III.

Particles of Matter

Ordinary matter makes up all the objects and materials familiar to life on Earth, including people, cars, buildings, mountains, air, and clouds. Stars, planets, and other celestial bodies also contain ordinary matter. The fundamental fermions that make up matter fall into two categories: leptons and quarks. Each lepton and quark has an antiparticle partner, with the same mass but opposite charge. Leptons and quarks differ from each other in two main ways: (1) the electric charge they carry and (2) the way they interact with each other and with other particles. Scientists usually state the electric charge of a particle as a multiple of the electric charge of a proton, which is 1.602 × 10^-19 coulombs (C). Leptons have electric charges of either -1 or 0 (neutral), with their antiparticles having charges of +1 or 0. Quarks have electric charges of either +’ or -€. Antiquarks have electric charges of either -’ or +€. Leptons interact rather weakly with one another and with other particles, while quarks interact strongly with one another.

Leptons and quarks each come in 6 varieties. Scientists divide these 12 basic types into 3 groups, called generations. Each generation consists of 2 leptons and 2 quarks. All ordinary matter consists of just the first generation of particles. The particles in the second and third generation tend to be heavier than their counterparts in the first generation. These heavier, higher-generation particles decay, or spontaneously change, into their first generation counterparts. Most of these decays occur very quickly, and the particles in the higher generations exist for an extremely short time (a millionth of a second or less). Particle physicists are still trying to understand the role of the second and third generations in nature.

A.

Leptons

Scientists divide leptons into two groups: particles that have electric charges and particles, called neutrinos, that are electrically neutral. Each of the three generations contains a charged lepton and a neutrino. The first generation of leptons consists of the electron (e-) and the electron neutrino (ν_e); the second generation, the muon (µ) and the muon neutrino (ν_µ); and the third generation, the tau (t) and the tau neutrino (ν_t;).

The electron is probably the most familiar elementary particle. Electrons are about 2,000 times lighter than protons and have an electric charge of –1. They are stable, so they can exist independently (outside an atom) for an infinitely long time. All atoms contain electrons, and the behavior of electrons in atoms distinguishes one type of atom from another. When atoms radioactively decay, they sometimes emit an electron in a process called beta decay.

Studies of beta decay led to the discovery of the electron neutrino, the first generation lepton with no electric charge. Atoms release neutrinos, along with electrons, when they undergo beta decay. Electron neutrinos might have a tiny mass, but their mass is so small that scientists have not been able to measure it or conclusively confirm that the particles have any mass at all.

Physicists discovered a particle heavier than the electron but lighter than a proton in studies of high-energy particles created in Earth’s atmosphere. This particle, called the muon (pronounced MYOO-on), is the second generation charged lepton. Muons have an electric charge of -1 and an average lifetime of 1.52 microseconds (a microsecond is one-millionth of a second). Unlike electrons, they do not make up everyday matter. Muons live their brief lives in the atmosphere, where heavier particles called pions decay into muons and other particles. The electrically neutral partner of the muon is the muon neutrino. Muon neutrinos, like electron neutrinos, have either a tiny mass too small to measure or no mass at all. They are released when a muon decays.

The third generation charged lepton is the tau. The tau has an electric charge of -1 and almost twice the mass of a proton. Scientists have detected taus only in laboratory experiments. The average lifetime of taus is extremely short—only 0.3 picoseconds (a picosecond is one-trillionth of a second). The tau has an electrically neutral partner called the tau neutrino. Scientists have detected tau neutrinos directly during experiments. Like the other neutrinos, the tau neutrino has an extremely small mass.

B.

Quarks

The fundamental particles that make up protons and neutrons are called quarks. Like leptons, quarks come in six varieties, or “flavors,” divided into three generations. Unlike leptons, however, quarks never exist alone—they are always combined with other quarks. In fact, quarks cannot be isolated even with the most advanced laboratory equipment and processes. Scientists have had to determine the charges and approximate masses of quarks mathematically by studying particles that contain quarks.

Quarks are unique among all elementary particles in that they have fractional electric charges—either +’ or -€. In an observable particle, the fractional charges of quarks in the particle add up to an integer charge for the combination.

The first generation quarks are designated up (u) and down (d); the second generation, charm (c) and strange (s); and the third generation, top (t) and bottom (b). The odd names for quarks do not describe any aspect of the particles; they merely give scientists a way to refer to a particular type of quark.

The up quark and the down quark make up protons and neutrons in atoms, as described below. The up quark has an electric charge of +’, and the down quark has a charge of -€. The second generation quarks have greater mass than those in the first generation. The charm quark has an electric charge of +’, and the strange quark has a charge of -€. The heaviest quarks are the third generation top and bottom quarks. Some scientists originally called the top and bottom quarks truth and beauty, but those names have dropped out of use. The top quark has an electric charge of +’, and the bottom quark has a charge of -€. The up quark, the charm quark, and the top quark behave similarly and are called up-type quarks. The down quark, the strange quark, and the bottom quark are called down-type quarks because they share the same electric charge.

Particles made of quarks are called hadrons (pronounced HA-dronz). Hadrons are not fundamental, since they consist of quarks, but they are commonly included in discussions of elementary particles. Two classes of hadrons can be found in nature: mesons (pronounced ME-zonz) and baryons (pronounced BARE-ee-onz).

Mesons contain a quark and an antiquark (the antiparticle partner of the quark). Since they contain two fermions, mesons are bosons. The first meson that scientists detected was the pion. Pions exist as intermediary particles in the nuclei of atoms, forming from and being absorbed by protons and neutrons. The pion comes in three varieties: a positive pion (p⁺), a negative pion (p^-), and an electrically neutral pion (p⁰). The positive pion consists of an up quark and a down antiquark. The up quark has charge +’ and the down antiquark has charge +€, so the charge on the positive pion is +1. Positive pions have an average lifetime of 26 nanoseconds (a nanosecond is one-billionth of a second). The negative pion contains an up antiquark and a down quark, so the charge on the negative pion is -’ plus –€, or -1. It has the same mass and average lifetime as the positive pion. The neutral pion contains an up quark and an up antiquark, so the electric charges cancel each other. It has an average lifetime of 9 femtoseconds (a femtosecond is one-quadrillionth of a second).

Many other mesons exist. All six quarks play a part in the formation of mesons, although mesons containing heavier quarks like the top quark have very short lifetimes. Other mesons include the kaons (pronounced KAY-ons) and the D particles. Kaons (Κ) and Ds come in several different varieties, just as pions do. All varieties of kaons and some varieties of Ds contain either a strange quark or a strange antiquark. All Ds contain either a charm quark or a charm antiquark.

Three quarks together form a baryon. A baryon contains an odd number of fermions, so it is a fermion itself. Protons, the positively charged particles in all atomic nuclei, are baryons that consist of two up quarks and a down quark. Adding the charges of two up quarks and a down quark, +’ plus +’ plus -€, produces a net charge of +1, the charge of the proton. Protons have never been observed to decay.

The neutrons found inside atoms are baryons as well. A neutron consists of one up quark and two down quarks. Adding these charges gives +’ plus -€ plus -€ for a net charge of 0, making the neutron electrically neutral. Neutrons have a slightly greater mass than protons and an average lifetime of 930 seconds.

Many other baryons exist, and many contain quarks other than the up and down flavors. For example, lambda and sigma (S) particles contain strange, charm, or bottom quarks. For lambda particles, the average lifespan ranges from 200 femtoseconds to 1.2 picoseconds. The average lifetime of sigma particles ranges from 0.0007 femtoseconds to 150 picoseconds.