RUI: An Inference Methodology to Illuminate Nonlinear Neutrino Flavor Transformation for Nuclear Astrophysics

探索宇宙爆炸背后的物理学对于理解可观测宇宙的构成至关重要。大质量恒星的爆炸是一些重元素核合成的候选地点——重元素是地球上生命的基石。然而，这些爆炸的重要方面很难通过核天体物理学的传统方法获得。这是由于现有代码缺乏对所需数学框架的适应性，以及计算费用。此外，物理学的重要特征仍然被人为地隐藏在用来描述它们的工具之外。推理（与常用术语“机器学习”相关）是另一种方法。在地球科学和神经生物学中，推理已经揭示了几十年来类似于阻碍核天体物理学进展的问题。由于这个原因，最近推理被引入了天体物理学，其中概念验证模拟已经取得了成功。这个项目超越了这些测试，将推理集成到更大规模的代码中，并处理真实的天体物理数据。在一个科学领域培养的创新，在扩展到不相关的领域时，可能具有变革性。这项研究的一部分是对本科生的培训，其中许多人的社会经济背景在科学领域的代表性不足。学生还参与喜剧科学外展，以建立沟通技巧。在这些高密度环境中，被传统技术“人为隐藏”的物理现象是在中微子味场中改变方向的反向散射。中微子是基本粒子，其“风味”决定了它们与其他粒子相互作用的方式。味道在很大程度上决定了中子与质子的比例以及能量和熵的沉积，从而在一定程度上决定了爆炸和核合成的机制。香味场的后向散射对爆炸有显著的影响。但它提出了一个两点边值问题：一个传统数值积分无法处理的框架。本项目应用统计数据同化（SDA）来阐明这一问题。SDA是一种贝叶斯推理方法，为数值天气预报而发明，用于预测稀疏采样的非线性系统。SDA非常适合求解边值问题，并且有望在计算效率上优于集成。该项目建立在先前的工作基础之上，这些工作表明：1)在解决方向变化的后向散射问题方面，SDA可以优于集成；2)比集成更有效地搜索参数空间；3)在数据真实而不是模拟的情况下，找到简单问题的解决方案。这些发现要求对SDA解决更复杂的参数估计问题和增加更大规模代码的能力进行更深入的研究。该项目推进了“宇宙之窗：多信使天体物理学时代”的目标，这是美国国家科学基金会未来投资的十大理念之一。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

Probing the physics underlying cosmic explosions is vital for understanding the makeup of the observable Universe. The explosions of massive stars are candidate sites for the nucleosynthesis of some heavy elements – the building blocks of life on Earth. Important aspects of these explosions, however, are difficult to access via traditional approaches in nuclear astrophysics. This is due both to a lack of adaptability of existing codes to the required mathematical framework, and to computational expense. Moreover, important features of the physics remain artificially hidden from the tools built to describe them. Inference (related to the common term “machine learning”) is an alternative methodology. In the geosciences and neurobiology, inference has for decades illuminated problems akin to those that hinder progress within nuclear astrophysics. For that reason, recently inference has been brought into astrophysics, where proof-of-concept simulations have been successful. This project builds beyond those tests, integrating inference into larger-scale codes and handling real astrophysical data. Innovations cultivated within one scientific arena can be transformative when expanded for disjoint fields. Integral to the research is the training of undergraduates, many with socio-economic backgrounds under-represented in science. Students also engage in comedic science outreach, to build communication skills. The physics noted as “artificially hidden” from traditional techniques is direction-changing backscattering in the neutrino flavor field in these high-density environments. Neutrinos are elementary particles whose “flavor” dictates the manner in which they interact with other particles. Flavor in large part sets the neutron-to-proton ratio as well as energy and entropy deposition, thereby in-part dictating the mechanism of explosion and nucleosynthesis. Backscattering in the flavor field can significantly shape the explosion. But it presents a two-point boundary-value problem: a framework that traditional numerical integration is ill-equipped to handle. This project applies statistical data assimilation (SDA) to illuminate this problem. SDA is a Bayesian inference methodology, invented for numerical weather prediction, to predict sparsely-sampled nonlinear systems. SDA is well-suited for solving boundary-value problems, and it is expected to outperform integration in computational efficiency. This project builds upon previous work that established that SDA can 1) outperform integration in terms of solving a direction-changing backscattering problem, 2) search parameter space more efficiently than integration, and 3) find solutions to simple problems where the data are real, rather than simulated. These findings call for a deeper examination of SDA’s ability to solve more complex parameter estimation problems and augment larger-scale codes.This project advances the objectives of "Windows on the Universe: the Era of Multi-Messenger Astrophysics", one of the 10 Big Ideas for Future NSF Investments.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.