What happens when neutrinos swap identities inside a supernova?
What happens when neutrinos swap identities inside a supernova?
当中微子在超新星内部发生身份转换时会发生什么？
Our basic understanding of core-collapse supernovae hasn’t changed in decades. Large stars burn through all the fuel at their cores and start creating heavier elements in reactions that consume energy. The lack of energy from these reactions allows gravity to pull the interior of the star in on itself, collapsing it into a neutron star or black hole. The energy released by this process then blows the remainder of the star apart. And, generally, that’s right. But there’s an entire busload of devils in the details. 几十年来，我们对核坍缩超新星的基本理解并没有改变。大质量恒星耗尽核心的所有燃料，开始通过消耗能量的反应产生更重的元素。这些反应导致的能量匮乏使得引力能够将恒星内部向内拉扯，使其坍缩成中子星或黑洞。这一过程释放的能量随后将恒星的其余部分炸开。总的来说，这是正确的。但细节中却隐藏着无数魔鬼。
The statistics of supernovae that we’ve observed indicate that the model may be seriously incomplete. And on the theoretical side, there are still plenty of uncertainties, including over some of the basics, such as whether all core collapses actually result in a supernova. A paper being released by Physical Review D provides what might be a potential explanation for the discrepancy: flavor-changing neutrinos. Neutrinos play a key role in our current models of supernovae, and right now, those models don’t take into account one of neutrinos’ most striking features: their ability to change identity. 我们观测到的超新星统计数据表明，该模型可能存在严重缺陷。在理论方面，仍存在许多不确定性，包括一些基本问题，例如是否所有的核坍缩最终都会产生超新星。发表在《物理评论 D》(Physical Review D) 上的一篇论文为这种差异提供了一种潜在的解释：中微子味变。中微子在我们目前的超新星模型中起着关键作用，但目前这些模型并未考虑中微子最显著的特征之一：它们改变身份的能力。
Supernovae and their discontents
超新星及其困境
We’ve observed plenty of supernovae, so it would seem like there would be little mystery left. But a number of observations suggest there are some subtleties that we might be missing. For example, if we compare the rate of star formation in the Universe to the frequency of supernovae, there’s a discrepancy; it appears we’re forming enough stars to fuel a much higher frequency of supernovae than we actually observe. Also, in cases where we can identify the progenitor star that exploded, we find too few red supergiants, indicating that they may be contributing to this discrepancy. 我们已经观测到了大量的超新星，因此似乎剩下的谜团已经不多了。但许多观测结果表明，我们可能遗漏了一些微妙之处。例如，如果我们比较宇宙中恒星的形成率与超新星的频率，就会发现存在差异；我们形成的恒星数量似乎足以支撑比我们实际观测到的高得多的超新星频率。此外，在能够识别爆炸前身星的情况下，我们发现红超巨星的数量太少，这表明它们可能导致了这种差异。
Other problems come from observations of gravitational waves generated by mergers of the black holes left behind after a supernova. These mergers suggest there’s a “mass gap” in black hole formation—a range of masses where there are fewer black holes than you’d expect from an even distribution. But that data is complicated by the fact that theorists haven’t definitively identified the conditions that determine when a neutron star tips over into a black hole instead. 其他问题来自对超新星遗留黑洞合并所产生的引力波的观测。这些合并表明黑洞形成中存在一个“质量间隙”——即在某个质量范围内，黑洞的数量比均匀分布所预期的要少。但这些数据很复杂，因为理论家尚未明确确定决定中子星何时坍缩成黑洞的条件。
Meanwhile, on the theory side, things have been in a bit of flux. As we’ve added ever more sophisticated physics to our models of supernovae, we’ve gone through periods where either everything blows up or nothing blows up. It has been harder to develop models that give us a good picture of why some stars blow up and others might not. Still, our best current models agree that neutrinos are essential to the process. 与此同时，在理论方面，情况一直处于波动之中。随着我们将越来越复杂的物理学加入到超新星模型中，我们经历过要么一切都爆炸、要么什么都不爆炸的时期。要开发出能够清晰解释为什么有些恒星会爆炸而有些则不会的模型，变得更加困难。尽管如此，我们目前最好的模型都一致认为，中微子对这一过程至关重要。
