Electron-Positron Annihilation and the New Particles Energetic collisions between electrons and positrons give rise to the unexpected particles discovered last November. They may help to elucidate the structure of more familiar particles by Sidney D. Drell hen matter and antimatter are few structureless entities that are truly of interactions they participate in, or, as Wbrought together, they can an­ fundamental. These constituent particles 'it is often put, according to the kinds of nihilate each other to form a have been named quarks. Different ver­ forces they "feel." The forces considered state of pure energy. A fundamental sions of the quark theory make different are the four fundamental ones that are principle of physics demands that the re­ predictions about what is to be expected believed to account for all observed in­ verse of that process also be possible: A in the aftermath of an electron-positron teractions of matter: gravitation, elec­ state of pure energy can (quite literally) annihilation, and it was hoped that the tromagnetism, the strong force and the materialize to form particles of ponder­ experiments would help to determine weak force. able mass. When the matter and anti­ which version is the correct one. In the everyday world gravitation is matter are an electron and a positron, As it turned out, the results of an ini­ the most obvious of the four forces; it the state formed by their annihilation tial series of experiments in 1973 and influences all matter, and the range over consists of electromagnetic energy. It is early 1974 were not in accord with any which it acts extends to infinity. For the a particularly simple state, since elec­ of the predictions. Then, last November, infinitesimal masses involved in sub­ tromagnetism is described by a well­ as the measurements were being repeat­ atomic events, however, its effects are tested theory and is believed to be un­ ed and refined, two very massive par­ vanishingly small and can be ignored. derstood. For some time physicists have ticles were unexpectedly discovered. By The electromagnetic force also has in­ been eager to learn just what kinds of coincidence the discovery of the first of finite range, but it acts only on matter particles are created when an electron the particles was announced simulta­ that carries an electric charge or current. and a positron collide at high energy. neously by· physicists at two laboratories The photon is the quantum, or carrier, During the past two years several experi­ studying quite different reactions. of the electromagnetic force, and when ments have provided a preliminary view two particles interact electromagnetical­ of the annihilation process; the results Four Forces ly, they can be considered to exchange a have annihilated expectations. It is as the photon or photons. In the classification British poet Gerald Bullett described the The existence of the new particles is of particles the photon is in a category arrival of spring: in itself a surprise, but even more re­ by itself: it has no mass and no charge markable is their extraordinary stability. and it does not participate in either Like a lovely woman late for her Although they decay to more familiar, strong or weak interactions. appointment less massive particles in a period that The strength referred to in the names She's suddenly here, taking us by conventional standards is very brief, of the strong and the weak interactions unawares, their lifetime is about 1,000 times longer is related to the rate at which the inter­ So beautifully annihilating than that of other particles of compara­ actions take place. The strong force has expectation. ble mass. This exceptional stability sug­ a short range: its effects extend only gests that the new particles are funda­ about 10-13 centimeter, or approximately The discoveries are the most startling mentally different from other kinds of the diameter of a subatomic particle such and exciting to emerge from high-energy matter. As yet their nature has not been as a proton_ When two particles that feel physics in a decade or more. satisfactorily explained, and their signifi­ the strong force approach to within this One reason for the great interest in cance remains a subject of lively specula­ distance, the probability is very high these experiments is that they provide a tion. Theories abound and physics is in a that they will interact, that is, they will means of testing a central concept of state of great ferment, but we cannot be either be deflected or they will produce modern particle physics: the notion that sure where the particles fit into the other particles_ In