Regular Series

Vol. 29 (1998), No. 9, pp. 2191 - 2560


Structure of Heaviest Nuclei

Acta Phys. Pol. B 29, 2191 (1998)

page 2191 •

abstract

Theoretical studies of the structure and the properties of heavy and superheavy nuclei are described. Such properties as mass and half-lives with respect to main decay modes are discussed. Even–even nuclei with proton number \(Z=82\)–120 and neutron number \(N=126\)–190 are considered. Main results obtained in recent years (in a macroscopic-microscopic approach) are illustrated.


Microscopic Structure of Charge-Exchange Spin–Isospin Modes Through Decay Measurements

Acta Phys. Pol. B 29, 2199 (1998)

page 2199 •

abstract

The study of particle decay of charge-exchange giant resonances (CEGR) furnishes information on their microscopic structure. Furthermore, \(\gamma \) decay of CEGR can help to unravel their structure. For example, the \(\gamma \)-decay of the Gamow–Teller (GT) resonance to the isobaric analogue state (IAS) points to a possible method to study quenching of GT strength. The study of \(\gamma \)-decay of the spin-flip dipole resonance (SDR) to GT and low-lying states will possibly allow to disentangle the different spin components. These aspects will be discussed in the light of recent (\(^{3}\)He,\(t\)) experiments at E(\(^{3}\)He) = 450 MeV and \(\theta = 0^{\circ }\) performed to study the proton decay of CEGR in \(^{208}\)Bi and \(^{12}\)N and the \(\gamma \) decay of the GT resonance, IAS and SDR in \(^{90}\)Nb.


Dynamics and Instabilities of Nuclear Fermi Liquid

Acta Phys. Pol. B 29, 2211 (1998)

page 2211 •

abstract

The kinetic theory of Fermi liquid is applied to finite nuclei. The nuclear collective dynamics is treated in terms of the observable variables: particle density, current density, pressure etc. The relaxation processes and the development of instabilities in nuclear Fermi-liquid drop are strongly influenced by the Fermi-surface distortion effects.


The Role of Fock Terms and Isovector Mesons in Relativistic Hartree–Fock Calculations for Neutron Rich Nuclei

Acta Phys. Pol. B 29, 2223 (1998)

page 2223 •

abstract

Density dependent relativistic Hartree–Fock theory has been extended to describe properties of exotic nuclei. The effects of Fock exchange terms and of \(\pi \)- and \(\rho \)-meson contributions are discussed. These effects are found to be more important for neutron rich nuclei than for nuclei near the valley of stability.


Effects of the Quasiparticle–Phonon Interaction in Magic and Non-Magic Nuclei

Acta Phys. Pol. B 29, 2231 (1998)

page 2231 •

abstract

The results of the recent \(^{40}\)Ca and \(^{58}\)Ni(\({\alpha ,\alpha '}\)) experiments at \(E_{\alpha }\! =\!\) 240 MeV are analyzed within a consistent microscopic model which takes into account both the effective particle–hole interaction and the quasiparticle–phonon one. A good agreement with the experiment for the isoscalar E0 EWSR was obtained for \(^{40}\)Ca. But due to using microscopic transition densities instead of phenomenological ones for \(^{58}\)Ni we obtained 71% of the EWSR instead of the noticeably less experimental value. Therefore, a part of the E0 strength may be hidden in the experimental background in the \(^{58}\)Ni experiments. An equation for the nuclear gap in non-magic nuclei which takes into account both a particle-particle interaction and the quasiparticle-phonon one has been obtained. The first results of its solution have been represented.


Configuration Mixing Effects in Isoscalar Giant Dipole Resonance

Acta Phys. Pol. B 29, 2239 (1998)

page 2239 •

abstract

Based on an explicit verification of the coupling matrix elements between the 1p1h and 2p2h states we propose a new method of selecting the most important 2p2h states responsible for fragmentation effects. In this way the dimensionality of the problem is reduced, such that the computation becomes feasible and the spreading of the strength is realistic, as verified by some tests of convergence. Calculations in \(^{208}\)Pb show that due to sizeable mixing effects only about \(50\%\) of the total isoscalar giant dipole resonance (ISGDR) \(3\hbar \omega \) strength is located in the energy region between 20 and 25 MeV. This is the energy region which currently is available in experiment. Even above 30 MeV we find about \(10\%\) of the total strength. This indicates that the current experimental evaluations of the ISGDR centroid energy may significantly underestimate its value.


