# why is there no ninth gluon?

A teacher of mine told me once that there were no ninth gluon because such a one should be white and interact infinitely far, and no one has been observed. Is there also a theoretical reason?

• A preprint with ninth gluon in the title: arxiv.org/abs/hep-ph/9606317 Commented Jan 25, 2012 at 22:13
• Interesting - it postulates that the color symmetry is broken $U(3)$ instead of $SU(3)$ and that the broken symmetry gives an extremely massive singlet gluon. Commented Jan 25, 2012 at 22:23
• See also this question and its answer on "colorless" gluons. Commented Jan 26, 2012 at 2:17
• @MarkBeadles Thanks for the flag, but they are not similar enough to be claimed duplicates.
– user68
Commented Jan 26, 2012 at 15:05
• Commented May 8, 2022 at 14:08

There is one color singlet, which is colorless and, as your teacher said, does not correspond to any gluon interaction. If it did it would interact with other color singlet states.

$$(r\bar{r}+b\bar{b}+g\bar{g})/\sqrt{3}$$

And then the eight remaining independent color states corresponding to the eight colors of gluons. One way of representing these as linearly independent states is:

$$(r\bar{b}+b\bar{r})/\sqrt{2}$$ $$-i(r\bar{b}-b\bar{r})/\sqrt{2}$$

$$(r\bar{g}+g\bar{r})/\sqrt{2}$$ $$-i(r\bar{g}-g\bar{r})/\sqrt{2}$$

$$(b\bar{g}+g\bar{b})/\sqrt{2}$$ $$-i(b\bar{g}-g\bar{b})/\sqrt{2}$$

$$(r\bar{r}-b\bar{b})/\sqrt{2}$$ $$(r\bar{r}+b\bar{b}-2g\bar{g})/\sqrt{6}$$

The states are not, as might be assumed, just blue-antired, red-antiblue, etc.

• why is the last state not colorless? Commented Dec 7, 2023 at 14:36