1

I am currently working my way through Howard Georgei's text on Lie Algebras in Particle Physics. I am having some trouble understanding how to go about decomposing a general tensor product into irreducible representations and counting the dimensions. One of the examples given is:

$$u^iv^j = \frac{1}{2}(u^iv^j + u^jv^i) + \frac{1}{2}\epsilon^{ijk}\epsilon_{klm}u^lv^m.$$

I understand that the first term on the right side transforms like a 6, as it is symmetric in the upper indices. I am having trouble understanding how the dimension for the second term has been counted. The textbook says that as this term has only 1 lower index, it transforms as $\bar{3}$. In doing so, we appear to ignore the $\epsilon^{ijk}$ term and only count the indices in the remaining. Why is this? I am also having trouble understanding how one uses this levi-civita to antisymmetrize in general.

For concreteness, a later example:

$$u^iv^j_k = \frac{1}{2}(u^iv^j_k + u^jv^i_k - \frac{1}{4}\delta^i_ku^lv^j_l - \frac{1}{2}\delta^j_ku^lv^i_l) + \frac{1}{4}\epsilon^{ijl}(\epsilon_{lmn}u^mv^n_k + \epsilon_{kmn}u^mv^n_l) + \frac{1}{8}(3\delta^i_ku^lv^j_l-\delta^j_ku^lv^i_l)$$

says that the right hand side is a $15\oplus\bar{6}\oplus 3$.

I would highly appreciate it if someone could explain what the three terms on the right hand side mean and how the counting is done.

Qmechanic
  • 201,751
Souroy
  • 153
  • 4
  • 4
    Georgi has done great things in his career but this book is not one of them. You would get more help from Lichtenberg’s text for this sort of stuff. There are also methods based on Young diagrams which can be useful: see this wiki page and also Campoamor-Stursberg, Rutwig, and Michel Rausch De Traubenberg. Group theory in physics: a practitioner’s guide. 2019. – ZeroTheHero Jul 31 '23 at 22:08
  • I recommend learning tensor product decomposition algorithms (and tables, and software) that work on any Lie group. Why learn $SU(3)$-specific techniques if next month you’re working with $SO(10)$ or $E_7$? – Ghoster Aug 01 '23 at 04:25
  • Possible duplicate: https://physics.stackexchange.com/q/102554/2451 – Qmechanic Aug 01 '23 at 08:17
  • You seem to have a typo in your later example; it doesn’t agree with the decomposition in the question this has been made a duplicate of. – Ghoster Aug 02 '23 at 05:02

0 Answers0