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Does invariance under infinite small transformation imply invariance to the finite one?

Physics Asked by Jānis Erdmanis on November 6, 2021

Let’s say that I have finite chiral transform and I would like to show invariance of Dirac’s Lagrangian when $m=0$ under it.

The chiral transform is defined as:
$$psi(x) rightarrow psi'(x) =e^{i alpha gamma_5} psi(x)$$
where the Dirac’s Lagrangian:
$$L = bar psi (x) (i hbar gamma^{mu} partial_{mu} – 0c) psi(x)$$

If I consider infinitsmall transformation of above:
$$psi(x) rightarrow psi'(x) =(1 + i alpha gamma_5) psi(x)$$
I obtain that Lagrangian transforms as:
$$L rightarrow L’ = L + O(alpha^2)$$
where $O(alpha^2)$ is term with with order $alpha^2$. Is it enough to say that Lagrangian is invariant under finite transformation?

One Answer

This is frequently good enough, but in your specific case its actually much easier to show this holds exactly in the finite case. I won't do this for you, but note that this is a global symmetry, i.e. alpha has no dependence on x. At most, you might also need to use the Baker-Campbell-Hausdorff formula.

Answered by jwimberley on November 6, 2021

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