The term “baryon” comes from the Greek word for “heavy”. Baryons are a class of particles that include protons and neutrons, which are the most massive particles in the known universe. The name was first proposed by Paul Dirac in 1931, when he suggested that all matter is made up of three types of elementary particles: baryons, leptons, and mesons.
Baryonic matter is the stuff that makes up stars, planets, and people—anything that isn’t an exotic particle like a quark or a neutrino. It’s what we normally think of as “matter”: atoms and molecules held together by electromagnetic forces. We now know that there’s more to the universe than just baryonic matter, but it still makes up most of what we can see and touch.
Protons and neutrons are both baryons. They’re made up of three quarks each: two “up” quarks (with charges +2/3) and one “down” quark (-1/3). Protons have all their quarks in different energy levels (or “flavors”), while neutrons have one down quark in the lowest level and two ups in higher levels. This gives protons a net charge of +1 and neutrons a net charge of zero.
All baryons interact via the strong nuclear force—the force that holds nuclei together. The strong force is mediated by bosons called gluons. Gluons have no electric charge, so they don’t interact with electrons or photons (the carriers of electromagnetism). Instead, they only interact with other gluONS AND WITH QUARKS AND ANTIQUARKS—the building blocks of protONS AND NEUTRONS AS WELL AS MESONS (which will be discussed later).
The strong force has a very short range—it only acts over distances smaller than about 10-15 meters (one femtometer). This means that it can only hold nuclei together; it can’t bind electrons to nuclei to form atoms or keep molecules from flying apart. The strong force is also much stronger than any other known force except for gravity; it takes a lot of energy to overcome it.