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Calculating equation of motion in gauge theories: using ordinary derivatives or covariant derivatives?

Physics Asked on December 26, 2020

For general gauge theories, the total Lagrangian density is given as $$L=-frac{1}{4}F^2+L_M(psi, Dpsi)$$ where $L_M(psi, Dpsi)$ is the matter field with the ordinary derivative replaced by the covariant derivative $D$. Here $psi$ is a generic matter field other than the gauge field.

Then, for calculating the equation of motion for the matter field $psi$, I am confused whether I have to calculate with $partial_mu phi$ or $D_mu psi$. That is, which one is correct?:

begin{equation}
frac{partial L}{partialpsi}-partial_mu frac{partial L}{partial(partial_mupsi)}=0,tag{1}
end{equation}

begin{equation}
frac{partial L}{partialpsi}-D_mu frac{partial L}{partial(D_mupsi)}=0.tag{2}
end{equation}

This kind of stuff have always confused me…so I desperately feel a need to clarify.

One Answer

  1. The principle of stationary action always implies the EL equations (1) with partial derivatives, so (1) is a safe bet.

  2. By imposing further conditions on the theory, the EL equations (2) with covariant derivatives may hold as well, cf. this related Phys.SE post.

Correct answer by Qmechanic on December 26, 2020

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