We propose the first practical method to detect atmospheric tau neutrino appearance at sub-GeV energies, which would be an important test of ${\nu }_{\mu }\to {\nu }_{\tau }$ oscillations and of new-physics scenarios. In the Jiangmen Underground Neutrino Observatory (JUNO; starts in 2024), active-flavor neutrinos eject neutrons from carbon via neutral-current quasielastic scattering. This produces a two-part signal: the prompt part is caused by the scattering of the neutron in the scintillator, and the delayed part by its radiative capture. Such events have been observed in KamLAND, but only in small numbers and were treated as a background. With ${\nu }_{\mu }\to {\nu }_{\tau }$ oscillations, JUNO should measure a clean sample of 55 events/yr; with simple ${\nu }_{\mu }$ disappearance, this would instead be 41 events/yr, where the latter is determined from Super-Kamiokande charged-current measurements at similar neutrino energies. Implementing this method will require precise laboratory measurements of neutrino-nucleus cross sections or other developments. With those, JUNO will have $5\sigma$ sensitivity to tau-neutrino appearance in five years of exposure, and likely sooner.

中文翻译：

---

大气 tau 中微子出现的新信号：JUNO 中的亚 GeV 中性流相互作用

我们提出了第一个实用方法来检测亚GeV能量下大气中tau中微子的出现，这将是一个重要的测试${\nu }_{\mu }\to {\nu }_{\tau }$振荡和新物理场景。在江门地下中微子观测站（JUNO；于 2024 年启动），活性中微子通过中性流准弹性散射从碳中喷射出中子。这会产生两部分信号：即时部分是由中子在闪烁体中的散射引起的，延迟部分是由中子的辐射捕获引起的。卡姆兰德曾观察到此类事件，但数量很少，并被视为背景。和${\nu }_{\mu }\to {\nu }_{\tau }$振荡，JUNO 应每年测量 55 个事件的干净样本；与简单的${\nu }_{\mu }$消失，这将是 41 次事件/年，后者是根据类似中微子能量下的超级神冈带电电流测量确定的。实施这种方法需要对中微子核横截面进行精确的实验室测量或其他进展。有了这些，JUNO 将拥有$5\sigma$在暴露五年内，甚至可能更早，对 tau 中微子出现的敏感性就会增加。