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 gravitationis 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 differentfrom 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-

© 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 10 10 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.

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 ME SON - 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. The classification is reflected in quantum numbers such interactions in which they participate. The hadrons take part in as lepton number, mu-ness and baryon number. The newly discov "strong" interactions; the leptons do not; the photon interacts only ered particles, psi(3095 ) and psi(3684), are mesons. Their most per electromagnetically. The hadrons are divided into mesons and plexing property is their lifetime, which is 1,000 times longer than baryons, which differ in their spin angular momentum and in other that of other particles of comparable mass, such as the rho meson.

excited state when the orbital configura hadrons was provided in the late 1960's of Technology and SLAC under the di tion of its electrons is changed, so a had by experiments in which high-energy rection of Jerome I. Friedman, Henry ron, by analogy, enters an excited state of electrons were scattered by protons and W. Kendall and Richard E. Taylor. higher energy (or mass) when the con neutrons. The experiments were per Once again the results inspired an anal figuration of its constituent quarks is formed at the Stanford Linear Accelera ogy with events on the atomic scale. In altered. tor Center (SLAC) by a group of investi 1911 Ernest Rutherford investigated the A further insight into the nature of gators from the Massachusetts Institute scattering of alpha particles by atoms and was led to predict the existence of a massive nucleus within the atom. Simi COLLISION larly, the SLAC experiments revealed an ENERGY internal structure within the individual hadron; the pattern of electron scatter v;;;c v;;;o 125 GeV i�g suggested the existence of pointlike - ) 0 substructures, which were named par 15 GeV .0005 GeV tons. Partons and quarks are believed to

v;;;c v;;;c be equivalent [see "The Structure of the - >< fit 30 GeV Proton and the Neutron," by Henry W. 15 GeV 15 GeV Kendall and Wolfgang K. H. Panofsky; SCIENTIFIC AMEIUCAN, June, 1971]. The leptons are a much smaller class COLLISION ENERGY in electron.positron annihilations depends on the initial energy and of particles than the hadrons. There are momentum of both particles. When one particle is stationary and the other has a velocity just four: the electron and its neutrino near (the speed of light in vacuum), the collision energy is only a small fraction of the en· and the muon and its neutrino (and their c ergy expended in accelerating the moving particle. When two particles collide with equal four antiparticles). The electron has a and opposite velocity, all their energy is made available for the creation of new particles. mass of about .5 MeV (million electron

© 1975 SCIENTIFIC AMERICAN, INC volts) and an electric charge of -1 unit. The muon has the same charge, but it is 207 times as massive as the electron. Both kinds of neutrino are without mass and charge. Because the electron and the POWER SOURCES muon are charged they can interact elec tromagnetically as well as by the weak force; the neutrinos feel only the weak force. Among the leptons no spectrum of ex cited states comparable to that of the hadrons has been discovered. Nor have ...__DETECTOR DETECTOR scattering experiments yielded any sug ___ gestion of an internal structure. The lep INTERACTION REGION tons are thus fundamentally different from the hadrons. They are not com posed of quarks or partons but are them FOCUSING AND selves apparently pointlike. It is possible they are analogues of the pOintlike con stituents of hadrons.

Quantum Numbers

Physicists identify and describe sub atomic particles by assigning them quan tum numbers. Each number designates a property that is conserved, or left un changed, when particles interact. Some quantum numbers, such as electric COUNTERROTATING BEAMS of electrons and positrons in the SPEAR storage ring are charge and spin angular momentum, re each confined to a small "bunch" a few centimeters long. Tbeir trajectories are determined fer to physical, measurable attributes of by the magnetic field and their energy is maintained by the input of radio.frequency power; the particle. Others are more abstract; a single field guides both electrons and positrons, since the oppositely charged particles reo spond oppositely to it. Moving at nearly the speed of light, the beams pass through each oth· they denote family resemblances among er several hundred thousand times per second. Each beam can have an energy of up to 4 particles and provide a valuable book Ge V (billion electron volts), so that total collision energies of up to 8 Ge V can be achieved. keeping system for classifying particles and their interactions in algebraic form. Baryons and mesons, for example, are distinguished by a property called bary on number. Baryons are assigned a val MUON ue of +1, antibaryons -1 and mesons 0; to say that baryon number is conserved is merely to say that baryons never turn SHOWER into mesons. The least massive baryon, the proton, cannot decay into the less TRIGGER massive mesons or leptons because in such a process baryon number would be SPARK changed. The proton is therefore stable. There are also conserved properties, or quantum numbers, of the leptons that ensure the stability of those particles. Lepton number is a quantity handled in the same way as baryon number. Its con servation prohibits the transformation of individual leptons into pure energy or into hadrons, just as hadrons are forbid FIELD MAGNET den to become leptons. For instance, a positron has the same charge as a proton, and it can be accelerated until its mass is MAGNETIC DETECTOR completely surrounds one of the interaction regions at SPEAR. equal to that of a proton, but no positron It consists of several layers of scintillation counters and spark chambers, which are here ex· has ever been observed to change into a tended along their axis for clarity. Both kinds of detector produce an electrical impulse proton. when a charged particle passes through them. Neutral particles cannot be detected.lnforma· In the same way a quantum number tion from the detectors is recorded electronically and employed to identify the particles, to named mu-ness divides the leptons into reconstruct their trajectories and to determine which events are the result of an electron· two groups. The muon and the muon positron annihilation and which are derived from other, extraneous sources. The momen· neutrino have mu-ness of + 1, their anti- tum of a particle is determined by measuring its deflection in magnetic field of detector.

