Family

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FAMILY

Investigations into the structure of matter at short distances have revealed three families (sometimes called generations) of elementary particles and antiparticles, identical in all respects except for the values of their masses. Stable matter consists only of particles in the first family. The members of the other two families are heavier unstable particles that existed at the earliest moments of the universe and are routinely produced in particle accelerators.

All family members share some attributes, such as mass, momentum, and spin-½, but their interactions differentiate them into two classes, quarks and leptons. Quarks are subject to strong, weak, and electromagnetic interactions, whereas leptons have no strong interactions. Each family consists of one negatively charged lepton, its associated neutral lepton, and two quark flavors, of fractional electric charges 2/3 and -1/3, each coming in three colors. The first family that makes up all matter contains the electron e with charge -1 and its associated neutrino ?_e, the charge 2/3 up quark (u ), and the down quark (d ) of charge -1/3. The second-family members are the negatively charged muon ? and its neutrino ?_?, the charm quark (c ), and the strange quark (s ). The third family is made up of the tau ? lepton and its neutrino ?_?, the top quark (t ), and the bottom quark (b ). The third-family members are the heaviest.

This classification is remarkably simple by historical standards. By the end of the nineteenth century, chemists and physicists had determined all matter to be arrangements of a finite number of electrically neutral atoms, each with definite chemical properties but an unknown structure. Their findings were summarized in Dmitry Mendeleyev's periodic table of elements. A century later, the nature of atoms, the forces that hold them together, and their interactions had all been understood. This knowledge was embodied in the Standard Model of elementary particle physics.

An earlier portrait of elementary particles, circa 1935, shows two nuclear particles, the neutron and proton, and two leptons, the electron and its associated neutrino. This picture of the first family stayed essentially unchanged for thirty-five years. In 1970 a Rutherford-type experiment was performed by a SLAC-MIT group at such high energy as to probe directly atomic nuclei. This produced an amazing result: neutrons and protons are not elementary but rather an assembly of fractionally charged particles, quarks, as earlier hypothesized by Murray Gell-Mann and George Zweig. Protons and neutrons each contain three quarks, uud and udd , respectively.

TABLE 1

The Three Families of Electricity Particles
credit: Courtesy of Pierre Raymond
Family	Quarks	Leptons
First	*u, d*	e ?_e
Second	c, s	?, ?_?
Third	*t, b*	?, ?_?

Quarks do not exist as free particles but are found only in tightly bound triplets, or quark-antiquark pairs. Quantum chromodynamics (QCD) explains this peculiar behavior in terms of the interactions of quarks and massless spin-1 gluons, which couple to the quark colors. This yields a new portrait of the first family: three colors of up and down quarks, the electron, and its neutrino. These are subject to strong interactions, described by eight massless gluons, and electromagnetic interactions, due to exchanges of massless stable photons ?. Weak interactions result from exchanges of two massive unstable particles, the charged W bosons and the neutral Z bosons, both predicted by the Standard Model and found experimentally at the European Laboratory for Particle Physics (CERN) in the early 1980s.

Over a seventy year period, two other families emerged, starting with the discovery in the 1930s of the muon in the by-products of cosmic-ray collisions with the atmosphere. I. I. Rabi's famous quip "who ordered that?" remains unanswered today. This was followed by the discovery of unstable nuclear particles, interpreted as containing a new quark flavor, the strange quark. The muon was found to have its own neutrino, but it was not until 1974 that evidence for the charm quark was produced at the Brookhaven National Laboratory (BNL) and at the Stanford Linear Accelerator Center (SLAC), in the form of a long-lived charm-anticharm bound state, called J/?, completing the second family. In 1977 the first sign of a third family, the heavy tau lepton, was also found at SLAC. The rest of the third family was discovered at the Fermi National Laboratory: the bottom quark in 1979, the top quark in 1995, and the tau neutrino in 2000.

There is no experimental sign of a fourth family of elementary particles, and there is a strong argument against its existence. The Z boson lifetime was measured with great precision at SLAC and CERN in the early 1990s, and all its decay channels have been identified with particle-antiparticle pairs of the first three families. A carbon-copy fourth family with a light neutrino would yield a new way for Z bosons to decay into the fourth neutrino-antineutrino pair, in disagreement with experiment: the Z boson counts the number of weakly interacting neutrinos and finds only three. There is no fourth family with a light neutrino!

The wide disparity between elementary particle masses remains a mystery and the subject of much theoretical research; for example, the recently discovered top quark is about 175 times heavier than hydrogen, whereas the up quark mass is two-hundredths that of hydrogen. The situation is similar for the charged leptons: the electron mass is 1/200 that of the muon, which is ten times lighter than the tau. Until 1998, when the Super-Kamiokande (SuperK) underground detector in Japan presented convincing evidence to the contrary, the three neutrinos were believed to have escaped this mass hierarchy as each appeared to have zero mass. In 2001 the Sudbury Neutrino Observatory (SNO) in Ontario, Canada, presented further evidence, corroborating a long string of earlier experiments that had found a deficit of solar neutrinos. According to SuperK, ?_?'s produced in the collision of cosmic rays with the atmosphere oscillate into ?_?'s, whereas SNO determined that electron neutrinos produced in the solar core oscillate on their way to Earth into other neutrino flavors, ?_?'s and/or ?_?'s, so that an electron-neutrino detector will see a deficit. Oscillations require neutrino flavors of different masses. In addition, neutrino massess are extremely tiny compared to that of their charged partners; for instance, ?_e is at least 1 million times lighter than an electron.

Much current research centers around the following questions: why do three families exist? Nature appears to operate like a bureaucracy, requiring triplicate copies, without any apparent reason. Is this a sign that further unification is needed? However, this tripling also brings about important new phenomena such as CP violation and neutrino oscillations. What is the origin of the quark and charged lepton mass hierarchies? Why are neutrino masses so tiny? What type of new matter awaits discovery?

As the investigation of the fundamental constituents and their interactions at higher energies continues at Fermilab's Tevatron and CERN's Large Hadronic Collider (LHC), physicists expect to find new types of matter. One noteworthy speculation, supersymmetry, suggests that the new matter is also organized in three families of elementary particles, with exactly the same attributes, except for spin! All elementary spin-½ particles of the three families have heavy counterparts with no spin. The spinless partners of the light spin-½ neutrinos, called sneutrinos, are naturally heavy and do not affect the Z boson lifetime. Such speculations await the verdict of experiments.