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## Main Question or Discussion Point

I have just read my first course on Quantum Field Theory (QFT) and have followed the book by Srednicki. I have peeked a bit in the books by Peskin & Schroeder and Ryder also but mostly Srednicki as this was the main course book. Now, I have to do a project in a topic not covered in the course and I have chosen Effective field theory (EFT), following the approach by Wilson. I have read the chapter(s) in Srednicki related to this topic a few times and understand (I think) the gist of the Renormalization Group (RG) and what it is about, but I can't say I understand the chapter on EFT (chapter 29 in Srednicki). I don't really understand what the EFT approach means and I was hoping that some of you could help me clear this up.

As I understand it, when we use the MS-bar renormalization scheme, the parameters in the lagrangian no longer represent the physical parameters (for example, the m term is not the physical mass) and we can find equations that tell us how the lagrangian parameters vary with the fake parameter μ (any final answer can't depend on μ). This can also be done with the RG approach in a more formal way (as I understand it, the result is the same - we get a group of equations that tell us how the lagrangian parameters vary).

However, the next chapter on EFT:s I struggle to understand. I get that we have a cut-off [itex]\Lambda[/itex] for the momentum and that we can try to see what the theory tells us at momenta well below the cut-off but then a new cut-off [itex]\Lambda_0[/itex] is introduced and I must say I don't understand the difference between the two.

Something I would also like to get some help with is how Wilson's approach with EFT:s relates to renormalization. Why does the EFT approach remove the necessity for a theory to be renormalizable?

Any help and clarifications is highly appreciated!

As I understand it, when we use the MS-bar renormalization scheme, the parameters in the lagrangian no longer represent the physical parameters (for example, the m term is not the physical mass) and we can find equations that tell us how the lagrangian parameters vary with the fake parameter μ (any final answer can't depend on μ). This can also be done with the RG approach in a more formal way (as I understand it, the result is the same - we get a group of equations that tell us how the lagrangian parameters vary).

However, the next chapter on EFT:s I struggle to understand. I get that we have a cut-off [itex]\Lambda[/itex] for the momentum and that we can try to see what the theory tells us at momenta well below the cut-off but then a new cut-off [itex]\Lambda_0[/itex] is introduced and I must say I don't understand the difference between the two.

Something I would also like to get some help with is how Wilson's approach with EFT:s relates to renormalization. Why does the EFT approach remove the necessity for a theory to be renormalizable?

Any help and clarifications is highly appreciated!