Baryon - Nomenclature

Nomenclature

Baryons are classified into groups according to their isospin (I) values and quark (q) content. There are six groups of baryons—nucleon (N), Delta (Δ), Lambda (Λ), Sigma (Σ), Xi (Ξ), and Omega (Ω). The rules for classification are defined by the Particle Data Group. These rules consider the up (u), down (d) and strange (s) quarks to be light and the charm (c), bottom quark (b), and top (t) to be heavy. The rules cover all the particles that can be made from three of each of the six quarks, even though baryons made of t quarks are not expected to exist because of the t quark's short lifetime. The rules do not cover pentaquarks.

• Baryons with three u and/or d quarks are N's (I = 1⁄₂) or Δ's (I = 3⁄₂).
• Baryons with two u and/or d quarks are Λ's (I = 0) or Σ's (I = 1). If the third quark is heavy, its identity is given by a subscript.
• Baryons with one u or d quark are Ξ's (I = 1⁄₂). One or two subscripts are used if one or both of the remaining quarks are heavy.
• Baryons with no u or d quarks are Ω's (I = 0), and subscripts indicate any heavy quark content.
• Baryons that decay strongly have their masses as part of their names. For example, Σ0 does not decay strongly, but Δ++(1232) does.

It is also a widespread (but not universal) practice to follow some additional rules when distinguishing between some states that would otherwise have the same symbol.

• Baryons in total angular momentum J = 3⁄₂ configuration that have the same symbols as their J = 1⁄₂ counterparts are denoted by an asterisk ( * ).
• Two baryons can be made of three different quarks in J = 1⁄₂ configuration. In this case, a prime ( ′ ) is used to distinguish between them.

• Exception: When two of the three quarks are one up and one down quark, one baryon is dubbed Λ while the other is dubbed Σ.

Quarks carry charge, so knowing the charge of a particle indirectly gives the quark content. For example, the rules above say that a Λ+
c contains a c quark and some combination of two u and/or d quarks. The c quark has a charge of (Q = +2⁄₃), therefore the other two must be a u quark (Q = +2⁄₃), and a d quark (Q = −1⁄₃) to have the correct total charge (Q = +1).