In theoretical physics, specifically quantum field theory, a beta function, β(g), encodes the dependence of a coupling parameter, g, on the energy scale, μ, of a given physical process described by quantum field theory.
It is defined as and, because of the underlying renormalization group, it has no explicit dependence on μ, so it only depends on μ implicitly through g. This dependence on the energy scale thus specified is known as the running of the coupling parameter, a fundamental feature of scale-dependence in quantum field theory, and its explicit computation is achievable through a variety of mathematical techniques.
The concept of Beta function was Introduced by Ernst Stueckelberg and André Petermann in 1953.
In this case, the non-zero beta function tells us that the classical scale invariance is anomalous.
Beta functions are usually computed in some kind of approximation scheme.
An example is perturbation theory, where one assumes that the coupling parameters are small.
One can then make an expansion in powers of the coupling parameters and truncate the higher-order terms (also known as higher loop contributions, due to the number of loops in the corresponding Feynman graphs).
Here are some examples of beta functions computed in perturbation theory: The one-loop beta function in quantum electrodynamics (QED) is or, equivalently, written in terms of the fine structure constant in natural units, α = e2/4π.
[2] This beta function tells us that the coupling increases with increasing energy scale, and QED becomes strongly coupled at high energy.
In fact, the coupling apparently becomes infinite at some finite energy, resulting in a Landau pole.
However, one cannot expect the perturbative beta function to give accurate results at strong coupling, and so it is likely that the Landau pole is an artifact of applying perturbation theory in a situation where it is no longer valid.
scalar colored bosons is or written in terms of αs =
Assuming ns=0, if nf ≤ 16, the ensuing beta function dictates that the coupling decreases with increasing energy scale, a phenomenon known as asymptotic freedom.
Conversely, the coupling increases with decreasing energy scale.
This means that the coupling becomes large at low energies, and one can no longer rely on perturbation theory.
of the Lie algebra in the representation R. (For Weyl or Majorana fermions, replace
, the above equation reduces to that listed for the quantum chromodynamics beta function.
This famous result was derived nearly simultaneously in 1973 by Politzer,[3] Gross and Wilczek,[4] for which the three were awarded the Nobel Prize in Physics in 2004.
Unbeknownst to these authors, G. 't Hooft had announced the result in a comment following a talk by K. Symanzik at a small meeting in Marseilles in June 1972, but he never published it.
[5] In the Standard Model, quarks and leptons have "Yukawa couplings" to the Higgs boson.
These Yukawa couplings change their values depending on the energy scale at which they are measured, through running.
The dynamics of Yukawa couplings of quarks are determined by the renormalization group equation:
This equation describes how the Yukawa coupling changes with energy scale
The Yukawa couplings of the up, down, charm, strange and bottom quarks, are small at the extremely high energy scale of grand unification,
is increased slightly at the low energy scales at which the quark masses are generated by the Higgs,
On the other hand, solutions to this equation for large initial values
cause the rhs to quickly approach smaller values as we descend in energy scale.
This is known as the (infrared) quasi-fixed point of the renormalization group equation for the Yukawa coupling.
Renomalization group studies in the Minimal Supersymmetric Standard Model (MSSM) of grand unification and the Higgs–Yukawa fixed points were very encouraging that the theory was on the right track.
So far, however, no evidence of the predicted MSSM particles has emerged in experiment at the Large Hadron Collider.