© 1975 SCIENTIFIC AMERICAN, INC particles have mu-ness of -1 and the -1, 0 and + 1. For each quantum num ard Wilson; SCIENTIFIC AMERICAN, Oc· electron, the positron, the electron neu ber the sum is zero. tober, 1973]. trino and the electron antineutrino all When matter and antimatter meet, It is also possible for the annihilation have zero mu-ness. An electron can their quantum numbers, being opposite, of an electron-positron pair to give rise transfer its mass and charge to more simply cancel. Properties such as charge to hadrons. The reaction is written massive particles and become an elec and lepton number are conserved, but e-e+ � hadrons, where e-e+ repre tron neutrino, but because mu-ness is they are also eliminated! Furthermore, sents the annihilating pair. It was during conserved it can never change simply from the state of energy thus created, an investigation of this process at SLAC into a muon or a muon neutrino. particles having virtually any combina that the new particles were discovered. Baryon number, lepton number and tion of quantum numbers can, at least in The particles were found by a group mu-ness are always conserved; certain principle, be formed, as long as the sum of 35 physicists from the Lawrence other quantum numbers, however, are of the quantum numbers of the products Berkeley Laboratory and SLAC, led conserved only in some interactions. remains zero. In particular, a pair or by Burton Richter, William Chinowsky, They are said to describe "approximate" more than one pair of particles and anti Gerson Goldhaber, Martin L. Perl and conservation laws. Several properties, for particles can be created. There is only George H. Trilling. The less massive of example, are conserved in strong inter one constraint on the process: the ener the two particles was discovered simulta actions but not in weak or electromag gies of a colliding particle and antipar neously at the Brookhaven National Lab netic ones. ticle do not cancel, and in the annihila oratory in experiments investigating a In addition to the several conserved tion energy must be conserved just as it process that is the inverse of the reaction quantum numbers, energy and momen would be in any other interaction. Mo studied at SLAC. The Brookhaven ex tum are always conserved. Energy and mentum must also be conserved. periments were performed by Samuel C. momentum can be transferred from one From these considerations it can be C. Ting and his colleagues from M.I.T. particle to another, but the sum of the seen that some processes that are for and Brookhaven; they studied the pro energies and momenta before an inter bidden in interactions of ordinary par duction of electron-positron pairs in col action must be exactly equal to the sum ticles are permissible in the annihilation lisions of hadrons. The detection of the afterward. of particle-antiparticle pairs. For exam particle by Ting's group was a technical In the annihilation of matter and anti ple, although an electron is prohibited tour de force: it is produced by their matter the arithmetic of quantum num by the conservation of mu-ness from be technique only once in 108 events. bers is particularly simple. The concept coming a muon (except with the emis of antimatter was introduced in 1928 by sion of an electron neutrino and a muon Particle Storage Rings P. A. M. Dirac, and it is now a firmly es antineutrino, a rare process), it is entire tablished principle that for every particle ly possible for an electronand a positron Because hadrons are much more mas there exists an antiparticle. All the quan to annihilate each other and for a muon sive than electrons, hadrons can be cre tum numbers of an antiparticle are "op and an antimuon to materialize from the ated only in annihilations at high energy. posite" those of the corresponding par energy state formed. It is necessary only In order to achieve the required collision ticle, that is, the sum of the quantum that momentum be conserved and that energies it has been necessary to build numbers of the particle and antiparticle the initial particles have sufficient energy machines known as particle storage is zero. The electron has lepton number to account for the rest mass of the muon rings. In these rings particles and anti +1, mu-ness 0 and charge -1; for the antimuon pair [see "Electron-Positron particles are made to circulate and then positron the corresponding values are Collisions.," by Alan M. Litke and Rich- to collide head on [see "Particle Storage Rings," by Gerard K. O'Neill; SCIENTIFIC AMERICAN, November, 1966]. Ordinary particle accelerators can pro duce electrons or positrons of very high energy, but when such a particle strikes an electron in a stationary target, little of that energy is liberated in the colli sion. This effect is a consequence of the special theory of relativity, which states that when a particle acquires a large total energy, it also acquires a large total mass (according to the equation

= E mc2, where c is the speed of light in vacuum). Once an electron or a positron has been accelerated to about 1 GeV (billion electron volts) its velocity is within a few centimeters per second of the speed of light. Any additional energy ANNIHILATION of an electron and a positron produces electromagnetic energy, or pho· imparted to it has negligible effect on its tons. The diagram depicting the event is called a Feynman graph (after Richard P. Feyn· velocity and goes almost entirely toward man); distance is represented on one axis and time on the other. If momentum is to be con· served, two photons must be emitted (a); the photons, like the initial particles, have equal increasing its mass. The two-mile linear and opposite momentum and thus net momentum of zero. When only one photon is created accelerator at SLAC can produce posi (b), it must have large energy but zero momentum, a condition that is impossible for a real trons with an energy, or mass, of 15 GeV. photon. The particle formed is called a virtual photon; it can never be detected and it The collision of such a particle with an quickly decays into real particles with zero total momentum. The production of two pho. electron at rest (which therefore has a tons is said to take place through the exchange of another virtual particle: a virtual electron. mass of about .0005 GeV , 30,000 times

