Mesons are unstable and decay , either through the weak decay of a quark in the charged cases, or because quark + antiquark annihilate in the neutral through the strong interaction.

Why don t quarks and antiquarks annihilate in mesons?

Answers and Replies

In baryons (e.g. proton) there are 3 'normal' quarks, so they are not going to annihilate themselves. Mesons, however, ARE made of quark-antiquark pairs, but they are not all made of the same flavour quark and anti-quark.

Why are mesons unstable?

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.