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Register a new userOn Alfven hypothesis about nuclear hydromagnetic resonances
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The atomic nucleus capability of responding by hydromagnetic vibrations, that has been considered long ago by Hannes Alfven, is re-examined in the context of current development of nuclear physics and pulsar astrophysics.
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We discuss the most effective energy range for charged particle induced reactions in a plasma environment at a given plasma temperature. The correspondence between the plasma temperature and the most effective energy should be modified from the one given by the Gamow peak energy, in the presence of a significant incident-energy dependence in the astrophysical S-factor as in the case of resonant reactions. The suggested modification of the effective energy range is important not only in thermonuclear reactions at high temperature in the stellar environment, e.g., in advanced burning stages of massive stars and in explosive stellar environment, as it has been already claimed, but also in the application of the nuclear reactions driven by ultra-intense laser pulse irradiations.
The extracted value for the $g^{eff}_{omega rhopi}$ effective coupling from experimental data, considering only the $rho$ meson, resumes not only the $rho$ meson effect but also all its additional radial excitation modes. By explicitly adding the radial excitations of the $rho$ meson, considering a particular form of the spectrum and relations among the couplings, we identify the single $g_{omega rho pi}$ and the $rho$ radial excitations effect in the $omega rightarrow pi^0 gamma$ decay. We obtain that the individual coupling is in the range $g_{omegarhopi}= 8.2 - 8.6 {text GeV}^{-1}$, which is about 40% smaller than the effective $g^{eff}_{omegarhopi}$. We verify the consistency with the chiral approach in the $pi^0 rightarrow gammagamma$ and $gamma^* rightarrow 3pi$ processes. Besides the model dependence, our description succeeds in exhibiting how each contribution came into the game. In particular, we show that for the $gamma^* rightarrow 3 pi$ decay, the usual relation $mathcal{A}^{VMD}_{gamma3pi}=(3/2)mathcal{A}^{WZW}_{gamma3pi}$, encodes all the vector contributions and not only the $rho$ meson one. In addition, we find that there is an almost exact (accidental) cancelation between the radial excitations and the contact term contributions.
The cross sections for neutrino scattering off the 12C and 16O nuclei are calculated within the framework of the continuum Random Phase Approximation. A model to consider also the final state interactions is developed. Total charge-conserving and charge-exchange cross sections for both electron neutrinos and antineutrinos have been calculated up to projectile energies of 100 MeV. The sensitivity of the cross sections to the residual interaction and to the final state interactions is investigated. A direct comparison between neutrino and electron scattering cross sections calculated under the same kinematic conditions is presented. We found remarkable differences between electromagnetic and weak nuclear responses. The model is applied to describe cross sections of neutrinos produced by muon decay at rest and in supernovae explosions.
We present a brief overview of recent developments in ab initio calculations of nuclear scattering and reactions with a focus on applications of the no-core shell model with continuum method.
We review the phenomenon of fine structure of nuclear giant resonances and its relation to different resonance decay mechanisms. Wavelet analysis of the experimental spectra provides quantitative information on the fine structure in terms of characteristic scales. A comparable analysis of resonance strength distributions from microscopic approaches incorporating one or several of the resonance decay mechanisms allows conclusions on the source of the fine structure. For the isoscalar giant quadrupole resonance (ISGQR), spreading through the first step of the doorway mechanism, i.e. coupling between one particle-one hole ($1p1h$) and two particle-two hole ($2p2h$) states is identified as the relevant mechanism. In heavy nuclei it is dominated by coupling to low-lying surface vibrations, while in lighter nuclei stochastic coupling becomes increasingly important. The fine structure observed for the isovector giant dipole resonance (IVGDR) arises mainly from the fragmentation of the $1p1h$ strength (Landau damping), although some indications for the relevance of the spreading width are also found.
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