Problems in Hypernuclear Physics

Acta Phys. Pol. B 29, 2245 (1998)

page 2245 •

abstract

The connection between the barion–barion interaction and the properties of hypernuclei, in particular \({\mit \Sigma }\) hypernuclei, is discussed. The inadequate accuracy of the so called low order Brueckner approximation is pointed out. The single particle \({\mit \Sigma }\) potential fitted to the pion spectrum measured in the \((K^-,\pi ^+)\) reactions appears to be repulsive. This eliminates some of the existing models of the barion–barion interaction.


The Rare \(\pi ^{+}\) Decay of \(^{4}_{\Lambda }\)He

Acta Phys. Pol. B 29, 2253 (1998)

page 2253 •

abstract

The \(\pi ^{+}\) emission from the weak decay of \(^{4}_{{\mit \Lambda }}\)He has been a puzzle for more than 30 years. We discuss the significance of two contributions to the decay rate that are due to pion charge exchange and due to a virtual \({\mit \Sigma }^{+}\) admixture of the initial \({\mit \Lambda }\)-hypernuclear state.


Weak Radiative Decays of Hyperons

Acta Phys. Pol. B 29, 2259 (1998)

page 2259 •

abstract

Weak radiative hyperon decays are briefly reviewed. We discuss the conflict between expectations based on Hara’s theorem on one side and experiment, quark, and VMD models on the other side. Two recent arguments against the interpretation of quark model results as a violation of Hara’s theorem are presented and their shortcomings are indicated. Phenomenological success of the VMD prescription is emphasized. It is stressed that the predictions of the VMD model are clear-cut and different from those of other approaches. The decisive role of the soon-to-be-processed results of the KTeV experiment on \( {\mit \Xi } ^0 \rightarrow {\mit \Lambda } \gamma \) and \( {\mit \Xi } ^0 \rightarrow {\mit \Sigma } ^0 \gamma \) asymmetries is pointed out.


full authors' list

S. Bianco, A.M. Bragadireanu, F.L. Fabbri, C. Guaraldo, M. Iliescu, T.M. Ito, V. Lucherini, C. Petrascu, M. Bregant, E. Milotti, A. Vacchi, E. Zavattini, M. Augsburger, D. Chatellard, P. Knowles, F. Mulhauser, L.A. Schaller, L. Schellenberg, H. Schneuwly, J.-P. Egger, D. Varidel, W.H. Breunlich, M. Cargnelli, B. Gartner, R. King, B. Lauss, J. Marton, J. Zmeskal, T. Ponta, S.N. Nakamura, T. Koike, R.S. Hayano, M. Hori, T. Ishikawa, T. Ishiwatari, M. Iwasaki, Y. Akaishi, G. Beer, A.C. Sanderson, R. Seki

Kaon–Nucleon Interaction Studied by Kaonic X Rays with DEAR at DA\(\Phi \)NE

Acta Phys. Pol. B 29, 2269 (1998)

page 2269 •

abstract

The scientific program and the experimental setup of the DEAR (DA\(\Phi \)NE Exotic Atom Research) experiment on the new \(\phi \)-factory DA\(\Phi \)NE of Laboratori Nazionali di Frascati, are presented. The objective of DEAR is to perform a \(1\%\) measurement of the shift, due to the strong interaction, of the \(K_\alpha \) line of kaonic hydrogen and a similar precision measurement, performed for the first time, on kaonic deuterium. The aim is to investigate the low-energy \(\bar KN\) physics and to understand SU(3) chiral symmetry breaking. DEAR looks as the major effort ever performed to study low energy \(\bar KN\) interaction, capable to produce a real breakthrough in the field.


Hard Photons Beyond Proton–Neutron Bremsstrahlung in Heavy-Ion Collisions

Acta Phys. Pol. B 29, 2277 (1998)

page 2277 •

abstract

We report on the study of extremely high energy photons, pions and etas, produced in intermediate energy heavy-ion collisions. Possibility of imaging the final-state phase space in these collisions by the Bose-Einstein correlations for photons is critically examined.


