# SU(3) gauge invariance in QCD

In QCD, the gauge-invariant lagrangian under the trasformation

$\psi \to \psi' = e^{ig T^a \theta^a(x)} \psi$

is written as:

$\mathcal{L} = \bar{\psi}(i\gamma^\mu D_\mu - m)\psi - \frac{1}{4}G^a_{\mu\nu}G_a^{\mu\nu}$

where the covariant derivative is:

$D_\mu = \partial_\mu - ig T^a G^a_\mu$

and the field strength tensor is defined as:

$G^a_{\mu\nu} = \partial_\mu G^a_\nu - \partial_\nu G^a_\mu + g f_{abc} G^b_\mu G^c_\nu$

If I impose the gauge-invariance, I find that the gauge field transforms as:

$G^a_\mu \to G'^a_\mu = G^a_\mu + \partial_\mu \theta^a$

Am I correct? I think I am, but if I look at how the field strength transforms, I expect it to remain invariant, but instead I find an extra term:

$G^a_{\mu\nu} \to G^a_{\mu\nu} + g_s f_{abc}(\partial_\mu \theta^b \partial_\nu \theta^c + \partial_\mu \theta^b G^c_\nu + \partial_\nu \theta^c G^b_\mu)$

Does this term vanish? Why? Or am I totally wrong on the transformation of the gauge field...?

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For nonabelian gauge fields the gauge field should transform as $A^a_\mu \rightarrow A^a_\mu + (D_\mu \theta)^a$. – DJBunk Jan 12 '13 at 0:39

## 1 Answer

The Gauge field transforms as $$G_{\mu}^{i}\frac{\lambda^{i}}{2} \rightarrow G_{\mu}' = uG_{\mu}u^{-1}+\frac{i}{g}u\partial_{\mu}u^{-1}$$ with $u \in SU(3)$ such that $$u^{-1}(x)=\exp (-i\alpha^i \lambda^i /2)$$ this can be expanded in a series for infinitesimal transformations.

EDIT:

expanding $u^{-1}$ $$u^{-1} \approx 1 -i\alpha^i \lambda^i/2 + \mathcal{O}(\alpha^2)$$ you can preform this expansion for both $u$ and it's inverse to first order and make sure to keep everything to first order in $\alpha$, it will include a commutator.

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Can you elaborate the expansion? Is the usual expression for the derivative of an exponential not valid for matrix exponentials? – Ganondolf Jan 11 '13 at 22:32
@Ganondolf see the update. – kηives Jan 11 '13 at 23:18
Uhm, I think I sorted out... Most probably I missed a commutator in the interaction term, which accounts for an extra term in the transformation of G. More important, as you suggested I should keep everything to first order, that should be the way to transform correctly the field strength tensor. I still don't know how exactly, but now I should be on the right way... Dropping the problem now for lack of time, but at least I saw the light at the end of the tunnel. Thank you. – Ganondolf Jan 12 '13 at 0:32