All mesons are unstable, with the longest-lived lasting for only a few hundredths of a microsecond. ... Because quarks have a spin 12, the difference in quark number between mesons and baryons results in conventional two-quark mesons being bosons, whereas baryons are fermions.

Why do mesons decay?

Mesons, for example the pion, decay because they are made from a quark and an antiquark. Particles and anti-particles don't get on very well, so as with an electron meeting a positron, they quickly annihilate or decay.

Are Pentaquarks stable?

The molecular picture also helps explain why the pentaquarks, although fleeting, appear to be more stable than expected, Karliner says. That's because packaging the charm quark in the baryon and anticharm quark in the meson separates them, keeping them from annihilating each other.