Nature. 2020 Apr;580(7803):339-344. doi: 10.1038/s41586-020-2177-0. Epub 2020 Apr 15.
The charge-conjugation and parity-reversal (CP) symmetry of fundamental particles is a symmetry between matter and antimatter. Violation of this CP symmetry was first observed in 1964, and CP violation in the weak interactions of quarks was soon established. Sakharov proposed that CP violation is necessary to explain the observed imbalance of matter and antimatter abundance in the Universe. However, CP violation in quarks is too small to support this explanation. So far, CP violation has not been observed in non-quark elementary particle systems. It has been shown that CP violation in leptons could generate the matter-antimatter disparity through a process called leptogenesis. Leptonic mixing, which appears in the standard model's charged current interactions, provides a potential source of CP violation through a complex phase δ, which is required by some theoretical models of leptogenesis. This CP violation can be measured in muon neutrino to electron neutrino oscillations and the corresponding antineutrino oscillations, which are experimentally accessible using accelerator-produced beams as established by the Tokai-to-Kamioka (T2K) and NOvA experiments. Until now, the value of δ has not been substantially constrained by neutrino oscillation experiments. Here we report a measurement using long-baseline neutrino and antineutrino oscillations observed by the T2K experiment that shows a large increase in the neutrino oscillation probability, excluding values of δ that result in a large increase in the observed antineutrino oscillation probability at three standard deviations (3σ). The 3σ confidence interval for δ, which is cyclic and repeats every 2π, is [-3.41, -0.03] for the so-called normal mass ordering and [-2.54, -0.32] for the inverted mass ordering. Our results indicate CP violation in leptons and our method enables sensitive searches for matter-antimatter asymmetry in neutrino oscillations using accelerator-produced neutrino beams. Future measurements with larger datasets will test whether leptonic CP violation is larger than the CP violation in quarks.
基本粒子的电荷共轭和宇称反转 (CP) 对称性是物质和反物质之间的一种对称性。这种 CP 对称性的破坏最早是在 1964 年观察到的，随后不久就确立了夸克弱相互作用中的 CP 破坏。萨哈罗夫提出，CP 破坏是解释宇宙中观测到的物质与反物质丰度不平衡所必需的。然而，夸克中的 CP 破坏太小，无法支持这种解释。到目前为止，在非夸克基本粒子系统中还没有观察到 CP 破坏。已经表明，通过称为轻子生成的过程，轻子中的 CP 破坏可以产生物质-反物质的差异。出现在标准模型带电电流相互作用中的轻子混合通过一个复杂的相位 δ 提供了 CP 破坏的潜在来源，某些轻子生成理论模型需要这个相位 δ。这种 CP 破坏可以通过μ子中微子到电子中微子振荡以及相应的反中微子振荡来测量，这是通过 Tokai-to-Kamioka (T2K) 和 NOvA 实验所建立的加速器产生的束流实验来实现的。到目前为止，中微子振荡实验并没有对 δ 值进行实质性的约束。在这里，我们报告了 T2K 实验观察到的长基线中微子和反中微子振荡的测量结果，该结果表明中微子振荡概率有很大的增加，排除了 δ 值导致观测到的反中微子振荡概率在三个标准偏差 (3σ) 内显著增加的情况。对于所谓的正常质量顺序，δ 的 3σ 置信区间是 [-3.41, -0.03]，对于倒转质量顺序，δ 的 3σ 置信区间是 [-2.54, -0.32]。我们的结果表明轻子中的 CP 破坏，我们的方法可以使用加速器产生的中微子束进行敏感的中微子振荡物质-反物质不对称性搜索。未来使用更大数据集的测量将检验轻子 CP 破坏是否大于夸克中的 CP 破坏。
