Mesons are an un-stable transient alliance between quarks and anti-quarks (matter and anti-matter) such as a positive pion, which comprises an up quark and anti down quark. The quark and anti-quark always have opposite colours, such that a meson is always colourless, or white. A baryon, with three quarks, is also always colourless because the three quarks are always coloured three different colours (like red, green and blue - or their anti-colours, cyan, yellow and magenta). But mesons, comprising two spin-half quarks, which can combine to produce a spin-0 or spin-1 meson, are bosons, with integral spin. Baryons, consisting of three spin-half quarks, can have a total spin of either ½ or 3/2, and thus have half-integral spin; they are fermions.

The diagram above shows three typical mesons, the positive pion (up/not down), negative pion (not up/down) and neutral kaon (not up/strange). In each of the above examples, the spins of the quark/anti-quark pair are paired up (i.e. opposite, and thus the two spins cancel to spin-0.

The diagram below shows mesons with the same combinations of quarks, but this time the quark/anti-quark pair have un-paired spins, resulting in a total spin of 1. Note the name changes, these are excited (higher energy) versions of the above mesons.

The electrical charge on a meson must be either zero, or plus (or minus) one, it cannot be fractional.

Mesons usually decay by the strong nuclear force within 10-20 seconds, but there are some which take 1010 times longer to decay via the weak force, by a process analogous to beta decay: the + meson (up/anti-down meson) decays by the weak force into neutrino and a similarly charged + (a heavy positron, 200 times heavier than an positron) in a process analogous to beta decay. The + then decays into a neutrino/anti-neutrino pair and a positron. A similar process befalls the the oppositely charged pion, which eventually becomes an positron plus a neutrino/anti-neutrino pair. The decay lifetime is about 2.6×10-8 s. Thus the electrical charge, always shown at top right, is conserved in this reaction.

Weak force decay of + meson (slow)

Thus we have mesons, which are hadrons, decaying into electrons and neutrinos, which are leptons; classic beta decay.

Strong force decay of 0 meson (quick)

A 0 meson decays by the strong force into two pairs of gamma rays before they swiftly change into two electron/positron pairs in the much shorter time of 0.8×10-16s.

The spin-0 mesons of the first two generations of quarks, u, d, s & c. [Pop Out Page]
Don red/blue 3D-spectacles to behold.

The above 3D-stereo diagram shows all 16 possible spin-0 mesons using only the first two generations of quarks, up and down plus strange and charm. There will be many more if the third generation, top and bottom, is included as well. The D and F mesons possess charm. The F mesons possess both charm and strangeness. The first generation octet occupies the equatorial plane, the K0, K+, +, K-bar0, K-, -, 0, and eta, . The four mesons in the middle, displayed as only one on the diagram, are depicted as being composed up up/not-up; down/not-down; strange/not-strange and charm/not-charm, but are in reality a chimera of several quarks. For instance, the 0, shown as up/not-up is actually the square root of (bracket) a half of up/not-up minus down/not-down (close-bracket).

N.B. Other than to show the relationship of the spin-0 mesons in the SU(4) symmetry group, a cubo-octahedron, this structure has no existence. A similar diagram can be made for the spin-1 mesons, but I haven't found all their names yet... The lifetime of spin-1 mesons will, in general, be much less that for spin-0 mesons.


The B-mesons, containing bottom quarks

Only recently have computing power and analytical method improved sufficiently to calculate the masses of mesons. It took two years and 500 billion calculations to compute the mass of the Bc meson at 6304 ± 22 MeV, which agrees with the measured value of 6287 ± 5 MeV.

Researchers have found a difference in the decay rate of B-mesons and the anti-B-meson, thus hinting, in some rare cases, at a slight difference between matter and anti-matter.