Composite particles, being composed of fermions, are classified as hadrons. Hadrons are divided into baryons (made of three quarks) and mesons (made of a quark and an anti-quark).
Nucleons in Atoms
The most notable baryons are the nucleons, which compose the center (nucleus) of atoms. If two up quarks (each with an electric charge of +2/3) and one down quark (with a charge of -1/3) bind together then the nucleon is a proton with a sum charge of +1.
Protons, being of like charge, repel each other – this is why the strong force is crucial to everyday matter. It holds protons together and allows for macrophysical systems to exist...people are macrophysical systems.
Protons and electrons exhibit electrical charge, where the proton is positive and the electron negative. The charges of the proton and electron are exactly inverse to each other, even though the proton has a mass roughly 1800 times larger than the electron.
If there are two down quarks and one up quark together the nucleon is neutral. This particle is fittingly called a neutron.
Neutrons have mass slightly larger than that of the proton (at 1 Dalton), but the difference is negligible and both are labeled with the relative mass of 1.
Protons and neutrons are given baryon numbers of +1, while the antiproton is assigned a -1. This baryon number must be conserved, similar in conception to the law of conservation of energy.
Other Baryons
Hyperons are composites of strange quarks. These particles are heavier than the nucleons and do not live nearly as long. A number of other baryons have also been observed, though they are not as numerous as nucleons.
There are many other variations of baryons, some hypothetical in nature, which play lesser roles in our view of the universe. These include the delta, sigma, lambda, xi, omega, and dibaryon varieties.
Mesons
Mesons are particles made of two quarks (a quark and an antiquark) and have zero spin. Though they are composed of quarks, they are called composite bosons because they carry the strong force.
Mesons have a weight between that of a proton and an electron. They are exchanged between protons and neutrons in close proximity.
The Pi-mesons, or pions, can have a positive, negative, or zero charge. Neutral pions are their own antiparticle.
All charges of pions have zero spin. They are also the lightest of the mesons.
K-mesons, or kaons, can have a positive, negative, or neutral charge (there are two types of neutral kaons). All kaons contain a strange quark or strange antiquark, and therefore have a property called strangeness. Strangeness is...strange...it is conserved in strong interactions and violated in weak interactions.
Other mesons, of little generic value, also exist. These include the Rho, Phi, Upsilon, and J mesons.
The Question of How
Both baryons and mesons are vital to the structure of everything within the universe. Potentially more important is not the question of how they combine or what they are, but rather why they behave in the manner they exhibit. They follow physical laws, yes, but what assigned the physical laws?
General References
1. Lederman, Leon. The God Particle
 |
