Could a tetraquark $q \bar{q} q \bar{q}$ be colorless? CERN just posted this article where it informs that it was found an hadron which cannot be classified within the traditional quark model. What other models are there to explain this result? Or is it possible to introduce a correction to the quark model to explain such find?
 A: There are three flavours of quarks in the fundamental $3$ representation of $SU(3)$, the QCD gauge group. Their antiparticles are in the conjugate representation $\bar3$ or $3^\star$.
QCD is confining; the quarks form bound, colorless states, which are singlets in $SU(3)$. 


*

*Mesons are $q\bar q$. The general tensor $3\times\bar 3$ can be decomposed into irreducible represetations; $3\times\bar 3 = 1 +8$. Note that this contains a singlet.

*Tetra-quarks are $q\bar q q\bar q$. Since $3\times\bar 3 = 1 +8$, $3\times\bar 3 \times 3\times\bar 3$ clearly contains a singlet.


Decomposition of an arbitrary tensor doesn't always contain a singlet e.g. $3\times3=\bar6+3$.
The discovery of a tetra-quark does not require the model of quarks and their interactions to be adjusted.
A: A tetraquark (and pentaquark) had been previously predicted as extensions of known mesons (and baryons, respectively). There is no reason why they cannot be colour neutral. e.g. in RBG the quarks could be
R 255 G 0 B 0
R 0 G 255 B 0
R 0 G 0 B 128
R 0 G 0 B 127
