The Leptons After 100 Years Martin L. Perl
Citation: Physics Today 50, 10, 34 (1997); doi: 10.1063/1.881960 View online: https://doi.org/10.1063/1.881960 View Table of Contents: http://physicstoday.scitation.org/toc/pto/50/10 Published by the American Institute of Physics
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Marietta Blau: Between Nazis and Nuclei Physics Today 50, 42 (1997); 10.1063/1.881996 THE LEPTONS AFTER 100 YEARS ver the course of a cen- Puzzles and mysteries abound: Are there in complicated particles such Otury, six leptons have as protons and TT mesons; the been discovered in the uni- more than just the six leptons already proton contains three quarks, verse of elementary particles. known; what is the intrinsic difference the ir meson contains one Three—the electron, muon quark and one antiquark. and tau—each have one unit between the electron, the muon and the We have never succeeded in of electric charge; the other tau; do neutrinos have mass? making or finding a single three—the neutrinos—have quark isolated by itself. no charge. Conversely, leptons, free of the Borrowing from Charles Martin L. Perl strong force, can be isolated Dickens, it can be said of the and studied individually. leptons that they are the best In thinking about defin- of particles, they are the worst of particles. ing the leptons, it is useful to remember that they are not The leptons are the best of the elementary particles force-carrying particles. This property separates them not because, being free of the complicated strong force, they only from the gluon and the photon, but also from the W can be isolated (figure 1), allowing many of their properties and Z particles that carry the weak force, and from the to be measured directly. Also, being free of the strong theorized graviton that is supposed to carry the gravita- force, they provide simple probes into atomic, nuclear and tional force. particle physics. But the leptons are also the worst of the elementary Lepton flavor conservation and neutrinos particles, for we do not know if their external simplicity The three charged leptons are distinguished from each hides important and intricate secrets. For example, after other not only by their very different masses (table 2 on many careful and clever experiments, we still do not know page 39), but also by a mysterious, at least to me, property the masses of any of the neutral leptons, the neutrinos. called lepton flavor. A student first learning about the We do not even know if the neutrino masses are zero or charged leptons would reasonably expect that the heavier not zero. muon and tau would decay through the electromagnetic The elementary particles in general are the smallest interaction: pieces of matter that we have been able to find. They are less than 10~16 meters in extent and perhaps have no detectable size.1 All the known elementary particles fall T1 -> e* + y (1) into two very general types. One type, consisting of the leptons and the quarks, is made up of spin-1/^ particles that obey Fermi-Dirac statistics and hence are called where y is a photon. Why not? The mass differences fermions. The other type consists of the bosons, which provide plenty of energy for the decay, the charge is have integral spin such as 0 or 1 and obey Bose—Einstein balanced and angular momentum can be balanced because statistics. The bosons are force-carrying particles. For the leptons have spin V2 and the photon has spin 1. But example, the gluon, spin 0, carries the strong force, and none of these decays has ever been seen! the photon, spin 1, carries the electromagnetic force. The measured upper limit on the probability of the /A -> ey decay is less than 10~10 compared to the observed I begin this review with one of the good aspects of decay of the /u, (equation 2 below). The analogous upper limit the leptons, the simplicity with which they can be defined. 