Blum Thomas, Chowdhury Saumitra, Hayakawa Masashi, Izubuchi Taku
Physics Department, University of Connecticut, Storrs, Connecticut 06269-3046, USA and RIKEN BNL Research Center, Brookhaven National Laboratory, Upton, New York 11973, USA.
Physics Department, University of Connecticut, Storrs, Connecticut 06269-3046, USA.
Phys Rev Lett. 2015 Jan 9;114(1):012001. doi: 10.1103/PhysRevLett.114.012001. Epub 2015 Jan 7.
The most compelling possibility for a new law of nature beyond the four fundamental forces comprising the standard model of high-energy physics is the discrepancy between measurements and calculations of the muon anomalous magnetic moment. Until now a key part of the calculation, the hadronic light-by-light contribution, has only been accessible from models of QCD, the quantum description of the strong force, whose accuracy at the required level may be questioned. A first principles calculation with systematically improvable errors is needed, along with the upcoming experiments, to decisively settle the matter. For the first time, the form factor that yields the light-by-light scattering contribution to the muon anomalous magnetic moment is computed in such a framework, lattice QCD+QED and QED. A nonperturbative treatment of QED is used and checked against perturbation theory. The hadronic contribution is calculated for unphysical quark and muon masses, and only the diagram with a single quark loop is computed for which statistically significant signals are obtained. Initial results are promising, and the prospect for a complete calculation with physical masses and controlled errors is discussed.
在构成高能物理标准模型的四种基本力之外，自然界新定律最令人信服的可能性在于μ子反常磁矩的测量值与计算值之间的差异。到目前为止，计算中的一个关键部分，即强子真空极化贡献，只能从量子色动力学（QCD，强力的量子描述）的模型中获得，而该模型在所需精度水平上的准确性可能受到质疑。需要进行具有系统可改进误差的第一性原理计算，并结合即将开展的实验，以果断解决这个问题。首次在这样的框架，即晶格QCD+QED和QED中，计算了产生μ子反常磁矩的轻子-轻子散射贡献的形状因子。采用了量子电动力学（QED）的非微扰处理方法，并与微扰理论进行了对比检验。针对非物理夸克和μ子质量计算了强子贡献，并且仅计算了获得具有统计显著性信号的单夸克圈图。初步结果很有希望，并且讨论了使用物理质量和可控误差进行完整计算的前景。
