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As I understand it, particles and antoparticles annihilate one another. But some mesons (and pions?) consist of a quark and its antiquark. How can they exist without the two particles annihilating each other?

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But they do decay into the channels available from conservation laws.

Annihilation happens when all the quantum numbers cancel. In the pio the charge and baryon number of the quark antiquark cancel each other, and the decay particles add up to zero quantum numbers.

The $\pi^0$ goes into two photons as soon (electromagnetic interaction rates) as it is produced. The $\pi^+$ and $\pi^-$ survive because of charge conservation, and they decay with the weak interaction.

This is the same as with proton antiproton annihilation. The baryon number and charge cancel, and the decay products have to add up to zero for these quantum numbers

This is the case with all mesons.

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  • $\begingroup$ But can decay really be considered annihilation? I understand annihiliation to be a separate concept...? $\endgroup$ Commented Apr 24, 2016 at 12:46
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    $\begingroup$ Annihilation happens when all the quantum numbers cancel. In the pio the charge and baryon number of the quark antiquark cancel each other, and the decay particles add up to zero quantum numbers. This is the same as with proton antiproton annihilation. The baryon number and charge cancel, and the decay products have to add up to zero for these quantum numbers . $\endgroup$
    – anna v
    Commented Apr 24, 2016 at 12:50

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