Helmholtz-Institut für Strahlen- und Kernphysik and Bethe Center for Theoretical Physics, Universität Bonn, D-53115 Bonn, Germany.
Department of Physics, North Carolina State University, Raleigh, North Carolina 27695, USA.
Nature. 2015 Dec 3;528(7580):111-4. doi: 10.1038/nature16067.
Processes such as the scattering of alpha particles ((4)He), the triple-alpha reaction, and alpha capture play a major role in stellar nucleosynthesis. In particular, alpha capture on carbon determines the ratio of carbon to oxygen during helium burning, and affects subsequent carbon, neon, oxygen, and silicon burning stages. It also substantially affects models of thermonuclear type Ia supernovae, owing to carbon detonation in accreting carbon-oxygen white-dwarf stars. In these reactions, the accurate calculation of the elastic scattering of alpha particles and alpha-like nuclei--nuclei with even and equal numbers of protons and neutrons--is important for understanding background and resonant scattering contributions. First-principles calculations of processes involving alpha particles and alpha-like nuclei have so far been impractical, owing to the exponential growth of the number of computational operations with the number of particles. Here we describe an ab initio calculation of alpha-alpha scattering that uses lattice Monte Carlo simulations. We use lattice effective field theory to describe the low-energy interactions of protons and neutrons, and apply a technique called the 'adiabatic projection method' to reduce the eight-body system to a two-cluster system. We take advantage of the computational efficiency and the more favourable scaling with system size of auxiliary-field Monte Carlo simulations to compute an ab initio effective Hamiltonian for the two clusters. We find promising agreement between lattice results and experimental phase shifts for s-wave and d-wave scattering. The approximately quadratic scaling of computational operations with particle number suggests that it should be possible to compute alpha scattering and capture on carbon and oxygen in the near future. The methods described here can be applied to ultracold atomic few-body systems as well as to hadronic systems using lattice quantum chromodynamics to describe the interactions of quarks and gluons.
阿尔法粒子（(^4He)）散射、三阿尔法反应和阿尔法俘获等过程在恒星核合成中起着重要作用。特别是，碳上的阿尔法俘获决定了氦燃烧过程中碳与氧的比例，并影响随后的碳、氖、氧和硅燃烧阶段。由于在吸积碳氧白矮星上的碳爆炸，它还极大地影响了热核型 Ia 超新星的模型。在这些反应中，准确计算阿尔法粒子和阿尔法类核——质子数和中子数相等的偶数核——的弹性散射，对于理解背景和共振散射的贡献很重要。由于与粒子数成指数增长的计算操作数的增加，到目前为止，涉及阿尔法粒子和阿尔法类核的过程的第一性原理计算是不切实际的。在这里，我们描述了一种使用晶格蒙特卡罗模拟的阿尔法-阿尔法散射的从头算计算。我们使用晶格有效场理论来描述质子和中子的低能相互作用，并应用一种称为“绝热投影方法”的技术将八体系统简化为两个团簇系统。我们利用辅助场蒙特卡罗模拟的计算效率和更有利于系统尺寸的比例，来计算两个团簇的从头算有效哈密顿量。我们发现晶格结果与 s 波和 d 波散射的实验相移之间存在有希望的一致性。计算操作数与粒子数的近似二次方比例表明，在不久的将来应该有可能计算阿尔法散射和碳、氧上的俘获。这里描述的方法可以应用于超冷原子少体系统以及使用晶格量子色动力学来描述夸克和胶子相互作用的强子系统。
