标题： 中子星合并模拟中捕捉中微子分布函数的难度 显示英文标题

标题： On the difficulty of capturing the distribution function of neutrinos in neutron star merger simulations

摘要： 中子星的碰撞是关于核物理的信息丰富来源。 特别是，合并后出现的千新星信号可以帮助我们阐明中子星在核合成中的作用，并向我们提供关于高于核饱和密度的物质性质的信息。 中微子的近似建模仍然是我们对这些可观测值进行预测能力的重要限制。 问题的一部分在于中微子的费米子特性。 根据泡利不相容原理，量子态中中微子数量的期望值$f_\nu$最多为 1。 任何产生中微子的过程都会被一个阻挡因子$(1-f_\nu)$所抑制。 最近专注于中微子物理的模拟大多使用灰两矩方法来演化中微子。 这种方法演化动量空间中$f_\nu$的积分，从而无法直接计算阻挡因子。 蒙特卡罗方法可能是一种有吸引力的替代方案，可以访问中微子的完整分布。 然而，它们的当前实现不足以估计$f_\nu$：在我们最近的模拟中，一个蒙特卡罗包在最坏情况下会导致$f_\nu$的估计值从$f_\nu=0$跳跃到$f_\nu\sim 10^5$。 虽然这令人担忧，但这种对中微子费米子特性的公然违反在模拟中使用的相互作用中并未产生重大影响，因为这些相互作用避免了对$f_\nu$的直接计算。然而，我们目前所处的建模水平已经使得这个问题无法再被忽视。在这里，我们讨论这一问题相对较简单的来源。然后我们表明，在合并模拟中理论上可以得到$f_\nu$的粗略估计，但它们将需要结合蒙特卡罗包的非直观加权方案，并在比合并模拟所用分辨率更低的尺度上对中微子分布进行平滑处理。 显示英文摘要

摘要： The collision of two neutron stars is a rich source of information about nuclear physics. In particular, the kilonova signal following a merger can help us elucidate the role of neutron stars in nucleosynthesis, and informs us about the properties of matter above nuclear saturation. Approximate modeling of neutrinos remains an important limitation to our ability to make predictions for these observables. Part of the problem is the fermionic nature of neutrinos. By the exclusion principle, the expected value $f_\nu$ for the number of neutrinos in a quantum state is at most 1. Any process producing neutrinos is suppressed by a blocking factor $(1-f_\nu)$. Recent simulations focused on neutrino physics mostly use a gray two-moment scheme to evolve neutrinos. This evolves integrals of $f_\nu$ over momentum space, preventing direct calculations of blocking factors. Monte Carlo methods may be an attractive alternative, providing access to the full distribution of neutrinos. Their current implementation is however inadequate to estimate $f_\nu$: in our most recent simulations, a single Monte Carlo packet causes, in the worst cases, estimates of $f_\nu$ to jump from $f_\nu=0$ to $f_\nu\sim 10^5$. While this is concerning, this brazen violation of the fermionic nature of neutrinos has been largely inconsequential as the interactions used in simulations avoid direct calculations of $f_\nu$. We are however reaching a level of modeling at which this problem can no longer be ignored. Here, we discuss the relatively simple origin of this issue. We then show that very rough estimates of $f_\nu$ can in theory be obtained in merger simulations, but that they will require a combination of unintuitive weighting schemes for Monte Carlo packets and smoothing of the neutrino distribution at coarser resolution than what the merger simulation uses.