Josef Speth

Experimental signatures of the Δ(1236)-hole quenching mechanism in pionic states

Abstract The Δ(1236)-hole model for the quenching of spin-isospin transition strength is investigated by explicitly including the Δ(1236) into the nuclear wave function. The angular momentum dependence of the quenching effect is discussed as a possible experimental signature of the model.

Isobar-hole and two-particle-two-hole effects in the spin response of 90Zr and 208Pb

Abstract We give a microscopic description of low-energy unnatural-parity excitations in 90 Zr and 208 Pb in a restricted Hilbert space spanned by one-particle-one-hole, two-particle-two-hole as well as one-isobar-one-hole excitations. In this model space it seems possible to reduce the effective two-body interaction to correlated one-pion and one-rho-meson exchange and yet get a good description of measured strength functions.

Giant Gamow-Teller resonances in nuclei described with realistic two-body interactions☆

Abstract Using effective interactions based on a Brueckner G -matrix in nuclear matter we calculate Gamow-Teller strength distributions in 48 Ca, 90 Zr and 208 Pb in local density approximation. The conventional RPA theory is extended to include 1p1h- as well as 2p2h-excitations in a consistent way. We obtain a very reasonable description of the centroid energies as well as the spreading width of the resonances in all three nuclei. The tails of the distributions typically contain 30–40% of the 3( N − Z ) sum rule and are mostly produced by central spin-isospin correlations. The tensor force only plays a minor role in shifting the στ -strength to higher energies.

Pionic ΔL = 1 charge-exchange modes in heavy mass nuclei

Abstract It is shown that the experimentally observed charge-exchange ΔL = 1 resonance in heavy mass nuclei is a superposition of all possible ΔL = 1, ΔS = 1 modes with spin-parity 0 − , 1 − , 2 − , and of one ΔL = 1, ΔS = 0 mode with spin-parity 1 − .

Critical analysis of a precritical phenomenon in connection with pion condensation

Abstract The (e,e′) form factor and (p,p′) cross section of the 1 + ( ΔT =1; 15.1 MeV) state in 12 C are calculated within the framework of the RPA. The effects of the many-particle many-hole configurations are also included. The results show that the precritical phenomenon is not necessary in order to reproduce the large momentum behaviour.

KN scattering and meson exchange

K+N scattering phase shifts have been calculated with a meson-exchange model based on vector-meson (ω, ϱ)- and Λ-baryon exchange. Fair agreement with empirical data has been found except for the isospin-1 P32 wave. This partial wave is remarkably improved by adding, instead of scalar-isoscalar σ-exchange, explicit second-order ϱ-exchange involving ΔK intermediate states.

Strangeness in nuclei

We present further results of our general program, which is to construct meson‐exchange potentials for hadronic systems involving strange particles. In this contribution we investigate the relationship between the free ΛN‐interaction and the effective interactions inside of a nucleus. These polarization effects are taken into account within a generalized Brueckner G‐matrix. Within this approximation we calculate the binding energy and effective mass of a Λ‐particle in nuclear matter as well as the Landau‐parameters of the ΛN‐system.

Deep inelastic scattering from the nuclear medium

Abstract We reexamine deep inelastic scattering from nuclei under assumptions commonly employed in the literature: that quarks remain confined in hadronic constituents of nuclei, that the nuclear cross section is the average of the free-space cross section of hadron i weighted by the probability of finding i in the nucleus, and that there are no final state interactions between the debris of hadron i and the rest of the nucleus. We develop a cluster expansion for the cross section of deep inelastic lepton scattering and the Drell-Yan process on nuclei. Using the “instant” from of dynamics, we find that large contributions to these processes arise from nuclear interactions and correlations. However, our theory differs in detail from other approaches, and we find that binding alone is unlikely to explain the EMC effect. Also, in contrast to many recent papers on the subject, we conclude that the contribution of pions (and other nonnucleonic consistuents of the nucleus) is strongly suppressed.

Correlations in hypernuclei

We investigate the relationship between the free ΛN‐interaction and the effective interaction inside of a nucleus. These polarization effects are taken into account within a generalized Brueckner G‐matrix. Within this approximation we calculate the Landau‐parameters in the ΛN‐system and compare the correlated ΛN‐ and NN‐wavefunctions in nuclear matter.

Effective nuclear interaction: Dirac versus conventional approach

Abstract The effective interaction in nuclear matter ( G -matrix) is calculated, in the relativistic (Dirac-Brueckner-Hartree-Fock) as well as in the conventional Brueckner-Hartree-Fock approach. Starting point is a nucleon-nucleon potential which, in addition to single-meson exchange, contains delta-isobar box diagrams with ππ- and πϱ-exchange, leading to Pauli blocking and dispersive effects. The implications for nuclear matter binding and Landau parameters are investigated.