page_title: Baryon
section_title: Isospin and charge
hierarchical_section_title: Baryon / Properties / Isospin and charge
context_page_description: In particle physics, a baryon is a type of composite subatomic particle which contains an odd number of valence quarks. Baryons belong to the hadron family of particles; hadrons are composed of quarks. Baryons are also classified as fermions because they have half-integer spin.
  The name "baryon", introduced by Abraham Pais, comes from the Greek word for "heavy", because, at the time of their naming, most known elementary particles had lower masses than the baryons. Each baryon has a corresponding antiparticle where their corresponding antiquarks replace quarks. For example, a proton is made of two up quarks and one down quark; and its corresponding antiparticle, the antiproton, is made of two up antiquarks and one down antiquark.
  Because they are composed of quark, -based particles, baryons participate in the strong interaction, which is mediated by particles known as gluons. The most familiar baryons are protons and neutrons, both of which contain three quarks, and for this reason they are sometimes called triquarks. These particles make up most of the mass of the visible matter in the universe and compose the nucleus of every atom.
context_section_description: The concept of isospin was first proposed by Werner Heisenberg in 1932 to explain the similarities between protons and neutrons under the strong interaction. Although they had different electric charges, their masses were so similar that physicists believed they were 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.
  This belief lasted until Murray Gell-Mann proposed the quark model in 1964 (containing originally only the u, d, and s quarks). The success of the isospin model is now understood to be the result of the similar masses of u and d quarks. Since u and d quarks have similar masses, particles made of the same number then also have similar masses. The exact specific u and d quark composition determines the charge, as u quarks carry charge + while d quarks carry charge −. For example, the four Deltas all have different charges (
  Δ⁺⁺
   (uuu), 
  Δ⁺
   (uud), 
  Δ⁰
   (udd), 
  Δ⁻
   (ddd)), but have similar masses (~1,232 MeV/c²) as they are each made of a combination of three u or d quarks. Under the isospin model, they were considered to be a single particle in different charged states.
  The mathematics of isospin was modeled after that of spin. Isospin projections varied in increments of 1 just like those of spin, and to each projection was associated a "charged state". Since the "Delta particle" had four "charged states", it was said to be of isospin I = . Its "charged states" 
  Δ⁺⁺
  , 
  Δ⁺
  , 
  Δ⁰
  , and 
  Δ⁻
  , corresponded to the isospin projections I₃ = +, I₃ = +, I₃ = −, and I₃ = −, respectively. Another example is the "nucleon particle". As there were two nucleon "charged states", it was said to be of isospin . The positive nucleon 
  N⁺
   (proton) was identified with I₃ = + and the neutral nucleon 
  N⁰
   (neutron) with I₃ = −. It was later noted that the isospin projections were related to the up and down quark content of particles by the relation:
  where the n's are the number of up and down quarks and antiquarks.
  In the "isospin picture", the four Deltas and the two nucleons were thought to be the different states of two particles. However, in the quark model, Deltas are different states of nucleons (the N⁺⁺ or N⁻ are forbidden by Pauli's exclusion principle). Isospin, although conveying an inaccurate picture of things, is still used to classify baryons, leading to unnatural and often confusing nomenclature.
caption: Combinations of three u, d or s quarks forming baryons with a spin-3/2 form the uds baryon decuplet