© 1975 SCIENTIFIC AMERICAN, INC smaller than the mass of the positron) is like the collision of a charging elephant with a mouse. The mouse may bounce back or it may be crushed, but in either case the elephant's kinetic energy will be little changed by the collision. In the case of the stationary electron and the 15-GeV positron only about .125 GeV would be made available to create new particles. That is less than 1 percent of the energy supplied to the accelerated particle, and it is too little to produce even a single pion, the least massive hadron. The result is quite diHerent when an electron and a positron movingwith equal velocity in opposite directions col TWO INTERPRETATIONS are possible when the product of an encounter between an lide head on. In this case the sum of the electron and a positron is another electron·positron pair. The initial pair may have annihi· momenta of the two particles is zero, and lated (a), producing a virtual photon that materialized into an electron and a positron. Or particles may have passed nearby and been scattered by the exchange of a photon all their energy appears in the products the (b). of the annihilatjon. The collision is anal ogous to the head-on encounter of two charging elephants. In the storage-ring facility at SLAC, which is called SPEAR, electrons and positrons can be made to collide with energies of up to about 4 GeV each, for a total energy of 8 GeV, enough to generate a rich spectrum of hadrons. To produce such collision en ergies with stationary targets would call for a particle beam having an energy of 64,000 GeV; the accelerator required would be some 6,000 miles long. The SPEAR storage ring was built un der the direction of John R. Rees and Richter. In it "bunches" of electrons and positrons circulate in opposite directions in a single toroidal chamber about 250 feet in diameter. They are confined by a magnetic field, and their energy is main MUONS AND HADRONS can be produced in electron.positron encounters only by the an· tained by the input of radio-frequency nihilation of the particles, not by scattering. The creation of muons (a) requires that the energy. The same magnetic field sustains quantum number mu·ness be changed for each of the particles, although the total mu·ness of both counterrotating bunches; because the pair remains zero. Hadrons are thought to be formed (b) through the creation of a par· tHe particles are oppositely charged they ton and an antiparton (or a quark and an antiquark). The partons interact to form hadrons. react oppositely to it [see top illustration

on page 53]. The beams collide at two regions on the perimeter of the ring, where detec tors have been installed to record and analyze the products of interactions. The

probability of even a single e-e + an nihilation in any one pass-through of the bunches is quite low, but because the particles' velocity is nearly that of light they pass through one another several hundred thousand times per second, so that an acceptable reaction rate is achieved. The beams of particles can be stored in the ring for minutes at a time.

Virtual Particles

What happens when an electron and NEW PARTICLES, psi (3095) and psi (3684), materializefrom the virtual photon when it has a positron meet and annihilate each oth exactly the right energy (3.095 Ge V and 3.684 Ge V respectively). They can decay through a vir. er with a combined energy of a few bil tual photon into leptons (a), such as an electron and a positron, or into hadrons (b). The had· lion electron volts? Because the particles rons most commonly produced by psi particles are pions, the least massive of the hadrons. 55

© 1975 SCIENTIFIC AMERICAN, INC are leptons they do not feel the strong energy and momentum , quantities that ma is for the annihilation to produce two force, and at the energies studied so far must be conserved in all interactions. photons that have equal but opposite the weak interactions are feeble enough For the photon, which has no mass and momenta, thereby satisfying the condi to be neglected. The particles are elec which travels at the speed of light, the tion that the sum of the momenta of the trically charged, however, so that they relation of momentum to energy is con products be zero. This reaction does in do feel the electromagnetic force, and stant: the momentum is a fixed fraction fact take place, and measurement of it is the energy produced by their annihila of the energy, equal to the energy divid of major interest. Generally, however, tion is (to a very good approximation) ed by c. This energy-momentum relation the annihilation process generates as few entirely electromagnetic. In other words, cannot be reconciled with the energy photons as it possibly can. The probabil the electron and the positron annihilate and momentum of the colliding particles. ity that an electron or a positron will in each other, canceling their electric In the storage ring the electron and posi teract with or produce a Single photon charges and lepton numbers, to produce tron move with equal energy but oppo is measured by a dimensionless number a very energetic photon (a gamma ray). site momentum, and the state formed by called the fine-structure constant, equal The photon emitted is not, however, a their annihilation must therefore have to about 1/137. For each additional pho "real" photon such as those that are ob large energy but zero momentum. A ton the probability is reduced by a high served in nature as the quanta of electro photon cannot have that combination of er power of the same factor. magnetic energy. It cannot be real be properties. The most likely outcome of the an cause it has the wrong proportions of One possible resolution of this dilem- nihilation is therefore the creation of a single photon. As we have seen, how ever, it cannot be a real particle; it is 10' called a virtual photon, and its most im portant characteristic is that it can never be observed; it can never emerge from the reaction as radiation. The virtual photon serves merely as a coupling be tween the initial electron-positron pair haVing zero total momentum and some subsequent ensemble of particles that must also have zero total momentum. The virtual photon cannot be ob served because it decays before it can be 103 detected. According to the uncertainty >- I- principle formulated by Werner Heisen :::i iD berg, the lifetime of a virtual particle is « CD necessarily too brief for the particle to 0 a: be observed. In the case of the recent n. en electron-pOSitron annihilation experi z 0 ments the virtual photon materializes in a: 0 less than 10-25 second into particles with « I the correct combination of energy and i momentum. + Q) I Q) Particle Production

