What's the reason behind the existence of mesons? (1 Viewer)

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I've read that mesons consist of a quark and an antiquark. So, here's my question. Why don't the quark and the anti-quark annihilate with each other (like they ususally do)?
For example, the pi(0) meson consists of the up and the anti-up quark and the eta meson consists of the down and the anti-down quark and even the eta-prime meson which is made up of a strange and an anti-strange pair of quarks.
So, what's the reason for the existence of these mesons?
What causes a quark and an anti-quark to come into a bound state and form a meson?
 
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jtbell

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The particles that you name do decay, right?

Consider that an electron-positron pair can also "live" for a short time in a bound state, called positronium.

Annihilation does occur, just not immediately.
 
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Apart from the neutral pion, all long-living mesons consist of a quark and a different antiquark, therefore they cannot decay via the electromagnetic interaction. They have to decay via the weak interaction, which is weak enough to give them some measurable lifetime and flight distance in experiments.
 

arivero

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The particles that you name do decay, right?

Consider that an electron-positron pair can also "live" for a short time in a bound state, called positronium.

Annihilation does occur, just not immediately.
That is not the whole history. The quarks aniquilate to a pair of gluons, which can not exist in a free form, this is very different that electron positron to fotons.
 
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Quark-antiquark pairs can decay into a pair of photons. This is the dominant decay for the neutral pion, as it cannot decay via the strong interaction.
Heavier quark-antiquark pairs can annihilate via the strong interaction and produce lighter quarks, which is usually the dominant decay process.
 
Quark-antiquark pairs can decay into a pair of photons. This is the dominant decay for the neutral pion, as it cannot decay via the strong interaction.
Heavier quark-antiquark pairs can annihilate via the strong interaction and produce lighter quarks, which is usually the dominant decay process.
Thanks! That pretty much explains it! :)
 

arivero

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Thanks! That pretty much explains it! :)
No sorry. That explains the _decay_ of mesons. The existence of mesons is/should be explained by QCD alone, without resourting to QED.
 
Mesons are typically formed when quarks/antiquarks are involved in collisions. If one quark is impelled away from the other particles, QCD colour confinement prevents its escape. As the quark tries to escape, the strength of the colour force between the quark and the remaining particles (of net opposite colour) does not decay by the inverse square law, but rather is believed to remain approximately constant. After a very short time, therefore, sufficient work will have been done against the strong attraction to enable a new quark/antiquark pair to be promoted from the vacuum. The anti-quark will have the opposite colour to the originally impelled quark so their colours now add up to zero and hence they can, finally, escape from the other particles.

But the newly created quark and anti-quark may or may not be the same flavour as the original quark. God plays dice. :-)
 

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