Every second, tens of billions of neutrinos produced by nuclear fusion in the Sun pass through every square centimetre of the Earth—and through our bodies—almost without interacting. Although they are one of the most abundant particles emitted by the Sun, their extremely weak interactions make the detection of these elusive particles one of the greatest experimental challenges in particle physics.
Today, during a seminar hosted by the INFN Laboratori Nazionali del Gran Sasso (LNGS), Italy, the XENON Collaboration announced the first observation of low-energy solar neutrinos scattering off electrons in the XENONnT detector. The measurement extends the frontier of direct neutrino observations down to neutrino energies of about 17 keV, the lowest neutrino energy threshold ever achieved. The detected signal is dominated by pp neutrinos, produced in the proton–proton fusion reactions that power the Sun and account for the vast majority of its neutrino emission.
XENONnT was originally designed for the direct search for particle dark matter in our Galaxy. At its heart is a dual-phase xenon Time Projection Chamber containing 5.9 tonnes of ultra-pure liquid xenon, installed at LNGS, 1,400 metres beneath the Gran Sasso massif. The detector is surrounded by water Cherenkov detectors that identify and reject cosmic-ray muons and neutrons, and it is capable of reconstructing the tiny flashes of light and ionization signals produced when a particle interacts with the xenon target.
Observing this feeble low-energy neutrino signal at 5σ—the statistical significance conventionally used in particle physics to claim a discovery—required not only reducing the detector backgrounds to unprecedented levels of purity but also quantifying them precisely. The dominant challenge comes from trace amounts of radioactive radon constantly released by detector materials. Over many years, the XENON Collaboration has pioneered techniques to suppress this background through extensive material screening and a dedicated online cryogenic distillation system that continuously removes radon from the xenon. The Collaboration identified and constrained every relevant background contribution at exceptionally low rates, including beta decays from lead and krypton isotopes, smaller contributions from material-induced gamma rays, as well as other subdominant components.
The result further expands the scientific reach of XENONnT. Following the recent observation of coherent elastic neutrino–nucleus scattering from higher-energy solar neutrinos, this new measurement demonstrates that the same detector can probe complementary aspects of neutrino physics, while continuing its primary search for dark matter. XENONnT is thus emerging as one of the world’s most sensitive observatories for rare low-energy particle interactions.
The measurement also builds upon a long tradition of solar-neutrino research at LNGS. GALLEX/GNO provided the first measurements of the low-energy solar neutrino flux using radiochemical techniques, while Borexino pioneered the real-time spectroscopy of individual solar neutrino interactions with a neutrino energy threshold of 335 keV. XENONnT now extends this legacy, lowering the solar-neutrino energy threshold to 17 keV.
Beyond this achievement, XENONnT offers a glimpse of the future of rare-event physics. The technologies developed to build and operate one of the cleanest particle detectors are laying the foundation for the next generation of liquid-xenon observatories. The planned XLZD experiment, with a target mass an order of magnitude larger than XENONnT, aims to extend the search for dark matter to the highest sensitivities ever achieved and to measure low-energy solar neutrinos with exceptional precision, while opening new opportunities in neutrino physics and the study of other extremely rare processes.
“This observation of low-energy solar neutrinos demonstrates how advances driven by the search for dark matter are opening entirely new windows on the Universe,” said Elena Aprile, Professor at Columbia University and spokesperson of the XENON Collaboration. “It shows that technologies originally developed to observe some of the rarest interactions in nature are now enabling us to explore fundamental questions well beyond their original scientific goals.”
The paper can be downloaded here (pdf).
每一秒，太阳核聚变产生的数百亿个中微子都会穿过地球上每平方厘米的面积，也会穿过我们的身体，而几乎不发生任何相互作用。中微子是太阳释放数量最多的粒子之一，但由于它们与物质的相互作用极其微弱，探测这些难以捉摸的粒子一直是粒子物理实验面临的重大挑战之一。
今天，在意大利国家核物理研究院格兰萨索国家实验室（INFN Laboratori Nazionali del Gran Sasso，LNGS）举办的专题报告上，XENON合作组宣布首次在XENONnT探测器中观测到低能太阳中微子与电子发生散射的信号。这一测量将直接观测太阳中微子的能量前沿推进至约17 keV，创造了迄今最低的中微子能量阈值。观测到的信号主要来自pp中微子，它们产生于驱动太阳发光发热的质子质子链聚变反应，同时构成太阳中微子通量的绝大部分。
XENONnT最初是为直接寻找银河系中的粒子暗物质而设计的。实验核心是一台双相氙时间投影室，其中装有5.9吨超纯液氙，安装在格兰萨索山体下方约1400米深处的LNGS地下实验室。探测器外围由水切伦科夫探测器环绕，用于识别并排除宇宙线缪子和中子产生的本底。XENONnT能够重建粒子与液氙相互作用时产生的极微弱闪烁光和电离信号。
要以5σ的统计显著性观测到这一极其微弱的低能中微子信号，需要将探测器本底降低到前所未有的水平，同时还必须对所有本底成分进行精确量化。在粒子物理中，5σ通常被视为宣布发现的统计标准。
其中最主要的挑战来自探测器材料持续释放的微量放射性氡。多年来，XENON合作组发展并完善了一系列抑制氡本底的技术，包括大规模材料放射性筛选，以及专门设计的在线低温蒸馏系统，该系统能够持续从液氙中去除氡。合作组还对所有相关本底来源进行了识别和约束，使其达到极低水平。这些本底包括铅和氪同位素的β衰变、探测器材料产生的少量γ射线，以及其他更小的本底成分。
这一结果进一步拓展了XENONnT的科学研究范围。此前，XENONnT已经观测到较高能太阳中微子引起的相干弹性中微子核散射。此次新测量表明，同一台探测器还能够研究太阳中微子的其他互补物理过程，同时继续开展其核心的暗物质直接探测任务。XENONnT正逐渐成为全球对稀有低能粒子相互作用最灵敏的观测装置之一。
这一测量也延续了LNGS在太阳中微子研究领域的悠久传统。GALLEX和GNO实验利用放射化学方法首次测量了低能太阳中微子通量，Borexino则率先实现了太阳中微子单次相互作用的实时能谱测量，其对应的中微子能量阈值为335 keV。如今，XENONnT进一步延续了这一研究传统，将太阳中微子的能量阈值降低至17 keV。
这一成果同时展现了稀有事例物理未来发展的一个方向。为了建造并运行世界上最洁净的粒子探测器之一而发展出的各项技术，正在为下一代液氙观测装置奠定基础。规划中的XLZD实验，其靶质量预计将达到XENONnT的约十倍，目标是把暗物质直接探测推进到前所未有的灵敏度，同时以前所未有的精度测量低能太阳中微子，并进一步拓展中微子物理以及其他极稀有过程的研究机会。
“这次对低能太阳中微子的观测表明，由暗物质探测推动的技术进步，正在为我们打开全新的宇宙观测窗口。”哥伦比亚大学教授、XENON合作组发言人Elena Aprile表示，“这说明，那些最初为探测自然界中最稀有相互作用而开发的技术，如今已经使我们能够探索远远超出其最初科学目标的基础物理问题。”
论文可在此处下载PDF。