Energetic Particles and Gammas from Low-Energy Nuclear Reactions

Acta Phys. Pol. B 29, 2285 (1998)

page 2285 •

abstract

We address pre-equilibrium model mechanisms for the cluster emission and compare them for reactions below 200 MeV. The conclusion about the proper mechanism is not unique yet, however. For the \(\gamma \) emission, the main attention is devoted to the nucleon radiative capture. The data suggest a shift of the effective position of GDR together with a new indication of its energy-dependent width.


Azimuthal Anisotropy of Neutral Mesons in Heavy Ion Collisions at 2 \(A\) GeV

Acta Phys. Pol. B 29, 2293 (1998)

page 2293 •

abstract

Unexpected differences between azimuthal distributions of \(\eta \) and \(\pi ^{0}\) mesons emitted at midrapidity were observed in collisions of 1.9 \(A\)GeV \(^{58}\)Ni+\(^{58}\)Ni and 2 \(A\)GeV \(^{40}\)Ca+\(^{\rm nat}\)Ca. The observed elliptic flow signal is negative for \(\eta \) mesons, indicating a preferred emission perpendicular to the reaction plane. In contrast the elliptic flow for \(\pi ^{0}\) mesons was observed in peripheral Ni+Ni collisions only.


Effective Field Theories for Nuclei and Dense Matter

Acta Phys. Pol. B 29, 2297 (1998)

page 2297 •

abstract

A recent development on the working of effective field theories in nuclei and in dense hadronic matter is discussed. We consider two extreme regimes: One, dilute regime for which fluctuations are made on top of the matter-free vacuum; two, dense systems for which fluctuations are treated on top of the “vacuum” defined at a given density, with masses and coupling constants varying as function of matter density (“Brown–Rho scaling”). Based on an intricate — as yet mostly conjectural — connection between the in-medium structure of chiral Lagrangian field theory which is a beautiful effective theory of QCD and that of Landau Fermi liquid theory which is an equally beautiful and highly successful effective theory of many-body systems, it is suggested that a chiral Lagrangian with Brown–Rho scaling in the mean field is equivalent to Fermi-liquid fixed point theory. I make this connection using electroweak and strong responses of nuclear matter up to nuclear matter density and then extrapolating to higher densities encountered in heavy-ion collisions and compact stars.


Medium Dependence of the Vector-Meson Mass: Dynamical and/or Brown–Rho Scaling?

Acta Phys. Pol. B 29, 2309 (1998)

page 2309 •

abstract

We discuss the similarities and differences for the theories of Rapp, Wambach and collaborators (called R/W in short) and those based on Brown-Rho scaling (called B/R), as applied to reproduce the dileptons measured by the CERES collaboration in the CERN experiments. In both theories the large number of dileptons at invariant masses \(\sim \) \(m_\rho \,/\,2\) are shown to be chiefly produced by a density-dependent \(\rho \)-meson mass. In R/W the medium dependence is dynamically calculated using hadronic variables defined in the matter-free vacuum. In B/R scaling it follows from movement towards chiral symmetry restoration due to medium-induced vacuum change, and is described in terms of constituent (or quasiparticle) quarks. We argue that the R/W description should be reliable up to densities somewhat beyond nuclear density, where hadrons are the effective variables. At higher density there should be a crossover to constituent quarks as effective variables scaling according to B/R. In the crossover region, the two descriptions must be “dual”. For the moment there is a factor \(\gtrsim \) 2 difference between the predicted number of dileptons from the two theories, B/R scaling giving the larger number. We show that a substantial factor results because in B/R, fluctuation is made about the “vacuum” modified by density, so that a different mass \(m_\rho ^*\) appears in the Lagrangian for each density, thereby rendering residual interactions between hadrons weaker, whereas R/W calculate a mass \(m_\rho ^*\) for each density with an effective Lagrangian defined in the zero-density vacuum, which has the free \(m_\rho \) in the Lagrangian and hence the coupling is strong. Thus more diagrams need to be incorporated in R/W to reduce the discrepancy. On the other hand, R/W include processes which may be additional to these of B/R. These constitute several (smaller) corrections. It is argued that the \(N^*\)-hole state \([N^*(1520) N^{-1}]^{1^-}\) is almost completely \(\rho \)-meson like in content; i.e., it is, to a good approximation, just the state \(\rho \left |0\right \gt \) that would be produced by the \(\rho \)-meson field acting on the nuclear ground state (finite temperatures are not expected to disturb this picture by much).