Neutrinos are produced in prodigious quantities both by the complex fusion reactions that take place during a supernova and by the formation of neutron star material at the heart of the collapse (which happens even if the collapse continues on to form a black hole). And those numbers matter for the fate of the material outside the core of the dying star. With fewer photons coming out of the core of the star, that material lacks the energy to resist the pull of gravity and starts rushing toward the core. On its way, it encounters the shock wave from the formation of a neutron star/black hole, which is rushing in the opposite direction. Left on its own, these forces roughly balance out, stalling the shock wave and letting gravity take over. 中微子在超新星爆发期间发生的复杂聚变反应中，以及在坍缩核心处形成中子星物质的过程中（即使坍缩继续形成黑洞，这一过程也会发生），都会产生惊人的数量。这些数量对于垂死恒星核心外部物质的命运至关重要。由于从恒星核心发出的光子较少，这些物质缺乏抵抗引力的能量，开始向核心冲去。在途中，它会遇到由中子星/黑洞形成产生的冲击波，该冲击波正向相反方向冲去。如果任其发展，这些力大致会相互抵消，导致冲击波停滞，从而让引力占据主导地位。
Neutrinos change the equation. While they tend not to interact with matter often, the sheer number of them rushing out ensures that enough bump into the material around the stalled shock wave. This transfers energy, heating it up enough to overcome gravity and allow the shock wave to escape, destroying the star. Failure of this process would, in contrast, allow almost the entire contents of the star to collapse into a black hole, killing the star without an explosion. 中微子改变了这个等式。虽然它们通常不常与物质发生相互作用，但大量涌出的中微子确保了有足够多的中微子撞击停滞冲击波周围的物质。这会传递能量，将其加热到足以克服引力，使冲击波得以逃逸，从而摧毁恒星。相反，如果这一过程失败，恒星几乎所有的物质都会坍缩成黑洞，导致恒星在没有爆炸的情况下死亡。
Flavorful
中微子味变
One potential problem with the models that show neutrino heating is that they treat neutrinos as a single factor. Neutrinos don’t want to be pinned down that way. There are three types, or flavors, of neutrino (electron, muon, and tau). But each particle is in a superposition of all three flavors and can shift among them in a process called flavor oscillation. So, even if the events inside the supernova produced nothing but electron neutrinos, they would likely oscillate among the two other identities multiple times before they reach the surface of the star. 显示中微子加热的模型的一个潜在问题是，它们将中微子视为单一因素。中微子并不想被那样定义。中微子有三种类型，或称“味”（电子、μ子和τ子）。但每个粒子都处于所有三种味的叠加态，并且可以通过一种称为“味振荡”的过程在它们之间转换。因此，即使超新星内部的事件只产生电子中微子，它们在到达恒星表面之前，也很可能会在其他两种身份之间多次振荡。
And that’s potentially a big deal, given that the neutrinos’ interactions with matter are influenced by their identity. “By modifying the relative spectra of electron neutrinos, electron antineutrinos, and heavy-lepton neutrinos,” the paper’s authors write, “[flavor oscillation] affects charged-current heating and cooling in the region behind the shock.” 考虑到中微子与物质的相互作用受其身份影响，这可能是一个大问题。论文作者写道：“通过改变电子中微子、电子反中微子和重轻子中微子的相对光谱，[味振荡] 会影响冲击波后区域的带电电流加热和冷却。”
The actual physics here is incredibly complicated, since the momentum of each neutrino will vary, as will where they are created relative to the center of the star. This will influence how far they can travel before experiencing a flavor oscillation and how many times they oscillate within the star. The researchers involved here, Mariam Gogilashvili and Irene Tamborra of the University of Copenhagen, aren’t going for a physically exacting model of all of this complexity. Instead, the two researchers use a simplified model where flavor changes occur roughly instantaneously, and the energy carried by neutrinos is divided evenly among the six types of neutrinos (electron, muon, tau, and their antiparticles). They apply this division of energy in their model as neutrinos travel through the space between the star’s core (where neutrinos are coupled with matter) and the higher-density matter of the shock wave. They test three different density cutoffs to define the shock wave, giving a range of potential estimates of the impact. 这里的实际物理过程极其复杂，因为每个中微子的动量都会有所不同，它们相对于恒星中心产生的位置也会不同。这将影响它们在经历味振荡之前能走多远，以及它们在恒星内部振荡的次数。参与这项研究的哥本哈根大学的 Mariam Gogilashvili 和 Irene Tamborra 并没有试图建立一个涵盖所有这些复杂性的物理精确模型。相反，这两位研究人员使用了一个简化模型，其中味变几乎是瞬间发生的，中微子携带的能量在六种中微子（电子、μ子、τ子及其反粒子）之间平均分配。他们在模型中应用了这种能量分配，模拟中微子穿过恒星核心（中微子与物质耦合的地方）和冲击波高密度物质之间的空间。他们测试了三种不同的密度截止值来定义冲击波，从而给出了影响的潜在估计范围。