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Comparative study of the direct $α+d$ $\rightarrow$ $^6$Li + $γ$ astrophysical capture reaction in few-body models

A comparative analysis of the astrophysical S factor and the reaction rate for the direct $ α(d,γ)^{6}{\rm Li}$ capture reaction, and the primordial abundance of the $^6$Li element, resulting from two-body, three-body and combined cluster models is presented. It is shown that the two-body model, based on the exact-mass prescription, can not correctly describe the dependence of the isospin-forbidden E1 S factor on energy and does not reproduce the temperature dependence of the reaction rate from the direct LUNA data. It is demonstrated that the isospin-forbidden E1 astrophysical S factor is very sensitive to the orthogonalization procedure of Pauli-forbidden states within the three-body model. On the other hand, the E2 S factor does not depend on the orthogonalization method. This insures that the orthogonolizing pseudopotentials method yields a very good description of the LUNA collaboration's low-energy direct data. At the same time, the SUSY transformation significantly underestimates the data from the LUNA collaboration. On the other hand, the energy dependence of the E1 S factor are the same in both methods. The best description of the LUNA data for the astrophysical S factor and the reaction rates is obtained within the combined E1(three-body OPP)+E2(two-body) model. It yields a value of $(0.72 \pm 0.01) \times 10^{-14}$ for the $^6$Li/H primordial abundance ratio, consistent with the estimation $(0.80 \pm 0.18) \times 10^{-14}$ of the LUNA collaboration. For the $^6{\rm Li}/^7{\rm Li}$ abundance ratio an estimation $(1.40\pm 0.12)\times 10^{-5}$ is obtained in good agreement with the Standard Model prediction.