Probing Chiral Symmetry Restoration with Heavy Ions

Acta Phys. Pol. B 29, 2323 (1998)

page 2323 •

abstract

It is discussed how chiral symmetry restoration manifests itself through mixing of vector and axial-vector correlators. The vector correlator is directly accessible in relativistic heavy-ion collisions. Within models of the vector correlator its implications for low-mass dilepton spectra are reviewed.


full authors' list

E.A. Pasyuk, C.L. Morris, J.L. Ullmann, J.D. Zumbro, L.W. Kwok, J.L. Matthews, Y. Tan

A Search for \({\mit \Delta }^-\) and \({\mit \Delta }^{++}\)Wave-Function Components in Light Nuclei Using the \((\pi ^+,\pi ^- p)\) and \((n,2p)\) Reactions

Acta Phys. Pol. B 29, 2335 (1998)

page 2335 •

abstract

We have studied the \((\pi ^+, \pi ^{\pm }p)\) reactions on \(^3\)H, \(^4\)He, \(^6\)Li and \(^7\)Li and the \((n,2p)\) reaction on \(^3\)He and \(^4\)He in quasi-free kinematics. A signature attributable to pre-existing \({\mit \Delta }\) components of the ground state wave function is observed.


Low Momentum Effective Theory for Few-Nucleon Systems

Acta Phys. Pol. B 29, 2339 (1998)

page 2339 •

abstract

Starting from a precise two-nucleon potential, I show how to use the method of unitary transformations to construct an effective potential that involves only momenta less than a given maximal value. I describe this method for an S-wave potential of the Malfliet–Tjon type. It is demonstrated that the bound and scattering state spectrum calculated within the effective theory agrees exactly with the one based on the original potential. Such a truely low momentum effective theory might pave the way for a consistent construction of effective chiral few-nucleon forces and for a systematic treatment of relativistic effects in few-body systems.


Chiral Symmetry Restoration and Parity Mixing

Acta Phys. Pol. B 29, 2349 (1998)

page 2349 •

abstract

We derive the expressions of the (isovector) vector and axial current from a chiral Lagrangian restricted to nucleons and pions. We show that in the heat bath certain terms can induce a mixing of the axial current into the vector one and vice versa. We generalize this concept to the case of a dense baryonic medium. We study the subsequent modifications of the axial nucleonic coupling constant and the pion decay one. They arise from a two pion exchange current of a new type. We discuss the link to the condensate evolution. The quenching of the axial coupling constant helps explaining the observed one of the Gamow–Teller sum rule.


Chiral Symmetry in Matter

Acta Phys. Pol. B 29, 2357 (1998)

page 2357 •

abstract

Soft-pion theorems are used to show how chiral symmetry constrains the contributions of low-momentum pions to the quark condensate, the pion decay constant and hadron masses, all of which have been proposed as signals of partial restoration of chiral symmetry in matter. These have contributions of order \(T^2\) for a pion gas or of order \(m_\pi \) for cold nuclear matter, which have different coefficients in all three cases, showing that there are no simple relations between the changes to these quantities in matter. In particular, such contributions are absent from the masses of vector mesons and nucleons and so these masses cannot scale as any simple function of the quark condensate. More generally, pieces of the quark condensate that arise from low-momentum pions should not be associated with partial restoration of chiral symmetry.