5 There are four known basic forces: electromagnetic, weak, for the probability of the T decays in equation 1 is 10~ . gravitational and strong.1 The leptons are acted upon by The nonobservation of the decays in equation 1 is the electromagnetic and weak forces in a well-understood explained by a rule that assigns a different type or flavor way described by electroweak theory. (See table 1 on page to each of the charged leptons and then states that, in a 36.) We usually assume that the leptons are acted upon reaction involving leptons, it is either very difficult or even by the gravitational force, as has been demonstrated for impossible for the lepton flavor to change. Briefly, the the electron, but not for the muon or tau. And although rule says that lepton flavor is conserved. But we have we have no experimental evidence for the action of gravity no understanding of the nature of lepton flavor or of how on the neutrinos, many models for the evolution and strictly it is conserved. Indeed it was wrong for me to structure of the universe assume both nonzero mass and use the word "explained" in the first sentence of this conventional gravitational interaction for neutrinos. No- paragraph; what we have here is a codifying of observa- tice that I have written "assume": Even defining the tions, not an explanation. In spite of lepton flavor conservation, the muon and leptons entails assuming an answer to some of their 6 secrets. tau do manage to decay with lifetimes of 2 x 10~ seconds and 3 x 10~13 seconds, respectively. They decay through Leptons do not interact through the strong force, and the weak interaction, with neutrinos preserving the lepton this property decisively separates them from quarks. The flavor conservation. Consider the negative muon. Its strong force between quarks compels them to be buried principal decay mode is lX~ —> e~ + v + v^, MARTIN PERL is a professor of physics at the Stanford Linear 1 Accelerator Center in Stanford, California. where vx is a neutrino and v~2 is an antineutrino. Specifi- cally, the vl is given the same flavor as the ixr and is
34 OCTOBER 1997 PHYSICS TODAY J2L1S97 Ammcjii JustiuM* of Physics, S-0031-9228-9710-030-8 FIGURE 1. LIGHTNING, the most dramatic display of leptons. Unlike quarks, which are subject to the strong force, leptons are easily separated from other particles, as happens here naturally. Their isolatability makes their properties relatively easy to study. (Photograph by Ian Symonds.)
denoted vM for muon neutrino. Thus, muon flavor is with hadrons. At high energies, when a v^ collides with conserved by being transferred from the /x" to the vM. And a proton p, we observe the creation of the e~ is compensated by the creation of an antielectron neutrino % with antielectron flavor. Thus, v^ + p —> vM + hadrons the decay is and IX- -» e~ + vM + %. (2) V + M P —> V-~ + hadrons. Similarly, the T" decays about 20% of the time through But we have never observed each of the analogous processes v^ + p —>ve + hadrons T e + vT
fl +VT or v^ + p —> e~ + hadrons. The remaining 60% of the time, the T~ decays into a tau neutrino, vT, and hadrons. Hadrons have zero lepton flavor. Examples are Neutrino masses and frustration The masses of the charged leptons are well known (table T" —> VT + 77" 2), although we have absolutely no understanding of why + T~ —» VT 7T~ TT the ix mass is about 200 times the e mass and the T mass is about 17 times the /x mass. We have measured only There are other observed and nonobserved interactions of upper limits for the neutrino masses (figure 2 and table the charged leptons that fit together with the nonobser- 2). Therefore, at present the neutrinos are distinguished vation of the decays in equation 1. For example, the from each other only by their associated charged lepton. reactions The upper limits come from the limited precision of the e+ + e~ -> /x+ + /x" technology of the experiments used to measure the neu- + e + e~ —> T+ + T" trino mass. (See box 1 on page 36.) Unfortunately, in all three cases the limited measurement precision of the energy are well known, and the second one is the way in which and momentum of the particles, combined with other experi- we produce tau's to study their properties. But reactions mental problems, has defeated the experimenter. It is very such as frustrating. e+ + e" x+ + e~ Individuals outside elementary particle physics may e+ + e~ justifiably wonder at our concern about neutrino masses. So what if the neutrinos have masses much smaller than have never been observed. their associated charged leptons? Most particle physicists Thus, each of the three known neutrinos is associated see two problems. First, suppose the neutrinos have zero with a specific charged lepton (summarized in table 2). mass like the photon. The photon's zero mass is related This association dominates the interaction of the leptons to a basic invariance property of the electromagnetic field.
