Measurements for Muon g-2 Hadronic Vacuum Polarization

目前对μ子异常磁矩a_mu = (g-2)/2的测量和理论预测存在大约3.5个标准差的差异。如果在更高的意义上得到验证，这将是新物理学的一个明确标志，并将彻底改变这个领域。新的费米实验室g-2实验将显著改善测量结果，但标准模型理论误差主要取决于强子真空极化（HVP）修正的确定，而标准模型理论误差必须与之比较。如果要在这一差异上取得进展，就需要同时改进对HVP贡献的确定。由于应用微扰QCD的相关能量尺度太低，解决这一问题的最可靠方法是测量过程e+e- ->强子从阈值到约3GeV，并通过色散关系计算HVP贡献。到目前为止，这个过程的主要通道是e+e- -> pi+ pi-。这是一项利用初始态辐射（ISR）方法精确测量e+e- -> pi+pi- / e+e- -> mu+mu-从阈值到3-5GeV能量的截面比的建议：即测量e+e- -> gamma pi+pi-和e+e- -> gamma mu+mu-其中伽马是一个ISR光子，在与光束的大角度处被探测和精确测量。BaBar使用其数据的前半部分（232/fb）发布了这一比率，这笔拨款将资助BaBar数据集的后半部分（245/fb）的分析和发布。BaBar数据集的后半部分使用了不同的μ子探测器系统，因此相对于现有出版物的增益不仅具有统计独立性，而且具有很大不同的系统不确定性，因为μ子系统用于将探测器中的μ子与介子清晰地分开。这种新的测量方法将大大改进HVP贡献的确定。该提案将支持一名现有的研究生，他正在测量e+e- -> mu+mu-，以及一名博士后，他将致力于完成BaBar数据集后半部分的e+e- ->伽马pi+ pi- / e+e- ->伽马mu+mu分析，并与之前的BaBar结果相结合。它包括要求一些旅行支持。出差将使申请人、学生和博士后能够参加BaBar会议和一些会议。出差支持也将使申请人履行其作为BaBar发言人的职责。

Current measurements and theoretical predictions of the muon anomalous magnetic moment, a_mu = (g-2)/2, are in disagreement by approximately 3.5 standard deviations. If verified at higher significance, this would be a clear signature of new physics and revolutionize the field. The new Fermilab g-2 experiment will improve the measurement significantly, but the Standard Model theoretical error, to which it must be compared, is dominated by the determination of the hadronic vacuum polarization (HVP) correction. Simultaneous improvements to the determination of the HVP contribution are required if there is to be progress on this discrepancy. Because the relevant energy scale is too low for applying perturbative QCD, the most reliable method for attacking this problem is to measure the process e+e- -> hadrons from threshold to approximately 3GeV and use that to calculate the HVP contributions though a dispersion relation. By far the dominant channel for this process is e+e- -> pi+ pi-. This is a proposal to make a precision measurement of the ratios of the cross section for e+e- -> pi+ pi- / e+e- -> mu+mu- from threshold to an energy of 3-5GeV using the initiate-state radiation (ISR) method: i.e. the measurement of e+e- -> gamma pi+pi- and e+e- -> gamma mu+mu- where the gamma is an ISR photon detected and precisely measured at large angles to the beam. BaBar published on this ratio using the first half of its data (232/fb) and this grant will fund the analysis and publication of the second half of the BaBar dataset (245/fb). The second half of the BaBar dataset uses a different muon detector system and therefore the gain over the existing publication is not only statistical independence, but a largely different systematic uncertainty, since the muon system is used to cleanly separate muons from pions in the detector. This new measurement will provide a substantial improvement on the determination of the HVP contribution. The proposal will support an existing graduate student who is measuring e+e- -> mu+mu-, and a postdoc who will be dedicated to completion to publication of the e+e- -> gamma pi+ pi- / e+e- -> gamma mu+mu analysis in the second half of the BaBar dataset and a combination with the previous BaBar result. It includes a request for some travel support. The travel will enable the applicant, student and postdoc to attend BaBar meetings and a few conferences. The travel support will also enable the applicant to fulfill his responsibilities as BaBar Spokesperson.