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A question involving chiral transformations and gamma matrices

Physics Asked on July 25, 2021

I’m looking at a calculation that involves an infinitesimal transformation of a Dirac fermion field:

$$Psi rightarrow e^{i beta gamma^5} Psi.$$

Then the conjugate field $bar{Psi} = Psi^{dagger} gamma^0$ transforms as $bar{Psi} rightarrow (e^{i beta gamma^5} Psi)^dagger gamma^0$. Then from here we get:

$$Psi^dagger e^{-i beta gamma^5} gamma^0.$$

So far I understand the steps, but I don’t how from here one jumps to
$$Psi^dagger gamma^0 e^{i beta gamma^5}.$$

Why does the sign in the exponential changes and the gamma matrix is suddenly on the right?

One Answer

As one usually does, write the exponential term with a power series expansion, $$Psi^{dagger} big(1 - i beta gamma^{5} + mathcal{O}(beta^2) big) gamma^0$$ then using the anticommutative properties ${gamma^5,gamma^{mu} } = 0$ you can move $gamma^{0}$ through the $gamma^{5}$ terms, picking up a minus sign in the process. You can check the higher order terms too.

Edit: I see this has also already been pointed out in the comments by Hannes too.

Correct answer by Eletie on July 25, 2021

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