Measurements for Muon g-2 Hadronic Vacuum Polarization

μ子反常磁矩a_mu =（g-2）/2的当前测量值和理论预测值相差约3.5个标准差。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，并将彻底改变该领域。新的费米实验室g-2实验将显著改善测量，但标准模型的理论误差，它必须进行比较，是由确定强子真空极化（HVP）校正。如果要在这一差异方面取得进展，就需要同时改进对高价值量贡献的确定。由于相应的能量标度太低，不能应用微扰QCD，解决这个问题的最可靠的方法是测量从阈到大约3GeV的e+e->强子过程，并利用它通过色散关系计算HVP贡献。到目前为止，这个过程的主要通道是e+e-> pi+ pi-。本文提出了用初始态辐射（ISR）方法精确测量从阈到3-5GeV能量的e+e- -> pi+ pi- / e+e- -> mu+mu-的截面比：即e+e-> gamma pi+pi-和e+e-> gamma mu+mu-的测量其中伽马是在与射束成大角度处检测并精确测量的ISR光子。BaBar使用其数据的前半部分（232/fb）发布了这一比率，这笔赠款将资助分析和发布BaBar数据集的后半部分（245/fb）。BaBar数据集的后半部分使用了不同的μ子探测器系统，因此与现有出版物相比，其增益不仅是统计独立性，而且是一种很大程度上不同的系统不确定性，因为μ子系统用于在探测器中干净地分离μ子和π子。这种新的测量方法将大大改善HVP贡献的测定。该提案将支持一名正在测量e+e- -> mu+mu-的现有研究生，以及一名博士后，该博士后将致力于完成BaBar数据集下半部分的e+e- -> gamma pi+ pi- / e+e- -> gamma mu+mu分析并与之前的BaBar结果相结合。其中包括要求提供一些旅行支助。旅行将使申请人，学生和博士后参加BaBar会议和一些会议。旅行支持也将使申请人能够履行其作为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.