R is the ratio of the hadronic cross section to the muon cross section in electron–positron collisions: where the superscript (0) indicates that the cross section has been corrected for initial state radiation.
R is an important input in the calculation of the anomalous magnetic dipole moment.
[1] Experimental values have been measured for center-of-mass energies from 400 MeV to 150 GeV.
[3] A simplified calculation of R yields where the sum is over all quark flavors with mass less than the beam energy.
[4] Usually, the denominator in R is not the actual experimental μμ cross section, but the off-resonance theoretical QED cross-section: this makes resonances more visibly dramatic than normalization by the μμ cross section, which is also greatly enhanced at these resonances (hadronic states, and Z boson).