Measurements for Muon g-2 Hadronic Vacuum Polarization

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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.