Meson is made up of one quark and one anti-quark

[iasc]

In my physics class my teacher said that a meson is made up of one quark and one anti-quark.
But when, for example, an electron are that close to a positron pair annihilation occurs.
Why does this not happen in a meson?

 

[mgb_phys]

It does, most have a lifetime of nanoseconds

 

[jtbell]

Also note that most mesons are made of a quark and a different kind of anti-quark. For example, a $\pi^{+}$ is made of an up quark and an anti-down quark. So they can't annihilate electromagnetically to photons, but have to go via the weak interaction.

 

[iasc]

Thank you for clearing that up.
She said something about the same force used to keep a nucleus together is also present in a meson, or something like that.
I didn't completely understand it

 

[Bob S]

Positrons annihilate with electrons, and electrons are ubiquitous. The average atom has one or two dozen (aluminum 13 iron 26). Pi mesons are the lightest strongly interacting mesons. Positive pions will stop and decay to a positive muon in about 26 nanoseconds; Negative pions will stop and often get captured in nuclear atomic orbits, and will get close enough to the nucleus to cause it to break up. Muons are leptons, with a lifetime of about 2 microseconds.

 

[xepma]

Thank you for clearing that up.
She said something about the same force used to keep a nucleus together is also present in a meson, or something like that.
I didn't completely understand it

Yes, the 'force' that glues the protons and neutrons together in an atom is an effective force (as opposed to a 'fundamental' force, like the strong nuclear force or the electromagnetic force). This glueing is accomplished, because the particles continously exchange mesons with each other which serve as a mediator; a "messenger" of the force, for lack of better terminology ;)

This is very similar to what holds an electron and an atom-core together. These particles are held together because they exchange photons, which is the mediator of the electromagnetic force. In the nuclear core the role of mediating particle is played by the mesons and the corresponding force is called the Yukawa interaction.