R (cross section ratio)

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R is the ratio of the hadronic cross section to the muon cross section in electron-positron collisions:

R = \frac{\sigma^{(0)}(e^+e^-\rightarrow \mathrm{hadrons})}{\sigma^{(0)}(e^+e^-\rightarrow \mu^+\mu^-)},

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.[2]

R also provides experimental confirmation of the electric charge of quarks, in particular the charm quark and bottom quark, and the existence of three quark colors.[3] A naive calculation of R yields

R = 3\sum_q e_q^2,

where the sum is over all quark flavors with mass less than the beam energy. eq is the electric charge of the quark, and the factor of 3 accounts for the three flavors of the quarks. QCD corrections to this naive formula have been calculated.[4]

[edit] Notes

  1. ^ S. Eidelman and F. Jegerlehner, Z. Phys. C67 585 (1995).
  2. ^ W.-M. Yao et al., J. Phys. G 33, 1 (2006) Section 40
  3. ^ W.-M. Yao et al., J. Phys. G 33, 1 (2006) Section 9.7
  4. ^ S.G. Gorishny et al., Phys. Lett. B259 144 (1991)
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