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Register a new userRho-Nucleon Tensor Coupling and Charge-Exchange Resonances
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Alfredo Pio Noronha Rodrigues Galeao
Publication date
2000
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and research's language is
English
Authors
C. De Conti
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The Gamow-Teller resonances are discussed in the context of a self-consistent RPA, based on the relativistic mean field theory. We inquire on the possibility of substituting the phenomenological Landau-Migdal force by a microscopic nucleon-nucleon interaction generated from the rho-nucleon tensor coupling. The effect of this coupling turns out to be very small when the short range correlations are not taken into account, but too large when these correlations are simulated by the simple extraction of the contact terms from the resulting nucleon-nucleon interaction.
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To describe the double-charge-exchange (DCE) processes, we have designed recently the $(pn,2p2n)$-QTDA model which fully includes the pairing correlations and four quasiparticle excitations. It has been applied in $2 u$ double beta decays (DBDs), and the double charge-exchange resonances (DCERs). Here we extend it to $ 0 u $ DBD and discuss the relationship between the nuclear matrix elements (NMEs), and the DCE reaction matrix elements (RMEs) with the same spin-isospin structure. We do it for all final $0^+$ states, even in the region of DCERs, where the DBD is energetically forbidden. As an example, we evaluate the DBD $^{76}$Ge $rightarrow ^{76}$Se, both for $2 u$ and $0 u$ modes, as well as the associated DCE sum rules, excitation energies within the $Q$-value window for DBD, and the $Q$-value itself. We find that the $0 u$ NMEs are correlated with the RMEs, both at low energy, and in the DCER region where most of the transition strength is concentrated. These findings occur in other nuclei as well and suggest that measurements of $0^+$ DCERs could provide useful information regarding the $ 0 u $ DBD. An analogous comparison and conclusion cannot be made for the $2^+$ states, since the $0 u$ NMEs and RMEs transition operators are not similar to each other in this case.
The contribution of the low-lying nucleon resonances $P_{33}(1232)$, $P_{11}(1440)$ $D_{13}(1520)$ and $S_{11}(1535)$ to the invariant mass spectra of di-electrons stemming from the exclusive processes $ppto pp e^+e^-$ and $pnto pn e^+e^-$ is investigated within a fully covariant and gauge invariant diagrammatical approach. We employ, within the one-boson exchange approximation, effective nucleon-meson interactions including the exchange mesons $pi$, $eta$, $sigma$, $omega$ and $rho$ as well as excitations and radiative decays of the above low-lying nucleon resonances. The total contribution of these resonances is dominant, however, bremsstrahlung processes in $pp$ and, in particular, $pn$ collisions at beam energies of 1 - 2 GeV are still significant in certain phase space regions.
Recent progress on the extraction of electromagnetic properties of nucleon resonance excitation through pion photo- and electroproduction is reviewed. Cross section data measured at MAMI, ELSA, and CEBAF are analyzed and compared to the analysis of other groups. On this basis, we derive longitudinal and transverse transition form factors for most of the four-star nucleon resonances. Furthermore, we discuss how the transition form factors can be used to obtain empirical transverse charge densities. Contour plots of the thus derived densities are shown for the Delta, Roper, S11, and D13 nucleon resonances.
We look for $DeltaDelta$ and $NDelta$ resonances by calculating $NN$ scattering phase shifts of two interacting baryon clusters of quarks with explicit coupling to these dibaryon channels. Two phenomenological nonrelativistic chiral quark models giving similar low-energy $NN$ properties are found to give significantly different dibaryon resonance structures. In the chiral quark model (ChQM), the dibaryon system does not resonate in the $NN$ $S$-waves, in agreement with the experimental SP07 $NN$ partial-wave scattering amplitudes. In the quark delocalization and color screening model (QDCSM), the $S$-wave NN resonances disappear when the nucleon size $b$ falls below 0.53 fm. Both quark models give an $IJ^P = 03^+$ $DeltaDelta$ resonance. At $b=0.52 $fm, the value favored by baryon spectrum, the resonance mass is 2390 (2420) MeV for the ChQM with quadratic (linear) confinement, and 2360 MeV for the QDCSM. Accessible from the $^3D_3^{NN}$ channel, this resonance is a promising candidate for the known isoscalar ABC structure seen more clearly in the $pn$$to $$dpipi$ production cross section at 2410 MeV in the recent preliminary data reported by the CELSIUS-WASA Collaboration. In the isovector dibaryon sector, our quark models give a bound or almost bound $^5S_2^{DeltaDelta}$ state that can give rise to a $^1D_2^{NN}$ resonance. None of the quark models used has bound $NDelta$ $P$-states that might generate odd-parity resonances.
Spin-isospin transitions in nuclei away from the valley of stability are essential for the description of astrophysically relevant weak interaction processes. While they remain mainly beyond the reach of experiment, theoretical modeling provides important insight into their properties. In order to describe the spin-isospin response,vcthe proton-neutron relativistic quasiparticle random phase approximation (PN-RQRPA) is formulated using the relativistic density-dependent point coupling interaction, and separable pairing interaction in both the $T=1$ and $T=0$ pairing channels. By implementing recently established DD-PCX interaction with improved isovector properties relevant for the description of nuclei with neutron-to-proton number asymmetry, the isobaric analog resonances (IAR) and Gamow-Teller resonances (GTR) have been investigated. In contrast to other models that usually underestimate the IAR excitation energies in Sn isotope chain, the present model accurately reproduces the experimental data, while the GTR properties depend on the isoscalar pairing interaction strength. This framework provides not only an improved description of the spin-isospin response in nuclei, but it also allows future large scale calculations of charge-exchange excitations and weak interaction processes in stellar environment.
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