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It seems to me like any sensible matter Lagrangian must obey the following constraints:

  • It must be invariant with respect to local $SU(N)$ transformations, diffeomorphisms of the space-time manifold, and CPT symmetry.

  • It must produce an Hamiltonian bounded from below.

Can one argue that because of the constraints above, the most general form of the lagrangian (excluding same-field interactions for the moment) should be this one? And if not, why would that be the case? \begin{equation} \mathcal{L} = \bar{\Psi}_{k}(\alpha D^{\mu}D^{\nu}\Omega^{kj}_{\mu\nu} + \beta\gamma^{\mu}_{kj}D_{\mu} + \lambda\gamma^{\mu}_{ka}\gamma_{\mu aj} + m\delta_{kj})\Psi_{j} \end{equation} Where we have\begin{equation} D_{\mu} = \nabla_{\mu} -igT^{a}A^{a}_{\mu}. \end{equation} As a curiosity, if this is the most general lagrangian, why is the $\lambda\gamma^{\mu}_{ka}\gamma_{\mu aj}$ term not commonly included? Moreover, can one make sense of the tensor $\Omega_{\mu\nu}^{kj}$ as a sort of rank-2 generalization of the gamma matrices? I would guess that for example $g_{\mu\nu}\delta^{kj}$ would produce the usual second-order derivative in some of the scalar lagrangians, but are there other tensors of this form?

Qmechanic
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Davyz2
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2 Answers2

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First of all, as pointed out in the other answer, The gamma matrices satisfy $\gamma^\mu\gamma_\mu=4\mathbb{I}$, so the $\lambda$ term reduces to the mass term.

I am concentrating on the $\alpha$ term: $$ \begin{equation} \bar{\Psi}_{k}(\alpha D^{\mu}D^{\nu}\Omega^{kj}_{\mu\nu})\Psi_{j} \end{equation} $$

Contrary to the claim of the other answer, such term with two derivatives is allowed, but there is a catch!

1, Symmetric case of $\Omega^{kj}_{\mu\nu}$

Let's look at the specific case of $\Omega^{kj}_{\mu\nu} = \eta_{\mu\nu}\delta^{kj}$ and ignore gauge field coupling, then the term reduces to the Klein–Gordon-like (applying simplified notation than OP) $$ \begin{equation} \alpha \bar{\psi}\partial^\mu\partial_\mu\psi \end{equation} $$

The modified/enhanced Dirac Lagrangian can be written as $$ L = i\bar{\psi}\not D\psi + \alpha \bar{\psi}\partial^\mu\partial_\mu\psi- m\bar{\psi}\psi \tag{1} $$ where the Klein–Gordon-like term $\alpha\bar{\psi}\partial^\mu\partial_\mu\psi$ is added to the original Dirac-like term $i\bar{\psi}\not D\psi$.

The Klein–Gordon-like term $\bar{\psi}\partial^\mu\partial_\mu\psi$ is a dimension-5 operator (3 from the the two dimension-$3/2$ spinors plus 2 from the two derivatives), and thus non-renormalizable. In principle, such non-renormalizable term IS allowed in the Effective Field Theory framework. However, given that the Klein–Gordon-like term is a dimension-5 operator, it is suppressed by a factor of: $$ \frac{E}{M_{Planck}} $$ where $E$ is the energy scale of the physics process in concern, and $ M_{Planck}$ is the Planck scale.

In other words, the Klein–Gordon-like term can be safely ignored under normal circumstances (i.e. when $E << M_{Planck}$), unless we are dealing with Planck energy scale physics such as early universe during big bang.

2, Anti-Symmetric case of $\Omega^{kj}_{\mu\nu}$

As for the other tensors form of $\Omega^{kj}_{\mu\nu}$, the anti-symmetrical portion of the gauge-covariant derivative $D^{\mu}D^{\nu}$ reduces to the 'non-minimal' interaction term originally suggested by Pauli: $$ \bar{\psi}F^{\mu\nu}\sigma_{\mu\nu}\psi $$ where $F^{\mu\nu}$ is the electromagnetic field tensor, and $\sigma_{\mu\nu}$ are Pauli matrices.

The Pauli term is dimention-5 too, therefore faces the same Planck-scale suppression issue as we discussed for the Klein–Gordon-like term.

3, Technical naturalness issue

Additionally, both the Pauli term $\bar{\psi}F^{\mu\nu}\sigma_{\mu\nu}\psi$ and the Klein–Gordon-like term $\bar{\psi}\partial^\mu\partial_\mu\psi$ break the axial symmetry $$ \psi \rightarrow e^{\theta i\gamma_5}\psi $$ Hence these terms are further suppressed due to t' Hooft's technical naturalness argument.

4, Other cases not considered in OP

And for that matter, there are other terms not considered in OP, such as three derivative term $$ \bar{\psi}D^{\mu}D^{\nu}\gamma_\mu D_{\nu}\psi \sim \bar{\psi}F^{\mu\nu}\gamma_{\mu}D_{\nu}\psi $$ or four derivative term $$ \bar{\psi}D^{\mu}D^{\nu}D_\mu D_{\nu}\psi \sim \bar{\psi}F^{\mu\nu}F_{\mu\nu}\psi $$ I will leave it to the readers as a home work excise as to whether they are (not) allowed.

See more details here.

MadMax
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Note that:

  1. The gamma matrices satisfy the identity $\gamma^\mu\gamma_\mu=4\mathbb{I}$, so the $\lambda$ term in your Lagrangian reduces to the mass term.
  2. When the covariant derivative acts on a spinor it produces an object with one spinor index and one Lorentz index, so you cannot apply it twice in the way you wrote (the covariant derivative of such an object would be different). This is the reason (among others) that the Lagrangian for Dirac spinors contains just a first order derivative.
J. Delaney
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