In finding a lagrangian for a left-handed spinor field , a textbook claims that a kinetic term such as $ \partial_{\mu} \psi^a \partial^{\mu} \psi_a =\epsilon^{ab}\partial_{\mu}\psi_a \partial^{\mu}\psi_b$ (where $\psi$ is a left-handed spinor field and $\epsilon^{ab}$is an antisymmetric tensor) is unacceptable because the corresponding hamiltonian is unbounded below. I cannot understand why this is true.
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