LEPTON UNIVERSALITY by J. RITCHIE PATTERSON “You know, it would be sufficient to really understand the electron.” —Albert Einstein LL OF US ARE FAMILIAR with the electron as the small companion of Athe atomic nucleus. It is the atomic nucleus that gives ordinary matter its weight, but the electron is responsible for the space that matter fills, for while it is small, the electron rules vast territories. The electron was first discovered thanks to the electric charge that it carries, and we rediscover the electron whenever we happen to get an electrical shock. BEAMLINE 7 Indeed, the root “electro-” comes As far as we know, the electron, It tells us that the electron is small from the Greek word meaning am- muon and tau are fundamental indeed: the agreement would be ber, so named because amber tends particles, and unlike molecules or spoiled if the radius of the elec- − to accumulate electric charge when atoms, they cannot be broken down tron were greater than about 10 14 rubbed, just as we do when we shuf- into smaller components. This is not centimeters. fle across a wool rug. the first time that we have thought Colliders provide an even more While the electron is unique in the we have found the fundamental powerful microscope into the elec- role that it plays in ordinary mat- building blocks of nature, and in the tron’s structure. By studying the ter, it is actually part of a trio. The past we have often been wrong. Or- deflections of electrons and anti- electron’s partners, called the muon dinary matter turned out to be made electrons when they collide with one and the tau, have the same electric of molecules, molecules of atoms, another, experimenters in Japan charge and seem to share its other atoms of nuclei and electrons, nuclei and Europe have probed the electron − properties as well. All its properties, of protons and neutrons, and, most on a scale as small as 10 17 cm, but that is, except one: their masses are recently, protons and neutrons of they see no sign of substructure. very different. The muon is heavier quarks. But the electron, muon and Equally precise studies of the muon than the electron by a factor of about tau appear to be indivisible: we see also yield null results. We know 200, while the tau is heavier by the no sign of constituents and they are much less about the tau, but so far, whopping factor of 3500. These three minuscule in size. In fact, surprising it too appears to be free of smaller particles are three of twelve known as it may seem, they may be point- constituents. fundamental particles of nature. like, with no spatial extent at all. The muon, which we write as µ, How do we measure the size of HE ELECTRON, MUON and was discovered in 1947 in cosmic the electron? Just as the electron tau are not alone. Each has a rays. At the time of its discovery, has electric charge, it also has an in- Tpartner called a neutrino, ν ν ν physicists had untangled quantum trinsic angular momentum that is written as e, µ and τ respectively. mechanics and understood atomic constant in time. We call this angu- The neutrinos are massless or near- structure and the nucleus. The puz- lar momentum “spin” and measure ly so, are electrically neutral, and are zles of nature had apparently been it in units of Planck’s constant h, insensitive to the strong force that solved, and many physicists were which, like the speed of light, is a binds atomic nuclei. As a result they ready to declare victory and retire. fundamental constant of nature. The are rarely detected. When they were The appearance of a new particle spin of the electron (and the muon first proposed by Wolfgang Pauli came as a surprise, and not an en- and tau) is h/2 . Like all circulating in order to explain the apparent tirely welcome one. As I. I. Rabi said electric charges, the electron’s spin loss of energy and momentum in at the time, “Who ordered this?” generates a magnetic field similar to radioactive decays, he apologized, “I The tau (τ) is a much more recent the one around the earth, the have done a terrible thing, I have discovery. It was found in 1975 by strength of which depends on the postulated a particle that cannot be a group led by Martin Perl at the spatial distribution of its charge. Cal- detected.” SPEAR particle accelerator at SLAC. culations of this magnetic field have Neutrinos may be hard to detect, This group observed that the col- been carried out for the electron us- but they are not rare. In fact, they are lision of an electron with an anti- ing the theory of quantum electro- produced abundantly in the sun, and electron (or “positron”) sometimes dynamics (QED) with the assumption more than 1013 pass through your produced particles in a configuration that the electron is pointlike. The re- body each second, and then contin- inconsistent with all known process- sults agree with the experimental ue though the earth and out the other es. Instead, it was exactly what one value within one part in one billion. side. Of these, only one per year would expect if new particles were This is the most precise test of the- interacts in your body, leaving a being produced that were similar to ory and experiment in physics, and brief ripple in its wake. Like the elec- electrons, but much heavier. the agreement is a triumph for QED. tron, muon and tau, the neutrinos 8 SPRING 1995 are believed to be carbon copies of “up” quarks and one “down” quark one another. All have the same spin while a neutron is made of one “up” νe νµ ντ and the same weak charge, and like quark and two “down” quarks). All µ τ the electron, muon and tau, they are other particles that we have observed ( e ) ( ) ( ) believed to be fundamental particles. (other than the leptons), such as the ν π What links the e and e as part- meson, are bound states of the u c t ners? In radioactive decays, we see quarks. d s b that the e is always accompanied by Like the leptons, the six quarks ( ) ( ) ( ) ν ν ν a e, but never, say, by a µ or a τ . seem to come in pairs, and their That the neutrino species are distinct masses range from very small in the The known fundamental particles was first demonstrated in 1962 by first generation to very large in the of nature. The upper six particles are Leon Lederman, Mel Schwartz, Jack third generation. In fact, the top the leptons and the lower six are the Steinberger and their collaborators quark weighs about as much as a quarks. Both the leptons and the quarks in an experiment which earned them gold nucleus. Why there should be are arranged into three generations of the Nobel prize. three generations, and the relation- two particles each (shown in brackets). ship between the lepton and quark HESE SIX PARTICLES, the elec- generations, are mysteries. tron, muon and tau plus their Could there be a fourth generation Tthree neutrinos are known as of quarks or leptons waiting to be “leptons,” a name derived from the discovered? Current evidence sug- Greek word λεπτοσ meaning small gests not. Data from the LEP accel- or light. (Had early particle physicists erator at CERN in Geneva, Switzer- known about the weighty τ, they land have shown that there are only might have chosen a different name!) three species of light (or massless) ν ν High energy physicists like to arrange neutrinos: these are the e, µ, and the leptons in a special way (see fig- ντ. Thus, if an additional generation ure on right), and refer to each lepton exists, its neutrino must be very mas- and its neutrino as a generation. The sive—a marked departure from the generations are ordered by the masses generations that we now know. of the charged leptons. In addition to the leptons, there is ORCES THAT CONTROL the in- another set of particles called quarks. teractions between particles are Quarks are similar to leptons, but un- Fas essential to nature as the par- like leptons, can interact via the ticles themselves. We know of four strong force. We know of six kinds forces: gravity; electromagnetism; (or “flavors”) of quark: “down,” “up,” the “strong” force, which binds to- “strange,” “charm,” “bottom,” and gether quarks into protons, neutrons, “top.” All of these are produced pro- π mesons or other, less common, par- lifically at accelerators except the ticles; and the “weak” force, which “top” quark, for which the first di- is responsible for the decay of ra- rect evidence was reported last year dioactive nuclei and for much of the by particle physicists at the Fermi activity in the sun. National Accelerator Laboratory lo- All of the forces operate in about cated outside Chicago. Quarks are the same way. Associated with each the building blocks of protons and one is a charge: electric charge for neutrons (a proton is made of two electromagnetism, mass for gravity, BEAM LINE 9 A particle physicist’s view of electromagnetic interactions. In the upper figure (a) two (a) charged particles, each represented by a solid line, scatter. A photon carrying e– e– energy and momentum is emitted by one and absorbed by the other. The lower figure (b) shows the less familiar annihilation of an electron and anti-electron.