Taxonomy of Particles

The Standard Model classifies all known particles (along with some undiscovered particles, which for theoretical reasons are believed to exist) into two broad categories: fermions and bosons. As discussed on the preceding pages, elementary bosons are “force-carrier” particles that transmit forces and energy from one place to another. Elementary particles of matter and antimatter—quarks, leptons, antiquarks, and antileptons—belong to a different class of particles called fermions, named for Italian physicist Enrico Fermi.

Fermions and bosons differ in several ways. One difference involves a fundamental property called spin. Spin is considered a form of angular momentum, because particles with spin behave similarly to macroscopic objects that are rotating. Unlike macroscopic objects, however, elementary particles always spin at the same speed. A particle’s spin orientation (i.e., the direction of its rotation) may change, but its magnitude does not: the particle can’t spin faster or slower. The magnitude of an elementary particle’s spin is one of its fundamental properties and is represented by a quantity called the spin quantum number. All elementary fermions have spin quantum number ½, while elementary bosons have spin quantum numbers of either 1 or 0.

Another important distinction is that fermions obey a principle of quantum mechanics called the Pauli exclusion principle, which says that identical fermions cannot occupy the same quantum state simultaneously. An important consequence of this principle is that two electrons must have opposite spin orientations in order to occupy the same orbital inside an atom, and no more than two electrons can occupy the same atomic orbital at a time, as we saw in chapter 4. The Pauli exclusion principle also limits the compression of matter inside certain types of stars, as we’ll see in chapter 8.In particular, the Pauli exclusion principle is responsible for the electron degeneracy pressure that stabilizes white dwarf stars, and it is also responsible for the neutron degeneracy pressure that stabilizes neutron stars. Bosons, on the other hand, do not obey the Pauli exclusion principle. An unlimited number of bosons with exactly the same properties may occupy the same physical space all at the same time.

Elementary fermions are further subdivided into quarks and leptons, along with their antimatter counterparts. Below is a complete taxonomy of all known elementary particles. The mass of each particle is shown in atomic mass units (1 u is approximately the mass of a proton), and electric charge is shown in units of elementary charge (+1 e is the charge of a proton). The spin quantum number is also shown.

That’s all the elementary particles there are, so far as we know, but there are hundreds of kinds of composite particles. All composite particles (non-elementary particles) are also classified as either fermions or bosons. Here are some of the most important categories and subcategories of composite particles:

• Hadrons - made of quarks and/or antiquarks held together by the strong force
  • Baryons - fermions comprised of three quarks
    • Protons - 2 up quarks and 1 down quark
    • Neutrons - 1 up quark and 2 down quarks
  • Mesons - bosons comprised of a quark and an antiquark (All mesons are unstable, because the quark and the antiquark quickly annihilate each other: they are converted into light. The longest-lived mesons only last for a few hundredths of a millisecond.)
• Nuclei - protons and neutrons held together by the strong force
• Atoms - electrons and a nucleus held together by the electromagnetic force
• Molecules - atoms held together by the electromagnetic force