Quark Model, Chiral Symmetry, Deformation of the Nucleon and the Nucleon–Nucleon Interaction

Acta Phys. Pol. B 29, 2367 (1998)

page 2367 •

abstract

A property of Quantum Chromodynamics (QCD) which should be included into effective models describing QCD at low energies is chiral symmetry. It is conserved if one assumes that the quark masses are zero. This symmetry is spontaneously broken, which leads to constituent quark and as the Goldstone Boson one obtains the pion and its chiral partner the \(\sigma \) meson. We use the linear \(\sigma \) model which has compared to the non-linear one the advantage, that one treats pions \(\pi \) and \(\sigma \) mesons not only on the chiral circle, but allows also fluctuations around it. The scale of this fluctuations is the \(\sigma \) meson mass. If one eliminates gluons in second order, one obtains the “Tuebingen chiral quark model” with effective gluon exchange between the quarks and with pions and sigma mesons. A confinement potential is added. With this model we describe the photo and electro-excitation of the nucleon into the delta resonance and the decay of this resonance into a nucleon and a pion. The angular distribution gives information about the C2/E2 admixture into the M1 transition from the nucleon to the delta resonance. The quadrupole contribution of this transition has been described in the past by d state admixture due to tensor forces from the gluon and the pion exchange. This yields values which are more than a factor 10 too small compared with recent data for the C2/E2 Sachs transition form factor. We show that meson and gluon pair exchange currents can explain the data without the need of a large nucleon or delta deformation. The same model is then used to describe the nucleon-nucleon phase shifts. An essential ingredient for the good agreement is to include to all orders couplings to \(\Delta \) channels. The \(^1S_0\) phase shift can only be described in agreement with the data if the coupling to the \(^5D_0\) nucleon delta channel is included.


full authors' list

H. Toki, H. Suganuma, M. Fukushima, K. Amemiya, A. Tanaka, S. Umisedo, T. Sakai

Dual Ginzburg–Landau Theory and Quark Nuclear Physics

Acta Phys. Pol. B 29, 2377 (1998)

page 2377 •

abstract

The fundamental building blocks of matter are quarks. Hence, it is fundamental to describe hadrons and nuclei in terms of quarks and gluons, the subject of which is called Quark Nuclear Physics. The quark–gluon dynamics is described by quantum chromodynamics (QCD). Our interest is the non-perturbative aspect of QCD as confinement, chiral symmetry breaking, hadronization etc. We introduce the dual Ginzburg–Landau theory (DGL), where the color monopole fields and their condensation in the QCD vacuum, play essential roles in describing these non-perturbative phenomena. We apply the DGL theory to various observables. We discuss then the connection of the monopole fields with instantons, which are the classical solutions of the non-abelian gauge theory.


The Interaction Between Constituent Quarks

Acta Phys. Pol. B 29, 2389 (1998)

page 2389 •

abstract

The symmetry structure and the dynamical mechanisms that underlie the effective interaction between constituent quarks are discussed. The need for a flavor dependent hyperfine interaction is emphasized. Such a hyperfine interaction arises from exchange of the octet of light pseudoscalar mesons, which are the Goldstone bosons of the approximate chiral symmetry of QCD, between the quarks. The role of gluon exchange between quarks is small, but gluonic “dressing” of the meson exchange interaction is important for eliminating the tensor component of the meson exchange interaction.


The Dynamics of \(\pi N\) Scattering and the Baryon Spectrum

Acta Phys. Pol. B 29, 2397 (1998)

page 2397 •

abstract

In the present analysis I will detail a procedure for calculating the baryon spectrum as a solution of an eigenvalue problem that generates both the mass and width of the state. This is illustrated for the case of the \({\mit \Delta }\) and Roper resonances.


Hadrons in Nuclear Medium and Color Transparency

Acta Phys. Pol. B 29, 2407 (1998)

page 2407 •

abstract

Coherent and incoherent propagations of hadrons in nuclear medium are discussed in connection with the “mass shift”, the nuclear transparency and the color transparency.


