Baryons
Baryons are a class of subatomic particles composed of three quarks.
They are members of the fermion family, meaning they possess half-integer spin and are subject to the Pauli exclusion principle.
The most familiar baryons are protons and neutrons, which together make up the nuclei of atoms. A proton contains two up quarks and one down quark, whereas a neutron contains two down quarks and one up quark.
The characteristics of a baryon - such as its mass, electric charge, and magnetic properties - are determined by the specific combination of quarks it contains and by the dynamics of the strong force that binds them together.
The study of baryons falls within the domain of quantum chromodynamics (QCD), the theory that describes the strong interaction - one of the four fundamental forces of nature.
Beyond protons and neutrons, there exists a wide variety of other, less stable baryons. These are typically created in high-energy collisions at particle accelerators or in extreme astrophysical environments. Such exotic baryons generally have extremely short lifetimes, decaying rapidly into lighter particles through various decay channels.
Baryon Number
The baryon number ( $B$ ) is a quantum number that tracks the net count of baryons within a system.
- each baryon (such as protons and neutrons) carries $B = +1$
- each antibaryon carries $B = -1$
- all other particles (leptons, mesons, photons, etc.) carry $B = 0$
A defining feature of baryons is that their total number is conserved in all known interactions.
$$ B_{\text{initial}} = B_{\text{final}} $$
This rule, known as the conservation of baryon number, states that in any reaction or decay, the difference between the number of baryons and antibaryons remains constant.
Example
A textbook example is the beta-minus decay of a neutron:
$$ n \rightarrow p + e^- + \bar{\nu}_e $$
In this decay, the neutron $(B = 1)$ transforms into a proton $(B = 1)$, plus an electron and an electron antineutrino, both with $B = 0$.
The overall baryon number therefore stays unchanged:
$$ B_{\text{initial}} = 1 \quad \Rightarrow \quad B_{\text{final}} = 1 + 0 + 0 = 1 $$
This conservation law is crucial for understanding the stability of matter and the bookkeeping of particles in nuclear and subnuclear processes.
Antibaryons
Antibaryons are the antiparticles of baryons. Whereas a baryon is built from three quarks, an antibaryon is composed of three antiquarks.
For instance, the antiproton ($\bar{p}$) contains two up antiquarks ($\bar{u}$) and one down antiquark ($\bar{d}$). The antineutron, by contrast, consists of two down antiquarks ($\bar{d}$) and one up antiquark ($\bar{u}$).
Like baryons, antibaryons belong to the family of fermions and therefore have half-integer spin. However, they carry charges opposite to those of their baryonic counterparts.
By convention, every antibaryon is assigned a baryon number of $B = -1$.
The conservation of baryon number requires that baryons and antibaryons are always produced or destroyed in pairs, ensuring that the overall balance remains unchanged.
The Instability of Antibaryons
Antibaryons are fundamental constituents of antimatter, together with positrons (antielectrons) and antineutrinos. Consequently, they cannot remain stable in ordinary matter.
When an antibaryon (such as an antiproton) meets a baryon (such as a proton), the two annihilate, converting their mass into other particles, typically mesons and photons.
$$ p + \bar{p} \;\;\rightarrow\;\; \pi^+ + \pi^- + \pi^0 $$
For this reason, antibaryons have only fleeting lifetimes in the visible universe. They are observed primarily in high-energy collisions at particle accelerators or in extreme astrophysical events.
And research into the rich and complex world of baryons continues…
