# How and why is the pentaquark stable if it consists of 4 quarks and an antiquark?

If matter and antimatter annihilate each other, why is the pentaquark stable since quarks are matter and the antiquark is antimatter?

• It's not stable. But quark and antiquark can only annihilate if they are the same flavor (e.g. up quark, up antiquark can annihilate; but not up quark, down antiquark). – Mitchell Porter Apr 2 '19 at 6:59
• You could ask the same question of literally any meson. All mesons contain one quark and one antiquark, yet they still last long enough to be measured. There's nothing particularly special about the pentaquark in this regard. – probably_someone Apr 2 '19 at 7:17
• Related: physics.stackexchange.com/q/219710/2451 and links therein. – Qmechanic Apr 2 '19 at 8:04
• – uhoh Jun 9 '19 at 16:08

It depends on whether or not the system corresponds to a $$SU (3)$$ singlet representation (a one dimensional irreducible representation). This is the color confinement postulate: (here somewhat sloppily) particles we can observe are in a singlet of $$SU (3)$$.
You can show by tensorial decomposition that the mesons and the baryons contain a singlet representation of $$SU (3)$$, the pentaquark should too. I once tried to do the math but did not end the computation.