full authors' list

Gerhard J. Wagner, R. Bilger, H. Clement, A. Denig, K. Föhl, J. Gräter, W. Kluge, J. Pätzold, R. Meier, M. Schepkin, G.J. Wagne, P.A. Amaudruz, A. Ambardar, R. Bilger, F. Bonutti, P. Camerini, J. Clark, H. Clement, L. Felawka, E. Friedman, J. Gräter, N. Grion, G.J. Hofman, P. Hong, M. Kermani, E.L. Mathie, R. Meier, D. Ottewell, O. Patarakin, R. Rui, M. Schepkin, M.E. Sevior, G.R. Smith, R. Tacik, G. Tagliente, G.J. Wagner, R. Bilger, W. Brodowski, H. Calén, H. Clement, C. Ekström, K. Föhl, K. Fransson, L. Gustafsson, S. Häggström, B. Höistad, A. Johansson, T. Johansson, K. Kilian, S. Kullander, A. Kupść, G. Kurz, P. Marciniewski, B. Morosov, J. Moehn, A. Mörtsell, W. Oelert, V. Renken, R. Ruber, M.G. Schepkin, U. Siodlaczek, J. Stepaniak, A. Sukhanov, A. Turowiecki, G.J. Wagner, Z. Wilhelmi, J. Zabierowski, A. Zernov, J. Zlomanczuk

Hunting the Dibaryon \(d'\)(2065)

Acta Phys. Pol. B 29, 2415 (1998)

page 2415 •

abstract

In order to explain the resonance-like excitation functions of pionic double-charge-exchange reactions near 50 MeV the existence of a narrow \(\pi NN\)-resonance \(d'\) with a mass of about 2.06 GeV has been postulated. The status of the various experiments that have been performed to establish the existence of the \(d'\) is reported.


Soft and Hard Structure of Nucleons as Seen in Deep Inelastic Scattering at HERA

Acta Phys. Pol. B 29, 2425 (1998)

page 2425 •

abstract

The old idea of using mesons in the nucleon as targets has been revived, developed further and applied successfully to production of leading baryons in deep inelastic scattering at HERA. I present a brief status report of the theory of leading baryon production placing an emphasis on the continuity between soft and hard scattering physics.


Resonant Quadrupole Amplitudes in the \(N\rightarrow {\mit \Delta }\) Transition

Acta Phys. Pol. B 29, 2437 (1998)

page 2437 •

abstract

A series of high precision, exclusive measurements on the \(N\rightarrow {\mit \Delta }\) transition involving polarized beams, out-of-plane detection and focal plane polarimetry in the H\((\vec {e},e^\prime p)\pi ^0\) and H\((\vec {e},e^\prime \pi ^+)n\) channels have been pursued at Bates during the last three years. They are geared towards the precise determination of the quadrupole amplitudes in the \(N\rightarrow {\mit \Delta }\) transition and the isolation of the coherent, competing processes (e.g. born terms, tails of higher resonances). The issue is of fundamental interest to hadronic physics as it pertains to interquark forces and the structure of hadrons. It is pursed intensively both theoretically and experimentally at many laboratories. The recent precise Bates electroproduction data do not support earlier claims for a strong Coulomb quadrupole amplitude. The available theoretical calculations fail to reproduce the isolated responses.


Proton Spin Content: Links to QCD Topological Susceptibility

Acta Phys. Pol. B 29, 2449 (1998)

page 2449 •

abstract

The part of the proton spin \({\mit \Sigma }\) carried by \(u, d, s\) quarks is calculated in the framework of the QCD sum rules in the external fields. The operators up to dimension 9 are accounted. An important contribution comes from the operator of dimension 3, which in the limit of massless \(u, d, s\) quarks is equal to the derivative of QCD topological susceptibility \(\chi ^{\prime } (0)\). The comparison with the experimental data on \({\mit \Sigma }\) gives \(\chi ^{\prime }(0)= (2.3 \pm 0.6) \times 10^{-3} ~ {\rm GeV}^2\). The limits on \({\mit \Sigma }\) and \(\chi ^{\prime }(0)\) are found from selfconsistency of the sum rule, \({\mit \Sigma } \gtrsim 0.05,~~ \chi ^{\prime } (0) \gtrsim 1.6 \times 10^{-3} ~ {\rm GeV}^2\). The values of \(g_A = 1.37 \pm 0.10\) and \(g^8_A = 0.65 \pm 0.15\) are also determined.