When the virtual photon decays, sev eral kinds of particles can be created. At the energies investigated so far pairs of electrons and positrons, pairs of muons and antimuons, and hadrons have all been observed. If the collision yields an electron-pOSi tron pair, the annihilation and rebirth of such a pair is phenomenologically indis tinguishable from the mere elastic scat

______1 0 ' L--- � tering of the incident electron and posi 3.00 3.05 3.10 3.15 tron. In the first case the pair disappears COLLISION ENERGY (BILLIONS OF ELECTRON-VOLTS) in forming a virtual photon and then re appears; in the second the two particles RESONANCE detected in electron-positron annihilations at SPEAR signifies the presence of a new particle, psi(3095). It was discovered last Novemher by a group of physicists un· are said to exchange a photon and are der the direction of Burton Richter, William Chinowsky, Gerson Goldhaber, Martin L. Perl deflected, so tEat their direction is and George H. Trilling. The resonance represents a greatly increased probability of interac· changed but the magnitudes of their en tion betw';en the colliding particles at the resonance energy. In this case the electrons and ergy and momentum are not [see top positrons are about 150 times as likely to annihilate each other and yield hadrons when their illustration on preceding pagel. combined energy is 3.095 Ge Vas they are at adjacent, "background" energies. The psi(3095) The creation of a muon-antimuon pair resonance is exceptionally narrow, which indicates that the lifetime of the particle is long. is not complicated by this ambiguity.

© 1975 SCIENTIFIC AMERICAN, INC Because mu-ness must be conserved the (J) pair can be created only through the an z o nihilation of the electron and the posi [[ o tron. It is also necessary, of course, that � 4 the e-e+ pair have enough energy to ac u.. o count for the mass of the muons . These o ..J reactions are of great interest because W 2 >= they provide a method of testing the W 11 validity of present theories of electro >

. . •• •• ) magnetism at high energy. In addition 3 f ff tt.t . ... f H.U f o t+ tt H+ .J. . •. H.f •. . .. +t.dH. w there is enormous interest in studying [[ the reactions that lead to the production 3.60 3.62 3.64 3.66 3.68 3.70 3.72 3.74 of hadrons. COLLISION ENERGY (BILLIONS OF ELECTRON VOLTS) Processes that involve only leptons and photons can be described entirely in PSI(3684) RESONANCE is neither as tall terms of the electromagnetic interaction. nor as narrow as that of the psi(3095) par· To predict hadron production, however, ticle. The graph (above) is at lower resolu· one must have a theory of the structure tion than the one on the opposite page, that of hadrons; as yet there is no completely is, the yield of the electron·positron colli· satisfactory theory. Most ideas of hadron sions was measured at fewer energies. The stmcture that are under consideration yield is expressed in relative numbers of today rely on the concept of partons or hadrons produced at each energy. Psi(3684) decays spontaneously into psi (3095) with quarks as constituents of the hadron. the emission of a pair of charged pions They predict that at high energy a virtual (le/t). Psi(3095) can then decay in turn, pro. photon can decay into a parton and an ducing other particles, such as an electron· antiparton (or a quark and an antiquark). positron pair or hadrons. Because psi(3684) The parton-antiparton pair is then trans can produce psi(3095) by decay, it has been formed by the strong interaction into suggested that the more massive particle hadrons that emerge in the aftermath of is an excited state of the less massive one. the collision. Even though partons and quarks have not been observed in isolation, many of eral versions of the quark theory predict ing a second round of measurements of their properties are described in detail different values for the hadron/muon the hadron/muon ratio made at many by theory; they are required to have cer ratio. In each case the value is calculated (and more closely spaced) values of the tain properties in order to explain the simply by adding the squares of the collision energy. The mass of the par properties of the families of hadrons that charges of all the quarks postulated by ticle has now been determined accurate have been observed. In addition the the model (see lower illustration on page ly as being 3.095 GeV, and the particle interpretation of the scattering experi 62]. The original formulation of the has been named by the SPEAR group ments performed at SLAC (which origi quark hypothesis, for example, predicts psi(3095). (The particle was given an nally led to the parton hypothesis) in a value of 2/3. Three of the more promi other name, J, at Brookhaven, but here dicates that at high energy quarks or nent variations on the theory give values I am adopting the SPEAR nomencla pa/tons behave as independent, point of 2, 10/3 and 4. ture.) A second, heavier particle was like entities, just as the leptons do. For Initial measurements of the ratio subsequently found at SPEAR, and it this reason it was expected that once the were made in 1973 at Frascati in Italy was designated psi(3684), to denote its total energy of the colliding particles ex and at the Cambridge Electron Accel mass of 3.684 GeV. The heavier particle ceeded a threshold value, suggested by erator in Cambridge, Mass.; they were can decay to form the lighter one, along the scattering experiments to be about 2 soon followed by the first round of with two pions. The Brookhaven workers GeV, the production of leptons and par measurements with the SPEAR storage have searched for the 3.684-GeV reso tons would obey similar mles. In particu ring. To the surprise of most particle nance, but they have found that it can lar it was predicted that above 2 GeV physicists, none of the predictions was not be detected by their methods, a fact the probability of producing hadrons confirmed; the experiments indicated that may provide a clue to the properties would vary with the collision energy in that the hadron/muon ratio is not con of the particle. the same way that the probability of pro stant at all. At 2 GeV the value appeared ducing a pair of muons varies. This rela to be about 2, and it increased gradually The New Particles tion was expressed mathematically by to about 5 at 5 GeV. In other words, stating that the ratio of hadrons to muon collisions at 2 GeV were twice as likely The new particles are unstable, and antimuon pairs would be constant and to produce hadrons as to produce muon like all other very massive and short independent of the collision energy. Be pairs, and at 5 GeV hadrons were about lived particles they are detected as fore the experiments were performed five times as likely. This disturbing de "resonances," or enhancements of the there was argument over what the nu velopment had not yet been accommo probability of an interaction [see "Reso mercial value of this ratio would be, dated by theory (and it still has not been) nance Particles," by R. D. Hill; SCIEN but there was little doubt that the ratio when the unexpected massive particles TIFIC AMERICAN, January , 1963]. As the would in fact be constant at all energies were encountered last November. energy of the colliding beams is in within reach of experiment. At the SPEAR storage ring the first creased in small steps a sudden peak in The disagreement arose because sev- of the new particles was discovered dur- the production of particles is observed at

