If matter and antimatter annihilate each other, why is the pentaquark stable since quarks are matter and the antiquark is antimatter?
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4$\begingroup$ 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). $\endgroup$– Mitchell PorterCommented Apr 2, 2019 at 6:59
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5$\begingroup$ 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. $\endgroup$– probably_someoneCommented Apr 2, 2019 at 7:17
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$\begingroup$ Related: physics.stackexchange.com/q/219710/2451 and links therein. $\endgroup$– Qmechanic ♦Commented Apr 2, 2019 at 8:04
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$\begingroup$ Phys.org: New evidence from LHC shows pentaquark has a molecule-like structure, PRL: Observation of a Narrow Pentaquark State, Pc(4312)+, and of the Two-Peak Structure of the Pc(4450)+ $\endgroup$– uhohCommented Jun 9, 2019 at 16:08
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1 Answer
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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.
Now I don't have phenomenological knowledge of the lifetime of the pentaquark and this could be a relevant reason as for why color confinement should not apply in the pentaq case.
