Purity of analog spin in isobaric analog resonances

Abstract

It is shown that the position of the pole of the inverse average S-matrix of an isolated isobaric analog resonance (IAR) is a very useful parameter, which allows one to distinguish between the weak and the strong coupling case of the finestructure distribution of the resonance. It gives also a direct measure of the purity of analog spin of the resonance. As an example the lowest IAR in 2~ is discussed.

Simple mode on a highly excited background: Collective strength and damping in the continuum

“…This natural estimate (inverse number of fine structure states within the spreading width) coincides with that used by von Brentano [10]. If the simple mode and the compund states have no common decay channels, the nonzero decay amplitudes are A c 0 = √ γ 0 (consider for simplicity a single direct decay channel) and A e n .…”

“…The general although schematic character of the analysis allows one to draw the conclusions concerning the manifestations of the simple mode in various situations. A typical example is given by the IAS which can be seen as a broad pole [10] or to be dissolved in the sea of the fine structure levels of another isospin. The analysis, analogous to that in [11], confirms the old idea [21,22] of increasing isospin purity of the IAS at high excitation energy.…”

“…The second expression reads Γ = Γ ↑ − Γ ↓ in the notations chosen in [10]. The state T > can be observed only if it decays before mixing, γ 0 > Γ s .…”

“…Saturation of the spreading width of the giant dipole resonance (GDR) in hot nuclei [3,4,5], "disappearance" of the collective strength of the GDR at high excitation energy [6], and existence and relatively narrow width of the double GDR [7,8,9] are just few bright examples. In the physics of isobaric analog states (IAS), one can mention the evidence for existence of the so-called "broad poles" [10], very weak fluctuations of the spreading widths of the IAS throughout the periodic table [11,12,13], and restoration of isospin purity at high excitation energy [2].…”

Super-radiant dynamics, doorways and resonances in nuclei and other open mesoscopic systems

“…This spreading width of the IAS increases with excitation energy and then saturates [114]. If at this excitation energy the lifetime of the T > state with respect to the decay outside is shorter than the lifetime of the background T < states, the IAS preserves its individuality [115,116]. One could expect that the purity of the IAS will rapidly be washed out at high density of the admixed states.…”