© 1975 SCIENTIFIC AMERICAN, INC the resonance energy; when the beam gy, they are more likely to interact than The height of the resonance peaks is energy is increased further, the produc they are at other energies. In the case related to their most remarkable fea tion drops off again. Such a pattern in of psi(3095) the probability that the elec ture-their narrowness. The width of a dicates that a particle exists with a mass tron and the positron will interact was resonance represents the uncertainty equal to the combined masses of the observed to increase by a factor of in the determination of the energy at colliding particles; when the colliding about 150 at the resonance energy com which the resonance occurs. This un particles have exactly the required ener- pared with its value at adjacent energies. certainty in the energy is in turn related to the lifetime of the particle by Heisen berg's uncertainty principle. The equa

tion that gives the lifetime is !:.E!:.t = h/27r, where !:.Eis a measure of the un certainty in the determination of the energy, !:.t is a measure of the lifetime and h is Planck's constant (approximate ly 6.6 X 10.27). Only for a stable parti 70 cle-one that has an indefinitely long lifetime, as the proton apparently does can the energy or mass be precisely de fined. In that case !:.tis large and !:.E is correspondingly small; the lifetime is

60 unlimited and the resonance width is vanishingly small, so that the energy of the particle can be known with arbi trarily great precision. The width of the psi(3095) resonance has been determined to be about 77 KeV 50 (thousand electron volts), which repre sents a very sharp peak. Substituting this value in the equation above yields

Cl a lifetime of about 10.20 second. The w-' psi(3684) resonance is somewhat broad >= w er, and that particle therefore decays > 40 faster, with a lifetime of about 10.21 w� second. a: Although 10.20 second is an almost unimaginably brief interval, far too brief to be measured directly, it is 1,000 times

30 longer than the expected lifetime of such a particle. All other heavy resonances are far broader (their width is typically measured in millions of electron volts rather than thousands), and the particles they represent decay in the characteris 20 tic time of the strong interaction: about 10.23 second. We are immediately com pelled to ask what properties of psi (3095) hold it together so long. Does it exhibit a new kind ,of structure? Is there a previously unknown quantum 10 number that nature wants to conserve but that must be changed when psi(3095) decays? Questions of this kind are among the most fundamental that can be asked in the physics of elemen tary particles. 2.75 3.00 3.25 3.50 Although the psi particles are them ENERGY (BILLIONS OF ELECTRON·VOLTS) selves mystifying, their discovery has to some extent simplified, if not clarified, HADRON COLLISIONS also result in the production of one of the new particles. The reso· the earlier measurements of the hadron/ nance is detected as an increased probability of the production of electron·positron pairs in muon ratio. It now appears that for ener the collision of protons with beryllium nuclei; this process is the inverse of electron.posi. an gies between 2 GeV and 3.8 GeV the tron annihilation. The production of the new particle in hadron collisions was observed by a group of physicists, directed by Samuel C. C. Ting, working at the Brookhaven National ratio is roughly constant at about 2.5; Laboratory. They discovered it simultaneously with the SPEAR group and named it ]. the only significantdeviations are those The psi( 3684) particle has proved to be undetectable in the reaction studied at Brook· associated with the two psi resonances. haven. The 3.095·GeV resonance is measured by the number of electron·positron pairs de. At higher energy the ratio increases, tected. Measurements at two calibrations of the detector are superposed (gray and black). reaches a broad peak at about 4.1 GeV,

