中微子直接模拟蒙特卡洛：非平衡衰变宇宙学遗迹的精确建模

中文摘要 AI 辅助

我们考虑了非标准宇宙学场景，其中衰变遗迹在兆电子伏特温度下注入能量，使中微子偏离平衡态。随后的中微子演化塑造了对宇宙辐射密度和原初轻元素丰度的预测。早期工作引入了中微子直接模拟蒙特卡洛（$\ u$DSMC）方法，该方法通过粒子相互作用的蒙特卡洛采样来描述中微子输运。在此，我们通过联合演化遗迹粒子群、中微子谱、电磁等离子体和宇宙膨胀，并将此演化与大爆炸核合成耦合，扩展了该方法。该框架包括遗迹的直接和逆衰变、中微子味转换、量子统计以及高能中微子产生π介子。为了揭示这些效应的物理影响并独立测试$\ u$DSMC，我们在多种非标准宇宙学场景中比较了几种中微子演化方法。我们发现与量子动力学或准经典玻尔兹曼方程的独立解具有极好的一致性。与这些计算相比，$\ u$DSMC包含了更广泛的物理过程，并且对于强非热中微子群体而言速度显著更快。这使得对非标准宇宙学进行广泛而稳健的参数扫描变得切实可行。我们表明，当偏离热平衡较大时，对中微子动量进行平均的近似可能给出定性错误的预测，包括有效中微子种类数变化符号的错误以及显著错误的原初核丰度。我们得出结论，扩展的$\ u$DSMC框架通过跟踪中微子热化及其可观测后果，能够对新物理进行可靠的宇宙学检验。

英文摘要

We consider nonstandard cosmologies in which decaying relics inject energy at MeV temperatures, driving neutrinos out of equilibrium. The subsequent neutrino evolution shapes predictions for the cosmic radiation density and primordial light element abundances. Earlier work introduced Neutrino Direct Simulation Monte Carlo ($ν$DSMC), which describes neutrino transport through Monte Carlo sampling of particle interactions. Here, we extend this method by jointly evolving the relic population, neutrino spectra, electromagnetic plasma, and cosmic expansion, and coupling this evolution to Big Bang nucleosynthesis. The framework includes direct and inverse decays of the relic, neutrino flavor conversion, quantum statistics, and pion production by energetic neutrinos. To reveal the physical impact of these effects and independently test $ν$DSMC, we compare several approaches to neutrino evolution across a range of nonstandard cosmological scenarios. We find excellent agreement with independent solutions of quantum kinetic or quasi-classical Boltzmann equations. Compared with these calculations, $ν$DSMC includes a broader range of physical processes and is substantially faster for strongly nonthermal neutrino populations. This makes broad, robust parameter scans for nonstandard cosmologies practical. We show that when departures from thermal equilibrium are large, approximations that average over neutrino momenta can give qualitatively incorrect predictions, including the wrong sign of the change in the effective number of neutrino species and substantially incorrect primordial nuclear abundances. We conclude that the extended $ν$DSMC framework enables reliable cosmological tests of new physics by following neutrino thermalization and its observable consequences.

发表机构

• Tohoku University（东北大学）
• The University of Tokyo（东京大学）
• CERN（欧洲核子研究中心）
• Taras Shevchenko National University of Kyiv（基辅塔拉斯·舍甫琴科国立大学）

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