OCTOBER 1997 PHYSICS TODAY 35 Similarly a zero mass for the neutrinos should signify Table 1. Definition of leptons and the differences something basic—but what? between leptons and quarks The second problem occurs if the neutrinos have nonzero Property Lepton Quark mass. We already know that Acted upon by the Yes Yes the ratio of the ve mass to the e mass is less than 3 x 10~5; electromagnetic force? what is the significance of Electric charge in units + 1, -lorO + 2/3,-2/3, +1/3 or-1/3 such a small number? In of 1.6 x 10"1 coulombs quark pairs the smallest mass ratio is that of the bot- Acted upon by the weak Yes for all Yes tom quark mass to the top force? known leptons quark mass, about 3 x 10~2. Acted upon by the Yes for electron, Yes for up and down I find that most workers r J assumed for other leptons quarks, assumed for in elementary particle phys- gravitational force? other quarks ics believe that the neutrinos No Yes have nonzero mass. But I Acted upon by the think some of this belief is strong force? emotional: There would Can be isolated as a Yes Never observed, seem to be nothing more to single particle? therefore taken as no learn about the neutrinos if they had no mass, unless a new Einstein is stimulated by the simplicity of zero to massive than its charged lepton: find a new general principle in particle physics. I am a 6 V bit skeptical about the hope for nonzero neutrino mass; nature would have to be particularly cruel to have set all the neutrino masses below the reach of present experi- L mental precision. But then, nature has been cruel to L' VV physicists in the past. A little later in this article I discuss an indirect way in which nonzero neutrino masses might be found; it is a more delicate way, a way that depends on lepton flavor nonconservation between neutrinos. When we discovered the tau in the 1970s, this model seemed even more reasonable. Are there more leptons? Since then, the powerful method used to discover the Since the discovery of the tau and the deduction of the tau at the electron-positron circular collider called discovery of the tau neutrino 20 years ago (see PHYSICS SPEAR, TODAY, + December 1995, page 17), there have been many, e + e" -» , many searches for additional leptons. Yet no more have been found. I am as surprised as anyone. When my has been used at ever-increasing energies to search for colleagues and I in the 1960s started thinking about the next charged lepton. In the reactions looking for charged leptons more massive than the muon, + + 3 e + e~ —> virtual photon —»L + Lr I was motivated by my sequential lepton model. I + + thought there was a long series of charged leptons and e + e~ -» virtual Z° or real Z° -> L + lr, associated neutrinos, with each neutrino being much less when the e+ and e~ have the same energy E and collide
Box 1: Measuring Neutrino Masses Directly
[~*he upper limit on the vM mass comes from the decay of the T. Then the upper bound on the vT mass is given by + v 2 T** ~* f^ + ii- mivjc < m{i)d- - E5v. + + If the TT decays at rest, the fi and vM are produced with equal Suppose ^ after measurement of many 5TT decays, we find and opposite momentum p. From energy conservation, that £^ never exceeds a maximum value £57r(max). then the -^pifP- _)_ rniuYc* + ^Ip2(p- + m(v )2c4 = miir)^ VT mass 1S given by
From this and our knowledge of m(ji) and 772(77) and the m{yr)
36 OCTOBER 1997 PHYSICS TODAY Lower limit on masses of leptons have also been fruitless up 6 2 io - of new charged leptons 1 TeV/c to a mass of about 80 GeV/c . Similarly, there have been specu- 10 lations about a massive stable neu- 10" trino N not associated with any Lower limit on masses charged lepton; or N might be stable 1C of new neutral leptons 1 GeV/c because it is associated with a more massive charged neutrino; or the de- 10" cay of N might violate lepton conser- vation. Once again, nothing has been 10 found, and the lower limit on m(N) is 2 1 1 MeV/c* 45 to 80 GeV/c , depending on the model I io used in the speculation. 