© 1975 SCIENTIFIC AMERICAN, INC 7 then stabilizes at a value of about 5 [see illustration at right]. Recent experi PSI Cf) 6 (3095) ments at SPEAR have confirmed that a: « the hadron/muon ratio remains approxi D- mately constant up to 6 GeV. The causes 0z 5 of the observed peculiarities near 4.1 :!:::l 0 Ge V are not yet understood. The broad � peak may or may not represent an ad Cf) 4 ditional resonance or perhaps several 0z a: resonances. 0 « f J: 3 LL Quantum Electrodynamics 0 0 I I � 2 III I I filII The interpretation of the puzzling a: I events revealed by electron-positron col lisions is made substantially easier by the fact that the initial state emerging from 1.0 2.0 3.0 4.0 5.0 the annihilation-the virtual photon-is COLLISION ENERGY (BILLIONS OF ELECTRON VOLTS) simple and well understood. Because it RATIO of hadrons to muon-antimuon pairs produced in electron-positron annihilations is formed in a purely electromagnetic was predicted to be constant at collision energies above 2 GeV. The SPEAR experiments process it is described by the theory of demonstrated that the ratio is roughly constant up to about 3.8 Ge V if the effectsintroduced quantum electrodynamics, one of the by the two psi particles are ignored. It rises to a peak at about 4.1 Ge V, then declines again most successful theories in physics. to a nearly constant value of approximately 5. Various versions of the quark model predict Quantum electrodynamics is the the different constant values for the ratio, but none of them explains the observed fluctuations. ory constructed by imposing the laws of quantum mechanics on the classical theory of electromagnetism. It is a ba centimeter, 'or roughly 1 percent of the In practice the interference is detect sic tenet of quantum electrodynamics diameter of a hadron. Even at that small ed by measuring the ratio of muon pairs that electrons, when they interact with scale experiment has so far revealed no to electron pairs produced at energies electromagnetic radiation, act as point Haws in the theory. near the psi(3095) resonance. Of particu charges. This assumption, however, may The quantitative verification of quan lar importance is the observed decline in be only an idealization that, although tum electrodynamics is the 20th-century the ratio just below the peak energy of useful at large distances, fails to describe parallel to the exciting voyage of Isaac the psi(3095) resonance. On fundamen events at subnuclear dimensions. Newton 300 years ago. The elegant sim tal theoretical grounds such an "inter Physicists are eager to test the theory plicity of nature was first comprehended ference dip" is expected only when the of quantum electrodynamics at ever when Newton realized that the same law electron-positron annihilation has two higher energy and with greater precision of gravity that governs the apple's fall possible final states and the two states in order to probe it in finer detail. It is to the ground also describes the motion are, with regard to the electromagnetic particularly important to know whether of the planets, hundreds of millions of interactions, interchangeable. In this in or not quantum electrodynamicsis valid miles apart. We have now also learned stance the twofinal states can only be for the very-high-energy collisions stud that nature applies the same laws of the photon and psi(3095), and the ex ied at SPEAR, since the theory is central electrodynamics both on a large scale, periment has the important implication to the interpretation of hadron structure. extending to about 80 earth radii, and in that psi(3095) has the same quantum Quantum electrodynamics has been a realm less than a millionth the size of numbers as the photon. tested at distances ranging from more an atom. Quantum electrodynamicshas This discovery enables us to specify than 250,000 miles (in measurements of been verified for distances encompassing some of the characteristics of the new the earth's magnetic field) to subatomic more than 25 orders of magnitude. particles. For example, psi(3095), like dimensions much smaller than 10-8 cen If a colliding electron-positron pair the photon, must have a spin angular timeter (in describing the detailed spec can create a psi(3095) resonance, then momentum of 1. Thus our detailed un trum of the hydrogen atom). The atomic of necessity the resonance particle, by derstanding of quantum electrodynam measurements have been made so pre the reverse process, must be able to de ics provides us with information on the cisely that theory and experiment agree cay into an electron-positron pair. (In new particles. to roughly four parts in 109• deed, that is how the particle was dis In interactions of electrons and posi covered by Ting and his colleagues at Color and Charm trons whose final products are either Brookhaven.) Psi(3095) decays in this electron-positron pairs, muon-antimuon way about 7 percent of the time, and There is more to the psi particles, pairs or two or more real photons only muon pairs are produced in another 7 however, than their quantum numbers the electromagnetic interaction makes a percent of the decay events. The re alone. In particular we want to know significant contribution. The entire proc maining 86 percent of the time the reso what kind of quarks they are made up ess, therefore, should be comprehended nance decays to yield hadrons. Since of. For now all attempts to deduce their by quantum electrodynamics and can both muon pairs and electron pairs are quark constitution must be considered be employed to test the theory. Another created through purely electromagnetic speculations only, because experiment application of the uncertainty principle forces, quantum electrodynamics should has not provided the evidence to dis demonstrates that at the energies avail describe these processes, and it is possi criminate conclusively among a welter able with existing storage rings such ble to determine to what extent the two of competing theories; nevertheless, two events probe distances as small as 10-15 mechanisms interfere with each other. proposals that have been given consid-

·59

© 1975 SCIENTIFIC AMERICAN, INC erable attention deserve discussion. Both the same orbital configuration, they have can appear in any of three colors-say were proposed to explain unrelated phe opposite spin. Our understanding of red, yellow or blue. nomena long before the psi particles atomic structure and of the periodic An incidental feature of the color were discovered. table of the elements is based on this hypothesis is that the addition of six Quarks, like the particles they com concept. Particles with integral spin more quarks to the original three enables pose, are assigned quantum numbers. (such as the mesons and the photon) are us to reformulate the quark theory with Allof them have spin angular momen not affected by the exclusion principle; integral charges rather than fractional tum of 1/2, for example, and baryon arbitrarily large numbers of them can be ones. A model of this kind was con number of 1/3. Of the original triplet assembled in the same state. (The half structed by Moo-Young Han of Duke of quarks proposed by Gell-Mann and integral-spin particles are said to obey University and Yoichiro Nambu of the

