Meson (from the
Greek 'middle' - its a middle weight). The meson is a class of particles that is, in general, heavier than the
lepton but lighter than those of the
baryon. The meson is a member of the
hadron class of particles and is composed of
quarks. However, unlike the baryons (of which
protons and
neutrons belong), the meson is composed of two quarks (instead of three), one of which is always an
antiquark.
The meson was predicted in 1935 by Hideki Yukawa (who theorized that it was responsible for the force holding the nucleus together). The meson was first detected in 1947 by C. F. Powell in secondary effects of cosmic rays (collisions between a cosmic ray and an atom in the atmosphere). The particle that Yukawa predicted was the pion.
All mesons consist of a quark bound to an anti-quark. The pion (π +), is composed of an up quark and a down anti-quark. The anti-pion (π-), is composed of an up quark is composed of an up anti-quark and a down quark. The pion also happens to be the lightest of the meson family of particles. There is also the neutral pion (π0) which is composed of a a strange combination of (uu + dd)/sqrt(2) - no, I don't understand it at all. A curious fact about the neutral pion is that it is its own anti-particle.
The very nature of the meson being composed of a quark and an anti-quark make it unstable. A charged pion decays with an average lifetime of 2.55 * 10-8 seconds into a muon of the same charge and a neutrino or anti-neutrino. A neutral pion will decay in 2.55 * 10-15 seconds, typically into a pair of photons, but will occasionally decay into a positron-electron pair and a photon.
There are on the order of 140 different types of mesons. Some of them are more stable than other - the kaon (K-) is made of a strange quark and an anti-up quark and is much heavier and long lived than other mesons. Currently, there is much research in the B-zero (B0) meson - it contains an anti-bottom quark and thus is much heaver than other mesons (about 50 times more massive than a pion and 10 times heavier than a kaon).
The pion has been used to predict the range of the the strong force. Using the uncertainty principle the and making the assumption that it is an exchange force (involves sending things back and forth) and involves neutral pions the equation:
h
R = cDt = -----
2mc
The range of 0.73 * 10
-15m results, or about 0.6 times the radius of a
proton.
Because the pion is composed of an up and anti down quark (anti-matter has the same mass as normal matter to within testable limits (a few billionth of a part)) it would be expected that the pion have a mass on the order of 2/3 of the mass of a proton (composed of two up quarks and a down quark) However, the mass of the pion was only 1/6 of the mass of a proton thus showing that the mass of hadrons depend on the dynamics of the particle and not just the components of it.