# Varying with respect to vierbein

1. Feb 22, 2013

Hi
Can anyone explain how to vary an action e.g $\int d^4 x e [\frac{1}{2}\partial_{\mu}\partial^{\mu} \phi + i\bar{\psi}\bar{\gamma}^{\mu}D_{\mu}\psi]$ w.r.t the vierbein?

Where e here is the determinant of the vierbein, and $D_{\mu}$ is = to $\partial_\mu + \frac{1}{4}\gamma_{\alpha \beta} \omega_{\mu}^{\alpha \beta}$

2. Feb 22, 2013

### Ben Niehoff

For the first term, I think you actually mean $\partial_\mu \phi \partial^\mu \phi = g^{\mu\nu} \partial_\mu \phi \partial_\nu \phi$. So remember that the metric is made of vierbeins.

For the second term, are you assuming the spin connection is a function of the vierbein, or is it an independent field? If the spin connection depends on the vierbein, then write down the expression for it in terms of the vierbein, and vary that.

For the determinant, same technique applies as for varying the metric determinant.

3. Feb 23, 2013

I know that the answer should be $T^{\mu \nu} = i/2 [\bar{\psi}\bar{\gamma}(\mu D_\nu)\psi - \bar{\psi}D(\mu \bar{\gamma}_\nu)\psi$, but i just don't see why