Zweig, the u quark has a charge of 2/3 Fermi-Dirac statistics, and they are University of Chicago in 1965. and the d and s quarks have a charge of called fermions; the integral-spin parti All versions of the color theory assume -1/3. Antiquarks, of course, have the cles obey Bose-Einstein statistics and are that in the known baryons the three opposite quantum numbers. According called bosons.) colors of quarks are equally represented; to these assignments, the baryons, being Quarks seem to violate these rules. as a result the particle exhibits no net made up of three quarks, must have a They must have spin of 1/2, but on the color. Similarly, the mesons are made up half-integral spin, a baryon number of other hand in constructing the baryons of equal proportions of red, yellow and +1 and a charge of +2, +1, 0 or -1. it is necessary to assume that two or blue quark-antiquark pairs and are also The mesons, as aggregates of a quark more are bound together in the same colorless. Indeed, some physicists have and an antiquark, must have an integral state. The paradox threatens to do vio speculated that in nature all particles spin, a baryon number of 0 and a charge lence to some basic and cherished theo may be colorless. One of a class of pro of +1, 0 or -1. retical principles. It can be resolved, posed interpretations of the psi particles This ingenious scheme neatly account however, simply by insisting that quarks suggests that they may be the first ob ed for all the particles and resonances do obey the exclusion principle. All that served states of colored matter. that had been observed when it was is necessary to make them conform to the The second important theory that has proposed, and it soon proved its predic rule is to endow them with a new quan been invoked to explain the psi particles tive power by postulating an unknown tum number having three possible val is called the charm hypothesis. It was resonance that was promptly discovered. ues, so that the quarks bound together proposed by a number of theorists in It contains a deeply disturbing peculiar in a baryon, although identical in all 1964 simply for the sake of symmetry. ity,however: the quarks are required to other properties, can differ in this new (In particle physics that is not a trivial be particles with a half-integral spin but one. The new property, first suggested motive, since every symmetry has an they do not behave as such particles are in 1964 by Oscar W. Greenberg of the associated conservation law.) In order to expected to. University of Maryland, is called color, construct a parallel to the four known All observed particles with a spin although it has nothing to do with vision leptons, the theory adds to the three of 1/2 obey Wolfgang Pauli's exclusion or the color of objects in the macroscopic original quarks a fourth, designated c principle, which demands that no two world; in this context color is merely a for charm. The c quark has a charge of be in an identical state. The electrons of label for a property that expands the 2/3, and it has + 1 unit of the new quan an atom, for example, always differ in at original ensemble of three quarks to tum number charm; all the other quarks least one quantum number; ifthey have nine. Each quark of the origin,,� triplet have zero charm. In 1970 the new quark and its quan

1.0o.--��--�-�-��rr-----""""""""------""""I tum number were given an important role in physics through the work of Shel (J) don L. Glashow, John Iliopoulos and z Luciano Maiani of Harvard University. 0 � �II: They invoked the charm quark in order 0 w .30 to explain the suppression of certain par w-' ticle decays that in the three-quark mod 0 � el should have proceeded normally. (J) z 0 :J Significance of the Particles ::< .10 0u. -_ ...... - If the c quark exists, of course, one 0 � would expect to observe charmed states II: of matter, such as a baryon made up of a c quark and two other quarks. No charmed particles have been observed, 3.000 3.085 3.090 3.095 3. 100 and hence it is believed that charmed COLLISION ENERGY (BILLIONS OF ELECTRON·VOLTS) quarks are much more massive than un charmed ones. The psi particles are not INTERFERENCE between entirely electromagnetic events and events tbat involve the believed to be charmed either, since they strong interaction is detected by measuring the ratio of muon pairs to electron pairs pro· duced in annihilation experiments. The presence of interference is an indication that psi. are produced electromagnetically and (3095) has the same quantum numbers as tbe photon. If the quantum numbers are the same, the electromagnetic interactions con theory predicts that the muon/ electron ratio will be depressed at energies just below the serve charm, but it has been suggested resonance energy (s olid colored line) ; if the numbers are different, there will be no inter· that they could be mesons consisting of

ference (b roken colored line). Experimentalresults (black dots and bars) show interference. a c quark and an anti-c-quark. That com-

© 1975 SCIENTIFIC AMERICAN, INC bination would not exhibit charm, be cause the charm quantum numbers of the two quarks would cancel. The first requirement of any theory of the new particles is that it explain their anomalously long lifetime. At the mo ment neither color nor charm seems to offer an entirely satisfactory explanation. If psi(3095) and psi(3684) are con sidered to be colored particles, then it is assumed that they live long because there are no other particles of lower mass to which they can transfer their color when they decay. In the strong in teractions color is conserved, so that a colored particle could not decay by the strong force. A particle might well be able to change color, however, in the electromagnetic interactions; the unit of color would be carried off by a pho ton, leaving decay products of colorless hadrons. As we have seen, the emission of a photon is suppressed by a factor of 1/137, and so the requirement that psi(3095) decay electromagnetically of fers a partial explanation of its stability. The actual suppression of the decay in volves a factor of about 1,000 instead of 137, but the discrepancy might be ac counted for. The hypothesis that the psi particles are colored is subject to experimental test by at least two methods. First, the gamma rays needed to carry off color in the electromagnetic decay should be de tected among the decay products of the particles. Second, ifthe psi particles do represent colored particles, they cannot be the only ones; all versions of the color theory predict families of associated col ored mesons, some of which would be electrically charged. The charmed-quark theory is simpler and therefore more appealing, but it of fers no compelling explanation for the psi lifetime. There is no known mecha nism that would prevent a meson made of a charmed quark and antiquark from simply h'ansforming itself into an ordi nary quark and antiquark. In this process there are no quantum numbers to be conserved, since in the particle-antipar ticle pair charm and all other properties cancel. The only apparent solution is to postulate a new law of nature, to declare arbitrarily that the decay of mesons made of a charmed quark and antiquark is inhibited by a factor of 1,000. There is precedent for such a rule in the theo retical treatment of other meson decays, but psi(3095) involves a larger inhibition factor. Moreover, it is a mystery why such rules should be required at all. In spite of this weakness the charm hypotheSis has attractive elements. The

