Let $-\partial^u\overline\Psi\partial_u\Psi-m^2\overline\Psi\Psi$ be a Lagrangian density. Here $\Psi$ is the Dirac spinor and $\overline \Psi$ is defined to be $\Psi^\dagger \gamma^0$. It is said that this Lagrangian is not Lorentz invariant. However, I cannot see why. For me, it has perfect form of being Lorentz invariant. Could anyone please explain why this Lagrangian is not Lorentz invariant?

  • 7
    $\begingroup$ "It is said" by whom? $\endgroup$ Jun 8 '18 at 13:27
  • 1
    $\begingroup$ It's a dimension 5 term $\endgroup$
    – FrodCube
    Jun 8 '18 at 14:00
  • $\begingroup$ @AccodentalFourierTransform my lecturer $\endgroup$
    – Keith
    Jun 8 '18 at 14:05
  • $\begingroup$ @Keith Perhaps it was a slip of the tongue. The Lagrangian is Lorentz invariant, but not renormalizable (in four dimensions). $\endgroup$
    – gj255
    Jun 8 '18 at 14:08
  • $\begingroup$ @gj255 It is renormalizable in any dimensions. The first term in N dimensions would imply $\Psi$ to be in dimensions $GeV^{N/2-1}$ which is the same for the second term because of the squared mass. It looks like a Klein-Gordon equation but has a four-component field with mixed energy eigenstates? $\endgroup$ Jun 8 '18 at 16:11

Let us first agree on the fact that $\bar{\psi}\psi$ is indeed a Lorentz invariant, so the mass term obviously is. For the second term, you could integrate by parts arrive to: $$\bar{\psi}(\Box - m^2)\psi$$ which is again Lorentz invariant since $\Box$ also is. Here the $\gamma^0$ is causing kinetic mixing originally if you want, but it looks Lorentz invariant.

So, I would say it looks like a weird mixture of 4 Klein Gordon fields. If you compute the equations of motion it seems every component of what you called a "Dirac spinor" satisfies a Klein-Gordon equation. If you are given this equation there is nothing forcing them to be Dirac spinors.

EDIT: What people are saying in the comments about 5th dimension operators is also true if you assume beforehand that those fields have the mass dimensions of a Dirac field, namely 3/2. But then again it makes no sense to declare them Dirac fields if there is no $\bar{\psi}\gamma^\mu\partial_\mu \psi$ term.


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