Meson

A meson is a particle (hadron) composed of a quark and an antiquark of opposite colors. Often a given quark-antiquark pair does not occur on its own, but instead is mixed with others so that the quarks have a superposition of flavors (as always, flavors more similar in mass mix more). The lowest energy mesons are the pseudoscalars (spin 0), where the quark and antiquark have opposite spin, and then the vectors (spin 1), where the quark and antiquark have parallel spin. Both come in higher energy versions where the spin is augmented by orbital angular momentum. Most of a meson's mass comes from binding energy, rather than the mass of the components. All mesons are unstable.

Mesons were originally predicted as carriers of the force that bind protons and neutrons together. When first discovered the muon was identified with this family from its similar mass and was named "mu meson", but did not show strong attraction to nuclear matter and is actually a lepton. Later the pion was discovered, which is the actual carrier and decays into the muon.

The various types of meson

Particle	Symbol	Anti- particle	Makeup	Rest mass MeV/c2	S	C	B	Half life
Pion	π+	π-	<math>\mathrm{u \bar{d}}</math>	139.6	0	0	0	2.60×10-8
Pion	π0	Self	<math>\mathrm{\frac{u\bar{u} + d \bar{d}}{\sqrt{2}}}</math>	135.0	0	0	0	0.83×10-16
Kaon[?]	K+	K-	<math>\mathrm{u\bar{s}}</math>	493.7	+1	0	0	1.24×10-8
Kaon	<math>\mathrm{K_S^0}</math>	<math>\mathrm{K_S^0}</math>	<math>\mathrm{\frac{d\bar{s} - s\bar{d}}{\sqrt{2}}}</math>	497.7	+1	0	0	0.89×10-10
Kaon	<math>\mathrm{K_L^0}</math>	<math>\mathrm{K_L^0}</math>	<math>\mathrm{\frac{d\bar{s} + s\bar{d}}{\sqrt{2}}}</math>	497.7	+1	0	0	5.2×10-8
Eta	η0	Self	<math>\mathrm{\frac{u\bar{u} + d\bar{d} - 2s\bar{s}}{\sqrt{6}}}</math>	548.8	0	0	0	<10-18
Rho	ρ+	ρ-	<math>\mathrm{u\bar{d}}</math>	770	0	0	0	0.4×10-23
Phi	φ	Self	<math>\mathrm{s\bar{s}}</math>	1020	0	0	0	20×10-23
D	D+	D-	<math>\mathrm{c\bar{d}}</math>	1869.4	0	+1	0	10.6×10-13
D	D0	<math>\mathrm{\bar{D^0}}</math>	<math>\mathrm{c\bar{u}}</math>	1864.6	0	+1	0	4.2×10-13
D	<math>\mathrm{D_S^+}</math>	<math>\mathrm{D_S^-}</math>	<math>\mathrm{c\bar{s}}</math>	1969	+1	+1	0	4.7×10-13
J/Psi	J/Ψ	Self	<math>\mathrm{c\bar{c}}</math>	3096.9	0	0	0	0.8×10-20
B	B-	B+	<math>\mathrm{b\bar{u}}</math>	5279	0	0	-1	1.5×10-12
B	B0	<math>\mathrm{\bar{B^0}}</math>	<math>\mathrm{d\bar{b}}</math>	5279	0	0	-1	1.5×10-12
Upsilon	Υ	Self	<math>\mathrm{b\bar{b}}</math>	9460.4	0	0	0	1.3×10-20

External links

• http://hyperphysics.phy-astr.gsu.edu/hbase/particles/meson#c1
• Authorative information on particle properties is compiled by the Particle Data Group http://pdg.lbl.gov