Flavor Asymmetry in Hyperons and Drell–Yan Processes

Acta Phys. Pol. B 29, 2459 (1998)

page 2459 •

abstract

SU(3), baryon octets and a meson cloud model are compared for the flavor asymmetry of sea quarks in the \({\mit \Sigma }^+\), as an example. Large differences are found, especially between SU(3) and the meson cloud model. We suggest Drell-Yan measurements of \({\mit \Sigma }^{+}-p\) and \({\mit \Sigma }^{+}- d\) to test the prediction of various models. We use the meson cloud model to predict both valence and sea quark distributions.


Low Energy Compton Scattering and Nucleon Structure

Acta Phys. Pol. B 29, 2467 (1998)

page 2467 •

abstract

The low energy virtual Compton scattering process \(eN\rightarrow e'N\gamma \) offers a new and potentially high resolution window on nucleon structure via measurement of so-called generalized polarizabilities (GPs). We present calculations of GPs within heavy baryon chiral perturbation theory and discuss present experimental efforts.


The Structure of Baryon Resonances in \(\pi N\) Scattering

Acta Phys. Pol. B 29, 2477 (1998)

page 2477 •

abstract

We present a coupled channel model for \(\pi N\) scattering, based on meson and baryon exchange processes. The inclusion of the reaction channels \(\pi N, \pi {\mit \Delta }, \sigma N\) and \(\eta N\) leads to a good description of \(\pi N\) phase shifts and inelasticities up to c.m. energies of 1600 MeV. The coupling between the channels \(\pi N, \pi {\mit \Delta }\) and \(\sigma N\) turns out to be large in the \(P_{11}\) and the \(N^*(1440)\) (Roper) resonance can be explained solely by meson-baryon dynamics, i.e. without the inclusion of a genuine \(N^*(1440)\) resonance. For the investigation of the \(N^*(1535)\) resonance, the model is extended by including the \(\eta N\) reaction channel. A genuine \(N^*(1535)\) resonance is needed for a quantitative description of the \(\pi N\) phase shift in the \(S_{11}\), but the background from meson and baryon exchanges is large and cannot be neglected.


Non-Perturbative VEVs from a Local Expansion

Acta Phys. Pol. B 29, 2487 (1998)

page 2487 •

abstract

We propose a method for the calculation of vacuum expectation values (VEVs) given a non-trivial, long-distance vacuum wave functional (VWF) of the kind that arises, for example, in variational calculations. The VEV is written in terms of a Schrödinger-picture path integral, then a local expansion for (the logarithm of the) VWF is used. The integral is regulated with an explicit momentum cut-off, \({\mit \Lambda }\). The resulting series is not expected to converge for \({\mit \Lambda }\) larger than the mass-gap but studying the domain of analyticity of the VEVs allows us to use analytic continuation to estimate the large-\({\mit \Lambda }\) limit. Scalar theory in \(1+1\) dimensions is analyzed, where (as in the case of Yang-Mills ) we do not expect boundary divergences.


The Balmer-Like Formula for Mass Distribution of Elementary Particle Resonances

Acta Phys. Pol. B 29, 2493 (1998)

page 2493 •

abstract

Elementary particle resonances have been systematically analyzed using all available experimental data. We have come to the conclusion that resonance decay product momenta and masses of resonances are to be quantized. The Balmer-like formula for mass distribution of elementary particle resonances has been obtained. These observations allow us to formulate a strategy of experimental searches for new resonances and systematize the already known one.


How Large is the \(\sigma \)-Nucleon-\(N^*(1440)\) Coupling Constant?

Acta Phys. Pol. B 29, 2501 (1998)

page 2501 •

abstract

The experimental information presently available on the \(\sigma \)-nucleon-N\(^*\)(1440) coupling constant is briefly discussed and the large uncertainty in this quantity is emphasized. We show that measurements of the associate photoproduction of a vector meson (\(\rho \)- or \(\omega \)-meson) and of the Roper resonance off proton targets near threshold could provide direct information on the strength of the scalar–isoscalar excitation of the \(N^*(1440)\) and hence on the magnitude of the \(\sigma N N^*\)(1440) coupling.


