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The strength function $ S_{\beta}(E) $ governs [1,2] the nuclear energy distribution of elementary charge-exchange excitations and their combinations like proton particle $({\pi}p)$-neutron hole $({\nu}h)$ coupled into a spin-parity $I^{\pi}$ : $[{\pi}p \otimes {\nu}h]I^{\pi}$ and neutron particle $({\nu}p)$-proton hole $({\pi}h)$ coupled into a spin-parity $I^{\pi} : [{\nu}p \otimes {\pi}h]I^{\pi}$. The strength function of Fermi-type $\beta$-transitions takes into account excitations $[{\pi}p \otimes {\nu}h]0^{+}$ or $[{\nu}p \otimes {\pi}h]0^{+}$. Since isospin is a quite good quantum number, the strength of the Fermi-type transitions is concentrated in the region of the isobar-analogue resonance ($IAR$). The strength function for $\beta$-transitions of the Gamow–Teller ($GT$) type describes excitations $[{\pi}p \otimes {\nu}h]1^{+}$ or $[{\nu}p \otimes {\pi}h]1^{+}$. Residual interaction can cause collectivization of these configurations and occurrence of resonances in $ S_{\beta}(E)$. In heavy and middle nuclei, because of repulsive character of the spin-isospin residual interaction [1,2], the energy of $GT$ resonance is larger than the energy of $IAR (E_{GT} > E_{IAR})$. One of the consequence of the Wigner spin-isospin $SU(4)$ symmetry is $E_{GT} = E_{IAR}$. $SU(4)$ symmetry-restoration effect induced by the residual interaction, which displaces the $GT$ towards the $IAR$ with increasing $(N-Z)/A$. In $^{6}Li$ nucleus (g.s. is the tango halo [3] state, IAR is the Borromean halo resonance) for low energy $GT$ phonon (reduced $GT$ strength $B(GT) \approx {5g_{A}}^{2}/4{\pi} (\Sigma($Ikeda sum rule$ ) = {6g_{A}}^{2}/4{\pi}$ we have $E_{GT} < E_{IAR}, E_{GT}-E_{IAR} = -3562.88 keV$, and $(N-Z)/A=0.33$ for $^{6}He$ ($^{6}He$ g.s. is the parent, Borromean halo state). Such situation may be connected with contribution of the attractive [4] component of residual interaction in this nucleus. It will be very interesting to find a region of atomic nuclei, where the $E_{} \approx E_{IAR}$ and spin-isospin $SU(4)$ symmetry determine the nuclear properties ($SU(4)$ region). Difference of the $E_{GT} - E_{IAR}$ energies as a function of the neutron [1,2] excess was analyzed. Datum for $^{6}He$ $\beta^{-}$ - decay was added to the data analyzed in [1,2].
Estimations shows that the value $Z/N \approx 0.6$ corresponds to the $SU(4)$ region. Different manifestations of the $SU(4)$ symmetry and possible experiments are discussed.
- Yu.V. Naumov, A.A. Bykov, I.N. Izosimov, Sov.J.Part.Nucl.14 (1983) 167.
- I.N. Izosimov, Physics of Particles and Nuclei 30 (1999) 131.
- I.N. Izosimov, JINR Preprint E6-2017-79, Dubna, 2017.
- Y. Fujita, et al., Phys. Rev. C 91 (2015) 064316, and references therein.
