Baryons are one of the two major families of hadrons, which are particles built from smaller constituents called quarks. They act as the primary building blocks for all visible matter. Baryons are complex, composite particles that experience the strong nuclear force, and their existence is necessary for the formation of atomic nuclei. Understanding baryons reveals how the stability of physical structures, from atoms to galaxies, is maintained.
Defining Baryons and Their Structure
Baryons are classified as a type of subatomic particle that is not fundamental, meaning they are composed of even smaller entities. They belong to the family of particles called hadrons, which are defined by their ability to interact via the strong nuclear force. This powerful, short-range force is what binds the components of a baryon together, overcoming the electromagnetic repulsion between them.
The defining characteristic of a baryon is its composition of three constituent particles known as quarks. These quarks are held in a perpetually confined state, never existing alone in nature due to the strong force. For every baryon, there is a corresponding antimatter partner, called an antibaryon, which is made up of three antiquarks instead.
Quark Flavors
Scientists currently recognize six different “flavors” of quarks:
- Up
- Down
- Strange
- Charm
- Bottom
- Top
Only the up and down quarks form the stable matter we observe daily; the other four flavors are heavier and rapidly decay. Baryons are also categorized as fermions, characterized by having a half-integer value of spin, such as \(1/2\) or \(3/2\).
The Essential Baryons: Protons and Neutrons
The proton and the neutron are the most recognized baryons, collectively known as nucleons, which form the atomic nucleus. The specific combination of three quarks dictates a baryon’s properties, including its mass and electric charge. The proton consists of two up quarks and one down quark, abbreviated as \(uud\).
The neutron, in contrast, is composed of one up quark and two down quarks, or \(udd\). The fractional charges of the quarks combine to give the proton its positive electrical charge and the neutron its neutral charge. These two baryons are the lightest in existence, which has profound implications for the stability of matter throughout the universe.
Protons are stable particles with a lifespan exceeding \(10^{34}\) years, but free neutrons are inherently unstable. An isolated neutron spontaneously undergoes beta decay, transforming into a proton, an electron, and an electron antineutrino. This decay is driven by the weak nuclear force because the neutron is slightly more massive than the proton. When bound in a nucleus, neutrons become stable and mediate the repulsive forces between the positively charged protons.
The Rule of Baryon Conservation
The stability of matter is governed by the fundamental law known as the conservation of baryon number. This law states that in any interaction, the total number of baryons before the event must equal the total number of baryons afterward. To track this, a quantum number called the Baryon Number (\(B\)) is assigned to all particles.
Baryons themselves are assigned a baryon number of \(+1\), while their antimatter counterparts, the antibaryons, are assigned a value of \(-1\). All other particles, such as leptons and the particles that mediate forces, are considered to have a baryon number of zero. Since quarks are the building blocks of baryons, each individual quark is assigned a fractional baryon number of \(1/3\).
This conservation principle explains why the proton is stable and why ordinary matter persists. Because the proton is the lightest known baryon, there is no less massive baryon it could decay into. For a proton to decay into a lighter particle, such as a lepton, the total baryon number of the system would change from \(+1\) to \(0\), which is forbidden by this conservation law.
This accounting system ensures that matter created in the early universe has not decayed away into energy or lighter particles. The stability of the proton, enforced by baryon number conservation, is necessary for the existence of atoms and the complex structures they form. Although speculative theories suggest potential violations, all currently observed physical processes adhere to this rule.