Relativity Damps OPEP in Nuclear Matter

Acta Phys. Pol. B 29, 2509 (1998)

page 2509 •

abstract

Using a relativistic Dirac–Brueckner analysis the OPEP contribution to the ground state energy of nuclear matter is studied. In the study the pion is derivative-coupled. We find that the role of the tensor force in the saturation mechanism is substantially reduced compared to its dominant role in a usual nonrelativistic treatment. We show that the damping of derivative-coupled OPEP is actually due to the decrease of \(M^*/M\) with increasing density. We point out that if derivative-coupled OPEP is the preferred form of nuclear effective Lagrangian nonrelativistic treatment of nuclear matter is in trouble. Lacking the notion of \(M^*\) it cannot replicate the damping. We suggest an examination of the feasibility of using pseudoscalar coupled \(\pi N\) interaction before reaching a final conclusion about nonrelativistic treatment of nuclear matter.


Possible Role of the \(\rho \)N Coupling in Pion Photo-Production

Acta Phys. Pol. B 29, 2519 (1998)

page 2519 •

abstract

We explore the role of the \(\rho N\) coupling in the pion photo-production reaction within a dynamical model approach based on a meson-exchange model of hadronic interactions. So far, all the existing dynamical models of pion photo-production consider only the \(\pi N\) intermediate states. Within such an approach, the pion photo-production reaction requires very soft hadronic form factors in order to reproduce the data. We show that the coupling to the \(\rho N\) intermediate states may allow the use of much harder form factors which are more in line with those used in the description of other processes.


Deep Inelastic Scattering and the Pion Light-Cone Wave Function

Acta Phys. Pol. B 29, 2527 (1998)

page 2527 •

abstract

Light-cone QCD sum rules are used to calculate the quark distribution function for the pion and place a new constraint on the pion twist-2 wave function. When combined with information available from analysis of other experimental data, we conclude that the wave function nearly achieves its asymptotic form.


Nonrelativistic Description of Heavy Quarkonia

Acta Phys. Pol. B 29, 2535 (1998)

page 2535 •

abstract

It is argued that a very good description of heavy quarkonia can be given in the framework of a nonrelativistic model, in spite of the fact that the relativistic corrections are expected to be significant. This suggests that most of the relativistic corrections can be absorbed into the phenomenological nonrelativistic potentials. How exactly this happens, is a difficult, open problem.


Meson Exchange Model of Meson–Meson Interactions

Acta Phys. Pol. B 29, 2539 (1998)

page 2539 •

abstract

We review the motivation for, and construction of, the Jülich meson exchange model of meson-meson interactions. The model employs both s- and t-channel exchanges of well-established mesons and gives a good quantitative fit to pseudoscalar–pseudoscalar meson scattering data from threshold to beyond 1.2 GeV. The model allows one to distinguish between “true” — i.e. , \(q\bar {q}\) — meson states and dynamical effects. In the framework of the model, the \(\rm f_{0}\) and \(\rm a_{0}\) appear as instances of dynamical effects. Focussing on the \(\pi \pi \) sector, limitationsof the model due to the lack of chiral symmetry are observed when the model is extended to pion interactions in nuclear matter. These are discussed, and one possible remedy is examined.


Study of \(\bar {p}p \to \phi \eta \) and \(\bar {p}p \to K^+K^-\eta \) Channels of Antiproton Annihilation at Rest at Three Hydrogen Target Densities

Acta Phys. Pol. B 29, 2547 (1998)

page 2547 •

abstract

The Okubo–Zweig–Iizuka (OZI) rule predicts that a ratio between the cross sections or annihilation frequencies of \(\phi \) and \(\omega \) meson production should be on the level of \(4\cdot 10^{-3}\cdot \). It is known that the predictions of the OZI rule are fulfilled in the hadronic interactions at a level of about \(10\%\). Recently large OZI rule violation has been found in some \(\bar {p}p\) annihilation channels in the experiments with stopped antiprotons at LEAR (CERN).


Strong Interaction Physics: from Quarks to Mesons, Baryons and Nuclei

Acta Phys. Pol. B 29, 2551 (1998)

page 2551 •

abstract

We summarize some of the important physics issues confronted at the NATO Advanced Research Workshop in Cracow. The topics addressed include modern challenges in understanding baryon and meson structure, hadronic interactions and hadron properties in dense matter.


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