The standard model of particle physics describes the vast majority of experiments and observations involving elementary particles. Any deviation from its predictions would be a sign of new, fundamental physics. One long-standing discrepancy concerns the anomalous magnetic moment of the muon, a measure of the magnetic field surrounding that particle. Standard-model predictions¹ exhibit disagreement with measurements² that is tightly scattered around 3.7 standard deviations. Today, theoretical and measurement errors are comparable; however, ongoing and planned experiments aim to reduce the measurement error by a factor of four. Theoretically, the dominant source of error is the leading-order hadronic vacuum polarization (LO-HVP) contribution. For the upcoming measurements, it is essential to evaluate the prediction for this contribution with independent methods and to reduce its uncertainties. The most precise, model-independent determinations so far rely on dispersive techniques, combined with measurements of the cross-section of electron–positron annihilation into hadrons^3,4,5,6. To eliminate our reliance on these experiments, here we use ab initio quantum chromodynamics (QCD) and quantum electrodynamics simulations to compute the LO-HVP contribution. We reach sufficient precision to discriminate between the measurement of the anomalous magnetic moment of the muon and the predictions of dispersive methods. Our result favours the experimentally measured value over those obtained using the dispersion relation. Moreover, the methods used and developed in this work will enable further increased precision as more powerful computers become available.

粒子物理学的标准模型描述了绝大多数涉及基本粒子的实验和观察。任何与其预测的偏差都将是新的基础物理学的标志。一个长期存在的差异涉及 μ 子的反常磁矩，这是该粒子周围磁场的量度。标准模型预测¹与测量结果不一致²紧密分布在 3.7 个标准差附近。今天，理论误差和测量误差具有可比性；然而，正在进行和计划中的实验旨在将测量误差减少四分之一。从理论上讲，误差的主要来源是前导阶强子真空极化 (LO-HVP) 贡献。对于即将进行的测量，必须使用独立方法评估对此贡献的预测并减少其不确定性。迄今为止，最精确、与模型无关的测定依赖于色散技术，并结合对电子-正电子湮灭成强子的横截面的测量^3,4,5,6. 为了消除我们对这些实验的依赖，我们在这里使用从头算量子色动力学 (QCD) 和量子电动力学模拟来计算 LO-HVP 贡献。我们达到了足够的精度来区分 μ 子异常磁矩的测量和色散方法的预测。我们的结果有利于实验测量值，而不是使用色散关系获得的值。此外，随着更强大的计算机的出现，这项工作中使用和开发的方法将进一步提高精度。