''R'' is the ratio of the
hadron
In particle physics, a hadron (; grc, ἁδρός, hadrós; "stout, thick") is a composite subatomic particle made of two or more quarks held together by the strong interaction. They are analogous to molecules that are held together by the ele ...
ic
cross section
Cross section may refer to:
* Cross section (geometry)
** Cross-sectional views in architecture & engineering 3D
*Cross section (geology)
* Cross section (electronics)
* Radar cross section, measure of detectability
* Cross section (physics)
**Abs ...
to the
muon
A muon ( ; from the Greek letter mu (μ) used to represent it) is an elementary particle similar to the electron, with an electric charge of −1 '' e'' and a spin of , but with a much greater mass. It is classified as a lepton. As wi ...
cross section in
electron
The electron ( or ) is a subatomic particle with a negative one elementary electric charge. Electrons belong to the first generation of the lepton particle family,
and are generally thought to be elementary particles because they have no kn ...
–
positron
The positron or antielectron is the antiparticle or the antimatter counterpart of the electron. It has an electric charge of +1 '' e'', a spin of 1/2 (the same as the electron), and the same mass as an electron. When a positron collides ...
collisions
In physics, a collision is any event in which two or more bodies exert forces on each other in a relatively short time. Although the most common use of the word ''collision'' refers to incidents in which two or more objects collide with great f ...
:
:
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
In quantum electrodynamics, the anomalous magnetic moment of a particle is a contribution of effects of quantum mechanics, expressed by Feynman diagrams with loops, to the magnetic moment of that particle. (The ''magnetic moment'', also called '' ...
. Experimental values have been measured for center-of-mass energies from 400 MeV to 150 GeV.
''R'' also provides experimental confirmation of the electric charge of
quark
A quark () is a type of elementary particle and a fundamental constituent of matter. Quarks combine to form composite particles called hadrons, the most stable of which are protons and neutrons, the components of atomic nuclei. All commonly o ...
s, in particular the charm quark and bottom quark, and the existence of three quark colors. A simplified calculation of ''R'' yields
:
where the sum is over all quark flavors with mass less than the beam energy. ''e
q'' is the electric charge of the quark, and the factor of 3 accounts for the three colors of the quarks.
QCD
In theoretical physics, quantum chromodynamics (QCD) is the theory of the strong interaction between quarks mediated by gluons. Quarks are fundamental particles that make up composite hadrons such as the proton, neutron and pion. QCD is a type o ...
corrections to this formula have been calculated.
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).
Notes
Scattering
Particle physics
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