Relationship between bare and renormalized beta functions

  • #1
Siupa
29
5
I'm looking at a proof of beta function universality in ##\phi^4## theory, and at one point they do the following step: after imposing that the renormalized coupling ##\lambda## is independent of the cutoff ##\Lambda##, we have
$$0= \Lambda \frac {\text{d} \lambda}{\text{d} \Lambda} = \Lambda \frac {\partial \lambda}{\partial \Lambda} + \Lambda \frac{\partial \lambda_0}{\partial \Lambda} \frac{\partial \lambda}{\partial \lambda_0} \implies 0 = - \mu \frac{\partial \lambda}{\partial \mu} + \beta^{(\text{B})}(\lambda) \frac{\partial \lambda}{\partial \lambda_0}$$
This seems to imply
$$\Lambda \frac{\partial \lambda}{\partial \Lambda} = -\mu \frac{\partial \lambda}{\partial \mu}$$
Why is this true? Where does it come from? ##\lambda_0## is the bare coupling and ##\mu## the arbitrary mass scale
 
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  • #2
@Siupa inline LaTeX needs to be enclosed in double dollar signs, not single ones. I have used magic moderator powers to edit your OP to fix this.
 
  • #3
Can you give the reference, where you got this from? What's the renormalization scheme used?

I have some RG stuff applied to ##\phi^4## theory in my QFT notes:

https://itp.uni-frankfurt.de/~hees/publ/lect.pdf Sect. 5.11
 

FAQ: Relationship between bare and renormalized beta functions

What is the difference between bare and renormalized beta functions?

The bare beta function is defined in terms of the bare parameters of a quantum field theory, which are the parameters appearing in the original, unrenormalized Lagrangian. The renormalized beta function, on the other hand, is defined in terms of the renormalized parameters, which are the physical, measurable quantities after renormalization. The renormalized beta function describes how these renormalized parameters evolve with the energy scale.

Why are beta functions important in quantum field theory?

Beta functions describe how the coupling constants of a quantum field theory change with the energy scale. This is crucial for understanding the behavior of the theory at different energy scales, such as in the context of asymptotic freedom or confinement in Quantum Chromodynamics (QCD). Beta functions help in predicting whether a theory remains well-behaved or becomes strongly coupled at high energies.

How are bare beta functions related to renormalized beta functions?

The relationship between bare and renormalized beta functions is established through the renormalization process. The bare beta function is typically expressed in terms of the bare coupling constants and the cutoff scale, while the renormalized beta function is expressed in terms of the renormalized coupling constants and the renormalization scale. The renormalization group equations connect these two descriptions, often involving counterterms to cancel infinities arising in the bare theory.

What role do counterterms play in the relationship between bare and renormalized beta functions?

Counterterms are introduced in the renormalization process to cancel the infinities that appear in the bare theory. These counterterms modify the bare parameters to yield finite, renormalized parameters. The beta functions of the renormalized theory are derived after these counterterms are included, ensuring that physical predictions remain finite and well-defined. Thus, counterterms are essential in transitioning from bare to renormalized beta functions.

Can the renormalized beta function be directly measured in experiments?

The renormalized beta function itself is not directly measurable, but its effects can be observed in experiments. For instance, the running of the coupling constants with energy, as predicted by the renormalized beta function, can be measured in high-energy particle collisions. These measurements provide indirect evidence of the renormalized beta function's behavior and validate the theoretical framework of quantum field theory.

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