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Sometimes I see Yukawa interaction term written as $$-g\bar{\psi} i \gamma^5 \phi \psi$$ and other times as $$-g \bar{ \psi} \gamma_5 \psi \phi $$ Which is the correct form?

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    $\begingroup$ The Wiki link answers your question. $\endgroup$ – ACuriousMind Nov 5 '14 at 0:41
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The version with the 'i' is correct. It is used in all references I checked, including printed books from pre-web days.

The term without the 'i' may have come from some author using uncommon conventions or definitions, perhaps defining $\gamma^5 = \gamma^0 \gamma^1 \gamma^2 \gamma^3$ instead of the widely used standard definition $\gamma^5 = i\gamma^0 \gamma^1 \gamma^2 \gamma^3$.

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Yukawa interaction can be described by two terms, one is scalar($-g\bar{\psi}\phi\psi$) and the other is pseudo-scalar($-g\bar{\psi}\gamma_5\phi\psi$). To make Lagrangian hermitian, we have to multiply the pseudo-scalar term with $i$.

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