Abstract
A new experiment is proposed to measure the running of the electromagnetic coupling constant in the space-like region by scattering high-energy muons on atomic electrons of a low-Z target. The differential cross section of the elastic process μe → μe provides direct sensitivity to the leading-order hadronic contribution to the muon anomaly aμ HLO . It is argued that by using the 150-GeV muon beam available at the CERN North Area, with an average rate of ~ 1.3 × 107 muon/s, a statistical uncertainty of ~ 0.3% can be achieved on aμ HLO after two years of data taking. The direct measurement of aμ HLO via μe scattering will provide an independent determination and consolidate the theoretical prediction for the muon g -2 in the Standard Model. It will allow therefore a firmer interpretation of the measurements of the future muon g -2 experiments at Fermilab and JPARC.
Highlights
The material presented here is largely based on the recently published paper of Ref. [1].The long-standing (3–4)σ discrepancy between the experimental value of the muon anomalous magnetic moment aμ = (g − 2)/2 and the Standard Model (SM) prediction, Δaμ(Exp − SM) ∼ (28 ± 8) × 10−10 [2, 3], is considered as one of the most intriguing indications of physics beyond the SM
By using analyticity and unitarity, it was shown [4] that the leading-order (LO) hadronic contribution to the muon g-2, aHμ LO, can be computed via a dispersion integral of the hadron production cross section in e+e− annihilation at low-energy
A novel approach is under study to measure the running of α in the space-like region which can be used to determine aHμ LO, the leading hadronic contribution to the muon g-2
Summary
The material presented here is largely based on the recently published paper of Ref. [1]. By using analyticity and unitarity, it was shown [4] that the leading-order (LO) hadronic contribution to the muon g-2, aHμ LO, can be computed via a dispersion integral of the hadron production cross section in e+e− annihilation at low-energy. With this technique, the present error on aHμ LO, ∼ 4 × 10−10 (corresponding to a fractional accuracy of 0.6%), constitutes the main uncertainty of the SM prediction. The error achieved by the BNL E821 experiment, δaEμxp = 6.3 × 10−10 (corresponding to 0.54 ppm) [6], is dominated by the available statistics. How the hadronic contribution to the running of α can be determined unambiguously through the t-channel μe elastic scattering, from which aHμ LO can be obtained, is sketched
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