Hadronic Light-by-Light Contribution to the Muon g-2

强子逐光对 Muon g-2 的贡献

• 批准号: 498966902
• 负责人: Dr. Igor Danilkin
• 金额: --
• 依托单位: Deutsches Elektronen-Synchrotron (DESY), Standort Zeuthen
• 依托单位国家: 德国
• 项目类别: Research Units
• 资助国家: 德国
• 项目状态: 未结题
• 来源: https://gepris.dfg.de/gepris/projekt/498966902?language=en
• 关键词: Hadronic; Light; Contribution; Muon

The anomalous magnetic moment of the muon — the muon g-2 — is serving as a precision test for the Standard Model (SM) of particle physics. However, at the moment this test does not seem to work out, which is a hint for physics beyond the SM. There is a discrepancy between the SM prediction and the experimental value of the muon g-2. Presently, this discrepancy is at the level of 4.2 standard deviations. Two dedicated experiments at Fermilab and J-PARC aim, respectively, to refine and independently confirm the experimental value in the coming years. To rigorously answer the question whether we observe New Physics in the muon g-2 experiments, the accuracy of the SM prediction must be improved to complement the anticipated precision of the new Fermilab experiment. This should be achieved in our Joint Research Project JRP.The accuracy of the SM prediction is limited by hadronic contributions. We distinguish the hadronic vacuum polarization (HVP) and light-by-light scattering (HLbL)  contributions. While the HVP is about 2 orders of magnitude larger than the HLbL, the theoretical uncertainty of the HVP is only a factor of 2 larger than the uncertainty of the HLbL. Due to the complicated non-perturbative nature of quantum chromodynamics (QCD), hadronic contributions are rather hard to calculate. In the absence of a simple QCD description, we employ data-driven dispersive evaluations and lattice QCD. Our main objective is to improve the prediction of the HLbL contribution. Firstly, we will consolidate our existing lattice QCD calculation of the HLbL contribution and reduce the statistical and the systematic errors from the present 15% accuracy level to 10% or better. The ultimate aim is a direct calculation with physical pion mass. Secondly, we will perform data-driven dispersive evaluations of the different single- and multi-meson channel contributions. Here, we employ new experimental data-sets from the BESIII and MAMI facilities, that we generate in Project TFF. The two independent predictions — from lattice QCD and phenomenology — could then be combined into a weighted average, as is done for the presently recommended HLbL value.Considering the HVP contribution, there is a tension of 2.1 standard deviations between the presently accepted data-driven estimate, using experimentally measured hadronic cross sections, and the  lattice QCD prediction by the BMW Collaboration. Clearly, an independent lattice QCD calculation with comparable overall precision is required to clarify the situation. Therefore, our other objective is to compute isospin-breaking corrections to the HVP contribution that arise from electromagnetic and strong interaction effects. These are required to control the overall uncertainty at the sub-percent level. For the QED corrections, we will again have two approaches: 1) direct lattice QCD calculation; 2) a Cottingham-type formula with lattice QCD results for the forward HLbL amplitudes, obtained in Project LBL, as input.

μ子的异常磁矩——μ子g-2——被用作粒子物理学标准模型（SM）的精确测试。然而，目前这个测试似乎没有成功，这是一个超越SM的物理暗示。μ介子g-2的SM预测值与实验值存在差异。目前，这一差异处于4.2个标准差的水平。费米实验室和J-PARC分别进行了两个专门的实验，目的是在未来几年内完善和独立确认实验值。为了严格回答我们是否在μ子g-2实验中观察到新物理的问题，必须提高SM预测的精度，以补充新费米实验室实验的预期精度。这应该在我们的联合研究项目JRP中实现。SM预测的精度受到强子贡献的限制。我们区分了强子真空极化（HVP）和逐光散射（HLbL）的贡献。虽然HVP比HLbL大2个数量级，但HVP的理论不确定度仅比HLbL大2个数量级。由于量子色动力学（QCD）复杂的非微扰性质，强子贡献相当难以计算。在没有简单QCD描述的情况下，我们采用数据驱动的色散评估和晶格QCD。我们的主要目标是改进对高水平白细胞贡献的预测。首先，我们将巩固现有的关于HLbL贡献的点阵QCD计算，并将统计误差和系统误差从目前的15%精度水平降低到10%或更高。最终目标是直接计算物理介子质量。其次，我们将对不同的单介子和多介子通道贡献进行数据驱动的色散评估。在这里，我们使用了来自BESIII和MAMI设施的新实验数据集，这些数据集是我们在TFF项目中生成的。两个独立的预测——来自晶格QCD和现象学——然后可以组合成一个加权平均值，就像目前推荐的HLbL值一样。考虑到HVP的贡献，目前接受的数据驱动估计（使用实验测量的强子截面）与BMW协作的晶格QCD预测之间存在2.1个标准差的张力。显然，需要一个具有相当整体精度的独立晶格QCD计算来澄清这种情况。因此，我们的另一个目标是计算由电磁和强相互作用效应引起的HVP贡献的同位旋断裂修正。这些都是将总体不确定性控制在次百分比水平所必需的。对于QED修正，我们将再次采用两种方法：1)直接点阵QCD计算；2)将LBL项目中得到的HLbL正演振幅的点阵QCD结果的cottingham型公式作为输入。