Quantifying alpha clustering in light nuclei from binding energies

What is the origin of nuclear clustering and how does it emerge from the nuclear interaction? While there is ample experimental evidence for this phenomenon, its theoretical characterization directly from nucleons as degrees of freedom remains a challenge, making it difficult to improve nuclear forces using clustering observables. In this work, a simple ratio of energies based on effective scale arguments is proposed to assess the quality of nuclear forces on alpha clustering (${ {}^{4}\text{He} }$) in bound and narrow ground-state nuclei. The proposed clustering ratio is parameter-free and correctly identify basic alpha clustering features in light nuclei. It is found that in nuclei ranging from ${ {}^{6}\text{Li} }$ to ${ {}^{14}\text{C} }$, state-of-the-art \textit{ab initio} nuclear forces underestimate the degree of alpha clustering in key nuclei such as ${ {}^{8}\text{Be} }$ and ${ {}^{12}\text{C} }$. Stringent constraints on binding energies are then provided by back-propagating 10\% relative uncertainties on the experimental clustering ratios using a parallel Markov chain Monte Carlo algorithm. It is demonstrated that the binding energies of the nuclei $^{6,7}$Li, $^7$Be, $^{10,11}$B, and $^{11}$C need to be obtained within tens of keV precision by nuclear forces to ensure a proper reproduction of basic alpha clustering features in light nuclei. This study provides a new and practical path to guide future optimizations of nuclear forces with a potential impact for medium-mass nuclei.

K. Fossez · 2020