For most of their existence, stars are fuelled by the fusion of hydrogen into helium. Fusion proceeds via two processes that are well understood theoretically: the proton-proton (pp) chain and the carbon-nitrogen-oxygen (CNO) cycle1,2. Neutrinos that are emitted along such fusion processes in the solar core are the only direct probe of the deep interior of the Sun. A complete spectroscopic study of neutrinos from the pp chain, which produces about 99 per cent of the solar energy, has been performed previously3; however, there has been no reported experimental evidence of the CNO cycle. Here we report the direct observation, with a high statistical significance, of neutrinos produced in the CNO cycle in the Sun. This experimental evidence was obtained using the highly radiopure, large-volume, liquid-scintillator detector of Borexino, an experiment located at the underground Laboratori Nazionali del Gran Sasso in Italy. The main experimental challenge was to identify the excess signal-only a few counts per day above the background per 100 tonnes of target-that is attributed to interactions of the CNO neutrinos. Advances in the thermal stabilization of the detector over the last five years enabled us to develop a method to constrain the rate of bismuth-210 contaminating the scintillator. In the CNO cycle, the fusion of hydrogen is catalysed by carbon, nitrogen and oxygen, and so its rate-as well as the flux of emitted CNO neutrinos-depends directly on the abundance of these elements in the solar core. This result therefore paves the way towards a direct measurement of the solar metallicity using CNO neutrinos. Our findings quantify the relative contribution of CNO fusion in the Sun to be of the order of 1 per cent; however, in massive stars, this is the dominant process of energy production. This work provides experimental evidence of the primary mechanism for the stellar conversion of hydrogen into helium in the Universe.

中文翻译：

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太阳中 CNO 聚变循环中产生的中微子的实验证据

在它们存在的大部分时间里，恒星的燃料都是氢融合成氦。融合通过两个理论上很好理解的过程进行：质子 - 质子 (pp) 链和碳 - 氮 - 氧 (CNO) 循环1,2。在太阳核心中沿着这种聚变过程发射的中微子是对太阳深处内部的唯一直接探测。之前已经对来自 pp 链的中微子进行了完整的光谱研究，该链产生了大约 99% 的太阳能。然而，还没有关于 CNO 循环的实验证据的报道。在这里，我们报告了对太阳 CNO 循环中产生的中微子的直接观察，具有很高的统计意义。该实验证据是使用 Borexino 的高辐射纯、大体积、液体闪烁体探测器获得的，一项位于意大利国家大萨索地下实验室的实验。主要的实验挑战是识别由于 CNO 中微子相互作用而产生的过量信号 - 每 100 吨目标每天仅比背景高出几个计数。过去五年中探测器热稳定性方面的进步使我们能够开发出一种方法来限制铋 210 污染闪烁体的速率。在 CNO 循环中，氢的聚变是由碳、氮和氧催化的，因此它的速率——以及发射的 CNO 中微子的通量——直接取决于太阳核心中这些元素的丰度。因此，该结果为使用 CNO 中微子直接测量太阳金属丰度铺平了道路。我们的研究结果将太阳中 CNO 聚变的相对贡献量化为 1% 左右；然而，在大质量恒星中，这是能量产生的主要过程。这项工作为宇宙中恒星将氢转化为氦的主要机制提供了实验证据。