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I don't understand the term "chiral fermion". To my understanding a fermion $\psi$ can be decomposed as the sum of its left and right (chiral) parts: $$\psi=\psi_L+\psi_R=\frac{1}{2}(1+\gamma^5)\psi+\frac{1}{2}(1-\gamma^5)\psi$$ and the trick is that if in the Lagrangian $m=0$ then the two chiral components propagate independently and the symmetry group of the theory doubles.

When one refers to a "chiral fermion" do they mean a $\psi$ that can be decomposed in its chiral parts (in which case, are there cases where this isn't possible and so the term "chiral fermion" is needed)?
Or do they mean that the symmetry group can be written as the product of the two chiral symmetry groups (in which case, why don't we just say "massless fermion")?

(to be clear, when I say "massive" I mean "with a mass term in the Lagrangian")

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    $\begingroup$ A chiral fermion is just a field which transforms in one of the Weyl representations of the Lorentz group, namely the $(\frac{1}{2},0)$ representation or the $(0,\frac{1}{2})$ representation. A Dirac fermion transforms in the direct sum of these two, and hence can be decomposed into two chiral fermions, but you could as well have a single chiral fermion. In the standard model while for the leptons we associate two chiral fermions so that in the end of the day when they get mass from the SSB they turn out to be Dirac fermions, the associated neutrinos are chiral and have just one chirality. $\endgroup$
    – Gold
    Commented Sep 27, 2021 at 17:47
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    $\begingroup$ In my understanding, it means Weyl fermion in general, so either $\psi_R$ or $\psi_L$. $\endgroup$
    – Kosm
    Commented Sep 27, 2021 at 17:50

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The answer is no.

For a non-chiral fermion $$ \psi = \begin{pmatrix} \psi_L \\ \psi_R\end{pmatrix} $$ We can associate a Dirac mass term $$ \overline{\psi_L}\psi_R + \overline{\psi_R}\psi_L $$ For a chiral fermion $\psi_L$, we can associate a Majorana mass term $$ \overline{\psi_L^c}\psi_L+\overline{\psi_L}\psi_L^c $$

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