10" We can go further in speculation. I We would still call a particle a lepton 10' if it were acted on only by the elec- jjj tromagnetic interaction—that is, if it 1 keV/c avoided the weak force as well as the strong force. Such a particle would 10 play havoc with the beautiful unified 10" theory of the weak and electromag- netic interaction, but no such lepton 10 1 eV/c has been found and there is no need to worry at present. FIGURE 2. LEPTON MASSES. This logarithmic plot shows the masses of the charged The search for new leptons has leptons and upper limits on the masses of the neutrinos. There is no lower limit expanded in yet other directions. For from experiment on the neutrino masses—indeed, they might be zero. The mass example, the known leptons all have unit MeV/c2 is equivalent to 1.8 x 10"30 kg. spin y% Could there be leptons with spin 0 or 1? Certainly we can main- tain the lepton definition as long as head on, the search can be made up to a lepton mass m(L) the particle avoids the strong interaction but is acted upon = E/c2. As I write this article, the LEP 2 electron-positron by the weak and electromagnetic forces. Indeed, the collider at CERN has been used to search up to E of about popular hypothesis in particle physics called supersym- 80 GeV. But no additional charged lepton has been found. metry predicts that every known lepton has a partner This means that if L exists, m(L) > 45 m(r), a larger ratio with spin 0. Thus, the partner of the spin-V2 electron is gap than exists between the T and the /x. called a slepton and has spin 0 but still has the electron Searches for neutral leptons are more difficult, be- charge ±q. Supersymmetric theory does not require that cause the reaction the particle partners have the same masses; and no e+ + e" —» virtual Z° —> supersymmetric partners have been found. The lower limits on the masses once again are 45 to 80 GeV/c2, cannot be detected directly. However, it can be detected depending on the details of the supposed decay process of indirectly when the reaction is carried out through a real the supersymmetric particle. Z°, e+ + e~ -» real Z° -H> VL + VL, because this reaction broadens the decay width of the Z°. Of course, m(vL) must be less than m(Z°)/2. Because m(Z0)—the mass of the Z°—is about 91 GeV/e2, there are no additional neutrinos with a mass of less than about 45 GeV/c2. These lower bounds for m(L) of about 80 2 2 GeV/c and for TO(VL) of about 45 GeV/c apply not only to leptons that follow the sequential lepton model, but also to hypothetical leptons with different properties. For example, we can consider a charged lepton L that has no 2 METERS associated neutrino, or a charged lepton L whose associated neutrino is more massive: m(vi) > m(L). If the lepton number of L is conserved, it will be stable; if it is not con- served, then L could decay to one or more of the known leptons. Searches for these sorts
FIGURE 3. LIQUID SCINTILLATOR APPARATUS used by Frederick Reines and Clyde Cowan to detect the electron antineutrino. (From ref. 7, 1960.)
OCTOBER 1997 PHYSICS TODAY 37 Technology and lepton discovery Frederick Reines and Clyde Cowan required still different technology.7 They showed that the electron antineutrino The reactions existed by using a nuclear reactor and large liquid scin- tillation counters. For that period in the history of nuclear physics, the liquid scintillation counter apparatus was immense (figure 3). may not be the best way to search for some kinds of new The first use of accelerators in the history of lepton leptons; they are certainly not the only way. Let's look discovery occurred in the early 1960s, when Melvin back at the history of lepton discovery. Schwartz and Bruno Pontecorvo independently proposed In the late 19th century research of William Crookes, the use of high-energy neutrinos to study the interaction Eugen Goldsmith, Heinrich Hertz, Walter Kaufman, of neutrinos with matter. Schwartz and his colleagues Philipp Lenard, Joseph Thomson and Emil Weichert that exposed thick-plate optical spark chambers to a vM beam led to the discovery of the electron, the cathode-ray tube from Brookhaven National Laboratory's Alternating Gra- was the primary apparatus.4 Working with the cathode- dient Synchrotron proton accelerator. The interaction of ray tube required understanding the late 19th century the v^'s with the spark chamber material led to muon technologies of gas discharges and vacuum pumps. In- production, but not electron production, thus showing that deed, one of Thomson's major contributions was his rec- the ve and vM are different and giving another demonstra- ognition that a good vacuum is required to produce an tion of lepton flavor conservation.8 electrostatic field inside the tube, a field that can deflect The next use of accelerators, and the last use in which the electrons making up the cathode ray.5 This accom- a new lepton was found, was my group's discovery of the plishment of Thomson