contrast, particles that the "herd" of supposedly elementary par­ scheme of things. interact electromagnetically are 10,000 ticles discovered during the past 25 years Subatomic particles can be classified times less likely to interact under the may actually be assemblages of only a in broad families according to the kinds same circumstances_ If the strongly in- 50 © 1975 SCIENTIFIC AMERICAN, INC teracting particles pass each other at the weak force, and even at that range but they are all more massive than the nearly the speed of light (3 X 1010 centi­ the probability that they will interact is leptons. meters per second), then they must in­ less than one in 1010• As a result of discoveries made over teract during the 10-23 second they are All particles except the photon are the past two decades the baryons and the within range of each other. That is the classified according to their response to mesons have become very large families characteristic time scale of the strong these two forces. Those that feel the of particles; there are in all more than interactions. strong force are called hadrons; those 100 known hadrons, most of them mas­ Compared with the strong force, the that do not feel the strong force but do sive and unstable. It was in an effort to weak force is feeble indeed: for collisions respond to the weak force are called lep­ explain this great proliferation of parti­ at low energy it is weaker by a factor of tons. Particles belonging to these two cles that the quark hypothesis was in­ about 1013• At higher collision energies families have quite different properties. troduced independently in 1963 by Mur­ the strength of the weak force increases, The hadrons are subdivided into two ray Cell-Mann and by Ceorge Zweig, but even at the highest energies yet classes called baryons and mesons. The both of the California Institute of Tech­ studied experimentally it is weaker than baryons include the familiar proton and nology. The quark model posits that the the strong force by a factor of about 1010• neutron (and it is the strong force that unstable particles are excited states of Moreover, the range of the weak force binds these particles together in atomic the stable ones. Baryons are thought to is at most a hundredth that of the strong nuclei). The mesons include such par­ consist of three quarks bound together force. Two particles must approach to ticles as the pion and the kaon; they are and mesons to consist of a quark and an within 10-15 centimeter in order to feel generally less massive than the baryons, antiquark. Just as an atom can enter an TWO·MILE-LONG ACCELERATOR at the Stanford Linear Accel­ erated and injected into a storage ring, where the particles and anti­ erator Center (SLAC) was employed to generate high-energy elec­ particles interact. The ring, called SPEAR, is to the right of the trons and positrons in the experiments that led to the discovery of largest buildings. In it beams of electrons and positrons are con­ the new particles. The main beam of the accelerator (extending un· fined in a single evacuated chamber. The beams circulate in oppo­ der the highway) propels electrons into target of tungsten, third site directions, guided by a magnetic field, and pass through each a a of the way down the accelerator. Some of the electrons collide with other twice each revolution. The two buildings straddling the ring tungsten nuclei, creating electron-positron pairs. The positrons and enclose detectors that surround the regions where particle-antipar­ the remaining electrons in the main beam are then further accel- ticle annihilations take place. The ring is about 250 feet in diameter. 51 © 1975 SCIENTIFIC AMERICAN, INC PARTICLE I SYMBOL I MASS I CHARGE I SPIN I LEPTON NUMBER I MU-NESS I BARYON NUMBER LIFETIME PHOTON STABLE y o o o o o ELECTRON e- .5 -1 1 +1 0 0 STABLE 2 POSITRON e .5 +1 1 -1 0 0 STABLE + 2 0 STABLE ELECTRON NEUTRINO v 0 1 +1 0 0 . en 2 z ELECTRON ANTINEUTRINO 0 0 1 -1 0 0 STABLE v 0 . 2 t- 6 a. MUON 106 -1 1 +1 +1 0 10- W-' I" 2 6 ANTIMUON 106 +1 1 -1 -1 0 10- 1"+ 2 0 +1 MUON NEUTRINO v 0 1 +1 0 STABLE � 2 MUON ANTINEUTRINO v 0 0 1 -1 -1 0 STABLE � 2 PROTON P 939 +1 1 0 0 +1 STABLE en 2 z ANTIPROTON 939 -1 1 STABLE P 1 0 0 - 0 2 >- 3 NEUTRON 939 0 0 0 +1 10 a: n 1 « 2 m 3 ANTINEUTRON 939 0 1 0 0 -1 10 n 2 + 8 1' 137 +1 0 0 0 0 10- 8 en PION - 137 -1 0 0 0 0 10- Z l' 0 0 15 a: 1' 137 0 0 0 0 0 10- c 23 « en p 750 +1 1 0 0 0 10- I Z + 23 0 RHO MESON - 750 -1 1 0 0 0 10- en p w 23 ::2' pO 750 0 1 0 0 0 10- 20 PSI 10- (3095) '" 3095 0 1 0 0 0 20 PSI (3684) '" 3684 0 1 0 0 0 10- SUBATOMIC PARTICLES are classified according to the kinds of properties.