reaction ${K}^{\ensuremath{-}}+(n, p)\ensuremath{\rightarrow}{\ensuremath{\Sigma}}^{\ensuremath{-}}+p$, and that in light nuclei, the major mode of multinucleon ${K}^{\ensuremath{-}}$ absorption in C,N,O appears to be on a pair of nucleons in an $\ensuremath{\alpha}$-particle cluster. Comparison of the characteristics of the ${K}^{\ensuremath{-}}$ multinucleon absorptions in light and heavy nuclei shows that $\ensuremath{\alpha}$-particle clusters are less likely to be involved in (Ag,Br) than in (C,N,O). An attempt to estimate the nuclear pair correlation function was also made.