III.

Types of Fermions

Two kinds of fermions exist. Scientists call one kind an elementary or fundamental fermion. Elementary fermions have no internal structure; that is, they cannot be divided into smaller particles. The other kind of fermion is called a composite fermion. These fermions are composed of an odd number (greater than one) of one type of elementary fermion, called a quark. Bosons hold the elementary fermions in composite fermions together, so composite fermions also contain bosons.

A.

Elementary Fermions

The two types of elementary fermions are called leptons and quarks. Leptons include electrons, the negatively charged particles that surround the nucleus of an atom. Quarks are the particles that make up protons and neutrons, the positively charged and neutral particles, respectively, that make up the nucleus of an atom.

There are two main types of leptons—charged leptons and neutrinos. Scientists classify these two types of leptons into three groups, or generations, with a charged lepton and a neutrino in each generation, for a total of six leptons. Everyday matter is made of just the first generation leptons. Leptons of the second and third generations are created during high-energy collisions of particles. On Earth these collisions occur in the atmosphere or in laboratories. Particles called cosmic rays from outer space collide with atoms or molecules in the atmosphere. Scientists use machines called particle accelerators to bring particles to high speeds and then send the particles at a target or another beam of particles to create collisions between particles. These collisions also occur in outer space, in stellar explosions called supernovas or in other violent astronomical events. Leptons of the second and third generations exist for only a fraction of a second before decaying into more stable particles.

The first generation leptons are the electron (e^-) and the electron neutrino (ν_e). An electron has an electric charge of –1 times an amount of charge scientists call the fundamental charge. The fundamental charge is defined as 1.602 × 10^-19 coulombs (C). An electron has a mass of 511 keV/c² (keV/c² stands for the unit kilo-, or 1000, electron volts divided by the speed of light squared). Physicists measure the mass of particles in these units because the masses of particles are so small that using kilograms or pounds would be unwieldy. An electron neutrino has no electric charge and has either zero mass or a mass so small that scientists have not yet been able to measure it.

The leptons of the second generation are the muon (µ) and the muon neutrino (ν_µ). Muons have electric charges of –1 fundamental charge and masses of 106 MeV/c². One MeV/c² is equal to 1 million eV/c². They are much heavier than electrons. Muon neutrinos, like all neutrinos, have no electric charge. They have no mass or a very small, as yet undetectable mass.

The third generation leptons are the tau lepton (t) and the tau neutrino (ν_t). The tau lepton has an electric charge of –1 and a mass of 1.77 GeV/c². One GeV/c² is equal to 1 billion eV/c². The mass of the tau lepton is almost twice that of the neutron, the heaviest particle in an atom. Like the other neutrinos, the tau neutrino has no electric charge and little or no mass.

Quarks are the other type of elementary fermion. Quarks differ from leptons in that their electric charges are fractions of the fundamental charge and in that they are never found alone. Quarks are always found in pairs or triplets. Like leptons, quarks come in two main types and three generations. Ordinary matter only includes the two quarks of the first generation. Like leptons of the second and third generations, quarks of the second and third generations are created in collisions between particles. The two main types of quarks are up-type quarks and down-type quarks, named after the two quarks of the first generation. Up-type quarks have electric charges of +’, and down-type quarks have electric charges of -€. The names of quarks do not describe any special characteristic of the quark; the names just allow scientists to differentiate between the different types.

In addition to electric charge, quarks have a characteristic called color charge. Color charge is similar to electric charge, but it is important to the strong force, instead of to the electromagnetic force. The strong force holds together the particles that make up the nuclei of atoms. Color charge does not refer to the colors of light and pigment that we see. Like the names of quarks, it is just a convenient way for scientists to describe the behavior of quarks. The three possible values for the color charge of a quark are red, blue, and green. Any quark can have any color charge. The color charge of a quark is constantly changing.

The two quarks of the first generation are the up quark and the down quark. Up quarks and down quarks combine to make up protons and neutrons, the particles that make up the nuclei of atoms. An up quark has an electric charge of +’ and a mass between 1.5 and 5 MeV/c². A down quark has an electric charge of -€ and a mass of between 3 and 9 MeV/c².

The two second-generation quarks are the charm and strange quarks. The charm quark is an up-type quark with an electric charge of +’ and a mass equal to between 1.1 and 1.4 GeV/c². A strange quark is a down-type quark with an electric charge of -€ and a mass between 60 and 170 MeV/c². The quarks of the third generation are the top and bottom quarks, originally called the truth and beauty quarks. The top quark is an up-type quark with an electric charge of +’ and a mass of approximately 170 GeV/c². A bottom quark is a down-type quark with an electric charge of -€ and a mass between 4.1 and 4.4 GeV/c².

B.

Composite Fermions

Elementary fermions combine to form other particles. A particle that contains an odd number of elementary fermions follows the same rules of behavior as elementary fermions do. Particles that contain an odd number of elementary fermions are called composite fermions because they are made up of more than one particle. A particle that contains an even number of fermions is a boson, because it follows the rules of behavior for bosons. The simplest type of composite fermions is made of three quarks. A particle made of three quarks is called a baryon. Baryons include the protons and neutrons of atoms. There is no limit to the size of a composite fermion, however. If a rock or a human body or a star contains an odd number of elementary fermions, it is classified as a composite fermion.

Protons and neutrons are made of up quarks and down quarks held together by a kind of boson called a gluon. Gluons transfer color charge from one quark to another. The constant exchange of gluons and color charges holds the particle together. A proton is made of two up quarks and one down quark. The sum of the quarks’ electric charges is +1, the proton’s electric charge. A proton has a mass of 938 MeV/c². A neutron is made of one up quark and two down quarks. The sum of the electric charges of the quarks is equal to zero, giving the neutron no electric charge. Neutrons have masses of 940 MeV/c².

Many other baryons exist, but most are unstable and decay quickly. Many of them contain quarks from the second and third generation. For example, groups of baryons called the lambda and sigma particles contain strange, charm, and bottom quarks.