© 1975 SCIENTIFIC AMERICAN, INC existence of resonances corresponding to already suggested mass regions where it and therefore exhibiting charm should a charmed quark and antiquark meson might be profitable to look for excited exist; the discovery of a charmed meson was predicted before the psi particles states of charmonium. Psi(3684) could would provide strong evidence for the were discovered. The existence of the be one of those states, but-there would theory. particle was discussed by Thomas W. have to be more. The transitionbetween In experiments completed so far none Appelquist and H. David Politzer of psi(3684) and psi(3095) with the emis of the phenomena that would confirm Harvard, who named the hypothetical sion of two pions conforms to theoretical the color or charm hypotheses have been entity charmonium. They also suggested expectations. The energy transitions be detected. Indeed, the failure to findthis that it could be formed in electron-posi tween other states have also been calcu supporting evidence has become an em tron annihilations. lated, and ifthey exist, it should be pos barrassment to both theories. The charm hypothesis is also suscepti sible to detect them. Finally, particles The new particles are not the only ble to experimental test. Theorists have incorporating just one charmed quark challenges to theory issuing from the recent discoveries. Some explanation is also required for the broad enhancement QUARK CHARGE BARYON NUMBER CHARM COLOR of hadron production in the vicinity of RED 4.1 GeV, and the questions raised by the u, r-- odd behavior of the hadron/muon ratio 2/3 0 YELLOW u uy + +1/3 r-- throughout the energy range have not L\, BLUE been resolved. The search for bumps d, RED and lines in the mass spectrum goes on I--- + / 0 YELLOW not only at SPEAR but also at the d dy -1/3 1 3 I---'- DORIS storage ring at the German Elec BLUE dt, tron Synchrotron inHamburg and the RED s, ADONE storage ring at Frascati. Other r-- - 0 YELLOW s Sy 1/3 +1/3 investigators are studying the production r-- BLUE of the psi particles through the interac sb tion of photons and hadrons and through c, RED I--- hadron-hadron collisions. (The last is the 2/3 + YELLOW C Cy + 1/3 +1 method employed by Ting and his col I--- cb BLUE leagues at Brookhaven.) In the experimental and theoretical QUARK HYPOTHESIS states that hadrons are not elementary particles but composites of investigations now under way many cur more fundamental entities called quarks. The original formulation of the theory, proposed rent concepts are being challenged; one, independently by Murray Gell·Mann and George Zweig, postulated three quarks, and u, d s. however, is not in question: that of Charge and baryon number (and other quantities not shown) are assigned to them accord· quarks themselves. The discovery of the ing to the principle that baryons are made up of three quarks and mesons of a quark and an psi particles has confirmed again the cen anti quark. Modifications of the theory add a fourth quark, which exhibits a property ar· c, tral importance of quarks as the consti bitrarily called charm, and propose that each quark exists in three states, distinguished tuent particles of hadrons. Whether or by another property, called color. Thus there could be three, four, nine, 12 or more quarks. not we shall ever see free quarks in the laboratory is another question; it is pos will PREDICTED VALUE sible that they always remain unob MODEL OF HADRON STRUCTURE OF HADRON·MUON PAIR RATIO served, exhibiting their physical reality only through their success in explaining GELL·MANN·ZWEIG MODEL 2 2 2 the structure of hadrons and the forces THREE QUARKS m + (-1) + (-1) = ! that act on them. FRACTIONAL CHARGE Furthermore, we have no assurance COLOR HYPOTHESIS that the quarks, whether there are three 2 2 or nine or 12 or more of them, are the THREE TRIPLETS OF QUARKS 3[m +(-1)2 + (-1) ]= 2 FRACTIONAL CHARGE fundamental particles of matter. In the 20th century physics has probed the COLOR HYPOTHESIS AND CHARM HYPOTHESIS 2 2 2 2 atom to discover the nucleus within, and THREE QUARTETS OF QUARKS 3[m +(-1) +H) +@ ]='J> has broken up the nucleus into its con FRACTIONAL CHARGE stituent particles. Those particles are HAN·NAMBU MODEL now interpreted as being composites of more basic entities, the quarks. It is not THREE QUARTETS OF QUARKS (1)2 + (1)2 + (_1)2 + (_1)2 = 4 INTEGRAL CHARGE unreasonable to imagine that we shall someday penetrate the quark and find an internal structure there as well. Only VARIANTS OF THE QUARK THEORY predict different values for the ratio of hadrons the experiments of the future can re to muon pairs produced in electron·positron annihilations. In each case the predicted value is equal to the sum of the squares of the charges of the quarks included in the theory. The veal whether quarks are the indivis Gell.Mann-Zweig model gives a value of 2/3 ; the color hypothesis would increase the ratio ible building blocks of all matter, the to 2; assuming that both color and charm exist would yield a value of 10/3. The scheme "atoms" of Democritus, or whether they devised by Moo·Young Han and Yoichiro Nambu eliminates the fractional charges com· too have a structure, as part of the end mon to other quark theories and predicts a value of 4 for the ratio. So far experimental less series of seeds within seeds en findings at SPEAR and other laboratories cannot be reconciled with any of the predictions. visioned by Anaxagoras.

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