Baryon

These particles make up most of the mass of the visible matter in the universe and compose the nucleus of every atom (electrons, the other major component of the atom, are members of a different family of particles called leptons; leptons do not interact via the strong force).

[9] However, in July 2015, the LHCb experiment observed two resonances consistent with pentaquark states in the Λ0b → J/ψK−p decay, with a combined statistical significance of 15σ.

This might include neutrinos and free electrons, dark matter, supersymmetric particles, axions, and black holes.

[12] Within the prevailing Standard Model of particle physics, the number of baryons may change in multiples of three due to the action of sphalerons, although this is rare and has not been observed under experiment.

Some grand unified theories of particle physics also predict that a single proton can decay, changing the baryon number by one; however, this has not yet been observed under experiment.

The concept of isospin was first proposed by Werner Heisenberg in 1932 to explain the similarities between protons and neutrons under the strong interaction.

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.

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.

Particles could be described with isospin projections (related to charge) and strangeness (mass) (see the uds octet and decuplet figures on the right).

As other quarks were discovered, new quantum numbers were made to have similar description of udc and udb octets and decuplets.

They are related to the number of strange, charm, bottom, and top quarks and antiquark according to the relations: meaning that the Gell-Mann–Nishijima formula is equivalent to the expression of charge in terms of quark content: Spin (quantum number S) is a vector quantity that represents the "intrinsic" angular momentum of a particle.

Particle physicists are most interested in baryons with no orbital angular momentum (L = 0), as they correspond to ground states—states of minimal energy.

This phenomenon of having multiple particles in the same total angular momentum configuration is called degeneracy.

Gravity, the electromagnetic force, and the strong interaction all behave in the same way regardless of whether or not the universe is reflected in a mirror, and thus are said to conserve parity (P-symmetry).

However, the weak interaction does distinguish "left" from "right", a phenomenon called parity violation (P-violation).

It turns out that this is not quite true: for the equations to be satisfied, the wavefunctions of certain types of particles have to be multiplied by −1, in addition to being mirror-reversed.

There are six groups of baryons: nucleon (N), Delta (Δ), Lambda (Λ), Sigma (Σ), Xi (Ξ), and Omega (Ω).