what's a meson

kevin mcd

New Blood
K
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May 14, 2008
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im sorry if this question has been answered a bajillion times, but im new to jref and the search layout is freaking me out.

anyways, i suppose this isnt too hard to answer. ive always been fascinated by the "exotic" particles and such that physicists believe are out there. one would be the meson.

from out of this world by stephen webb, i understand it is (a quark and antiquark), but im confused as to how such a particle could exist. he mentioned a "bound" state, but failed to define it. (maybe that has something to do with it???)

from my most basic knowledge (and trust me, although i read up on modern physics, half the time i am in way over my head), i know that a quark-antiquark will annihilate to create photons. so how can a meson even be considered a particle if it annihilates itself? or does it? i have no clue.

any help would be grand.
 
It depends on which meson we're talking about. Most of them are not the same antiquark as quark (e.g. an up and anti down is a pi+). The pi0 on the other hand is an up and anti up (or down and anti-down). It decays by annihilating with itself, to give 2 photons (nearly always). Because it can decay in this way (ie the EM force rather than the weak force) its lifetime is ~8 orders of magnitude shorter than that of the charged pions.

ETA: To be precise, the mean life of the pi0 is 8.4*10-17 s.
The mean life of the charged pions is 2.60*10-8 s.
 
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thanks for all your help. but just for clarification....

so if they are not the same flavor of quark/anti... they do not annihilate? so if a Up and anitDown met, they would not annihilate, as in the meson just decays otherwise? also... can it decay into w or z boson? thanks again.
 
Yes, they cannot annihilate with each other to give 2 gammas unless they are the same flavour.
None of the mesons are heavy enough to decay to a real W or Z. But you can think of a charged pion as decaying via a virtual W (of the appropriate charge) to a muon/antimuon and an antineutrino/neutrino. And similar for the heavier mesons.
 
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