We assert that nonlinear features of fast neutrino-flavor conversion (FFC) can be qualitatively different between core-collapse supernovae (CCSNe) and binary neutron star mergers (BNSMs). This argument arises from recent global FFC simulations in BNSM, in which fast flavor swap (FFS) emerges in very narrow spatial regions, whereas neutrinos in CCSN tend to evolve toward flavor equipartition. In this paper, we provide the physical mechanism of FFS based on a colliding neutrino beam model. Neutrinos/antineutrinos can undergo FFS when they propagate in ambient neutrino gas that propagates in the opposite direction and also has the opposite sign of ELN-XLN, where ELN and XLN denote electron- and heavy-leptonic neutrino number, respectively. Such environments can be naturally realized in BNSMs, whereas they are unlikely in CCSNe unless the neutrino sphere is strongly deformed aspherically. Our study exhibits the diversity of nonlinear dynamics of FFC.

中文翻译：

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高能天体物理环境中的快速中微子味道交换

我们断言，快速中微子味道转换（FFC）的非线性特征在核心塌缩超新星（CCSNe）和双中子星合并（BNSM）之间可能有本质上的不同。这一论点源于最近 BNSM 中的全球 FFC 模拟，其中快速味道交换（FFS）出现在非常狭窄的空间区域，而 CCSN 中的中微子倾向于演化为味道均分。在本文中，我们提供了基于碰撞中微子束模型的 FFS 物理机制。中微子/反中微子在环境中微子气体中传播时会经历 FFS，中微子气体以相反的方向传播，并且具有相反的 ELN-XLN 符号，其中 ELN 和 XLN 分别表示电子中微子数和重轻子中微子数。这种环境可以在 BNSM 中自然实现，而在 CCSNe 中则不太可能实现，除非中微子球发生强烈的非球面变形。我们的研究展示了 FFC 非线性动力学的多样性。