Friday, February 17, 2017

quantum mechanics - Relation between Wilson approach to renormalization group and 'standard' RG



While studying renormalization and the renonormalization group i felt that there wasn't any completely satisfying physical explanation that would justify those methods and the perfect results they get. Looking for some clarity i began to study Wilson's approach to renormalization; while i got a lot of insight on how a QFT works and what's the role of quantum fluctuations ecc i could not find a direct clear connection between the "standard" approach and the Wilson one. I'll try to be more specific:


To my understanding Wilson approach says (very) bascially this: given a quantum field theory defined to have a natural cutoff $\Lambda$ and quantized via path integrals (in euclidean spacetime)$$W=\int \mathscr D\phi_{\Lambda} \; e^{-S[\phi]}$$ it is possible to study the theory at a certain scale $\Lambda_N<\Lambda$ by integrating, in an iterative fashion, off high momentum modes of the field. Such rounds of integration can be viewed as a flow of the paramters of the lagrangian which is bounded in it's form only by symmetry principles. For example, given a certain $$\mathscr L_0=a_0\; \partial_{\mu}\phi+b_0\;\phi^2+c_0\;\phi^4$$ we will get something like $$\mathscr L_N=\sum_n a_n \;(\partial_{\mu}\phi)^n+b_n\phi^n+\sum_{n,m}c_{nm}(\partial_{\mu}\phi)^n(\phi)^m$$ where the new parameters $a_n \quad b_n \quad c_{nm}$ have evolved from the original paramters via some relation which depends fromt the cutoff in some way. Now from some dimensional analysis we understeand that the operators corresponing to these parameters organize themselves in three categories which are marginal, relevant and irrelrevant and this categories are the same as renormalizable, super renormalizable and non renormalizable. Then there is the discussion about fixed points and all that stuff needed to have a meaningfull perturbative expansion ecc.


My question(s) is (are):


How do i put in a single framework the Wilsonian approach in witch the relations are between the parameters at the scale $\Lambda_N$ with those of the lagrangian $\mathscr L_0$ and their renomrmalization goup flow descibes those changes in scale with the "standard" approach in which we take $\Lambda\rightarrow+\infty$ and relate the bare paramters of the theory $g_0^i$ with a set of parameters $g_i$ via renormalization prescriptions at a scale $\mu$ and then control how the theory behaves at different energy scales using callan-symanzik equation ?


How different are the relations between the paramters in the wilson approach and the one in the "standard" approach? Are these even comparable?


What is the meaning (expecially in the wilsonian approach?) of sending $\Lambda$ to infinity besides of getting completley ridden of non renormalizable terms in the theory?


Is, in the standard approach, giving a renormalization prescription which experimentally fixes the paramters $g_i$ at a scale $\mu$ basically the same as integrating from $\Lambda\rightarrow +\infty$ to the scale $\mu$ in the wilsonian approach?


I'm afraid i have some confusion here, any help would be appreciated!




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