Editing Meson (section)

==== Original isospin model ====
The concept of isospin was first proposed by [[Werner Heisenberg]] in 1932 to explain the similarities between protons and neutrons under the [[strong interaction]].<ref name="Heisenberg (1932)">
{{cite journal
 |last=Heisenberg |first=W. |author-link=Werner Heisenberg
 |year=1932
 |title=Über den Bau der Atomkerne
 |journal=[[Zeitschrift für Physik]]
 |volume=77 |issue=1–2 |pages=1–11
 |doi=10.1007/BF01342433
 |bibcode = 1932ZPhy...77....1H
 |s2cid=186218053
 |language=de
}}</ref> Although they had different electric charges, their masses were so similar that physicists believed that they were actually the same particle. The different electric charges were explained as being the result of some unknown excitation similar to spin. This unknown excitation was later dubbed ''isospin'' by [[Eugene Wigner]] in 1937.<ref name="Wigner (1937)">{{cite journal |last1=Wigner |first1=E. |year=1937 |title=On the Consequences of the Symmetry of the Nuclear Hamiltonian on the Spectroscopy of Nuclei |journal=[[Physical Review]] |volume=51 |issue=2 |pages=106–119 |bibcode=1937PhRv...51..106W |doi=10.1103/PhysRev.51.106 }}</ref>

When the first mesons were discovered, they too were seen through the eyes of isospin and so the three pions were believed to be the same particle, but in different isospin states.

The mathematics of isospin was modeled after the mathematics of [[Spin (physics)|spin]]. Isospin projections varied in increments of 1 just like those of spin, and to each projection was associated a "[[Quantum state|charged state]]". Because the "pion particle" had three "charged states", it was said to be of isospin {{nowrap| {{mvar|I}} {{=}} 1 .}} Its "charged states" {{SubatomicParticle|Pion+}}, {{SubatomicParticle|Pion0}}, and {{SubatomicParticle|Pion-}}, corresponded to the isospin projections {{nowrap| {{mvar|I}}{{sub|3}} {{=}} +1 ,}} {{nowrap| {{mvar|I}}{{sub|3}} {{=}} 0 ,}} and {{nowrap| {{mvar|I}}{{sub|3}} {{=}} −1 }} respectively. Another example is the "[[rho meson|rho particle]]", also with three charged states. Its "charged states" {{SubatomicParticle|rho+}}, {{SubatomicParticle|rho0}}, and {{SubatomicParticle|rho-}}, corresponded to the isospin projections {{nowrap| {{mvar|I}}{{sub|3}} {{=}} +1 ,}} {{nowrap| {{mvar|I}}{{sub|3}} {{=}} 0 ,}} and {{nowrap| {{mvar|I}}{{sub|3}} {{=}} −1 }} respectively.