For a long time, Standard Big-Bang Nucleosynthesis (SBBN) was the only method to evaluate the baryonic density in the Universe, by comparing observed and calculated light-element abundances (4He, D,3He and 7Li). However, the study of Cosmic Microwave Background (CMB) anisotropies has provided very recently a new tool for the precise determination of the baryonic density, which can be compared to the results obtained from SBBN. The compatibility of these two studies would lead to a more convincing evaluation of this fundamental cosmological parameter. The R-matrix framework assumes that the space is divided into two regions: the internal region (with radius a), where nuclear forces are important, and the external region, where the interaction between the nuclei is governed by the Coulomb force only. Although the R-matrix parameters do depend on the channel radius a, the sensitivity of the cross section with respect to its choice is quite weak. The physics of the internal region is parameterized by a number N of poles, which are characterized by energy Eλ and reduced width ˜γλ. In a multichannel problem, the R-matrix at energy E.

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