Sunday, November 6, 2016

What is classical Lagrangian? The Bare one or renormalized one? Are counterterms quantum corrections to renormalized Larangian?



EDIT




  1. When one talks about the “classical Lagrangian” of a field, does one mean the renormalized Lagrangian with physical/renormalized masses and physical/renormalized couplings and without counterterms?




  2. If yes, does it therefore mean that the bare Lagrangian is the "quantum corrected" Lagrangian of the theory which includes quantum corrections in the form of countertems (where $\mathcal{L}_{bare}=\mathcal{L}_{renorm}+\mathcal{L}_{counter}$)?




If not, why is that? Why is it wrong to say that "renormalized Lagrangian without the counterterm" $\mathcal{L}_{renorm}$ as the classical Lagrangian. This reference advocates the claim of question 1. This reference says, bare Lagrangian is different from classical Lagrangian. 12.11 is the bare Lagrangian. It says, below Eq. 12.13, classical Lagrangian doesn't have counterterms, and counterterms are added as quantum corrections to the classical Lagrangian. This is explained in the paragraph above eqn. 12.6. Also the footnote around Eqn. 12.14. Can you help me with this?



3. Do the counterterms have any physical effect? In particular, I have heard that quantum corrections can trigger spontaneous symmetry breaking (SSB) even if the theory is classically unbroken (say, in massless $\phi^4$ theory which is unbroken classically). Since, the counterterms, as I understand, are quantum contributions to the classical Lagrangian, I wonder, whether it is the counterterms which are responsible in some way in triggering the SSB?




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