Abstract
After a brief introduction to the muon anomalous magnetic moment, the current status of the ( g − 2) µ measurement is given. The three standard deviations of discrepancy between a µ calculations and the direct measurement by the E821 experiment at the Brookhaven National Laboratory (USA) calls for more precise measurements of the e + e − hadronic cross section. This article contains a not inclusive review of the main results and the state of the art on the hadronic cross section measurements from BABAR and other experiments.
Highlights
After a brief introduction to the muon anomalous magnetic moment, the current status of the (g − 2)μ measurement is given
In the Standard Model (SM) the anomalous muon magnetic moment is split into different contributions: aSμ M = aμQED + awμeak + ahμad
The major contributions come from the lowest order (LO) hadronic vacuum polarization, some higher order vacuum polarization, and a light-by-light contribution where the hadronic loop has four photon propagators connected to the QED diagram
Summary
The traditional way to measure the hadronic e+e− cross section as function of energy is to use data from e+e− scan experiments, collected by changing the beam energy in all the accessible energy range. Very large data sets exist in the energy region below 1.4 GeV collected by SND and CMD-2 experiments at VEPP-2M in Novosibirsk (Russia). This value very little data exist, collected mainly by DM1 and DM2 experiments at DCI collider in Orsay (France). The Initial State Radiation (ISR) method, used by BABAR at the PEP-II accelerator [6] in Menlo Park (USA) and other experiments like KLOE at DAΦNE in Frascati (Italy) or, more recently, BESIII at BEPCII in Beijing (China), takes advantage of the high precision allowed by the large data sets and the high detector performances in an energy region previously unreachable
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