This is probably an amateur question but it's caught my curiosity...

If annihilation occurs when a particle meets its antiparticle, then why is the neutral pion meson allowed to exist when it is composed of either up-antiup quarks, down-antidown quarks or strange-antistrange quarks?

look at the events which can create a neutral pi meson and it's lifespan, this very quick decay includes the annihilation of the quarks involved

This post was edited by brmv on Apr 3 2014 06:10am

So is the neutral pion even technically a particle? I thought annihilation occurred instantaneously upon contact

Does a similar thing happen during electron-positron annihilation?

what is a particle?
so far one cannot observe a neutral pi meson directly but based on calculations within the model it is a particle that needs to exist to complete he model and decay results have been observed to confirm it's existence
are quarks particles? depends on the viewpoint
the currently accepted model and observations confirm elementary particles to have certain features which are best explained by a composition by quarks (which cannot be observed)
never forget that we have a working model here with best match between observation and mathematical modelling

and yes, something similar happens during electron-positron annihilation
it is not as if they meet and then nothing is there
what is left behind depends on the energy with which they meet
http://physics.weber.edu/schroeder/feynman/AnnihilationTalk.pdf

Well annihilation is what generally happens to neutral pions, so I don't understand what you are asking.

Should be careful when using the word instantly in this context. A better adverb might be quickly.

So does the annihilation occur between two neutral pions or between the two quarks within the pion?

Is the short decay time of the neutral pion the time it takes for the constituent quarks to collide? And while they are in the process of colliding, is the pion able to interact with other things like a proton does, for example? Or would it be the respective quarks who each have their own independent interactions?

no, there is no interaction required for the neutral pi meson to decay
re quarks please read my post#4
don't forget that prior to the capability to measure time in extremely short intervals 'annihilation'/'decay' was seen as 'instant'

The two quarks. The composite system of a neutral pion has no net charge or color, and don't last long so that does not seem to leave much room for complicated interactions with other "stuff".

This is good to know tyty