resolved the long-standing puzzle tau using the electron-positron circular collider SPEAR.9 of why a cathode ray was deflected in a magnetic field But that was not the only new technology; we also used but appeared not to be deflected in an electric field. one of the first large-solid-angle particle detectors (figure The muon was discovered in cosmic rays by following 4). This detector, built by my colleagues from the Stanford the mystery of penetrating radiation—particles that pass Linear Accelerator Center and Lawrence Berkeley Na- through the atmosphere with little interaction compared tional Laboratory, consisted of a central magnetostrictive to the interactions expected from electrons or protons. wire spark chamber surrounded by sandwiches of plastic The penetrating radiation puzzle began in the 1920s. But scintillation counters and lead plates, these sandwiches it was not until the 1930s and early 1940s that the new allowing us to detect photons and identify electrons. Then technologies of triggered cloud chambers and coincidence came the coil producing an axial magnetic field, and circuits finally led to the identification of the muon and outermost layers of iron and spark chambers for muon its separation from the pion.6 identification. The discovery of the electron antineutrino by And so each lepton was discovered using a different experimental technology. Perhaps this was simply be- cause the discoveries Muon wire chambers stretched over 80 years. Or perhaps leptons are so elu- sive that a new technology is 4- Shower counters (24) required for each discovery. Coil Perhaps the next charged *~ Trigger counters (48) lepton, call it A, is so massive that it is beyond the energy reach of present or near-fu- ture e+ + e" —> L+ + Lr search technology. The mass need only be above several 1000 GeV/c2 to be out of reach. There are two methods, using different technologies, that could lead to the discov- ery of the A if—and this is a crucial if—the A already ex- ists in matter. Thus, the A would have been created in the early universe and would have to be sufficiently stable to still exist. One of the technologies involves using mass spec-
FIGURE 4. TAU DETECTION apparatus. This large-solid-angle detector, built by a team from the Stanford Linear Accelerator Center and Lawrence Berkeley National Laboratory, played a key role in the discovery of the tau.
38 OCTOBER 1997 PHYSICS TODAY 10 trometry to look for a very Drop generator heavy nucleus that contains + the A . In a typical search Falling drop just produced experiment, the researchers Image of falling drop look for very heavy hydrogen Falling drop being measured in water—that is, they look for A2O. Although the basic apparatus is a mass spec- trometer, the sample is some- times first enriched using techniques such as electroly- sis. So far, such experiments Lower electric plate - Falling drop already measured Lens CCD face of video camera have had no success. A 1993 Stroboscopic light source search covered A masses up to about 1500 GeV/c2 and set FIGURE 5. FRACTIONAL CHARGE detection scheme, showing apparatus being used to search a concentration upper limit for massive leptons or other massive particles possessing fractional electric charge. Small liquid of 10"16 A's per hydrogen drops, 7 jjim in diameter, fall through air. The air resistance and the small drop size cause the atom in deep seawater.11 drops to rapidly attain a terminal velocity proportional to the force on the drop. A vertical The other technology for electric field that periodically changes direction makes it possible to measure two terminal searching for very heavy lep- velocities, one for each electric field direction. The difference of the two terminal velocities is tons is part of the more gen- proportional to the charge on the drop. The sum of the two terminal velocities is eral search in bulk matter for proportional to the drop mass. free particles, not bound quarks, with fractional elec- tric charge.12 Such a particle might have a charge of 0.1 q and speculation that neutrinos may not obey perfect lepton or 2/3 q or Trq, where q is the magnitude of the electron flavor conservation.14 The broadest evidence comes from charge. My colleagues and I have been carrying out a measurements of the electron neutrino flux reaching Earth general search for particles with fractional electric charge from the Sun. In the nuclear fusion reactions that power using a highly automated version of the Millikan oil drop the Sun, ve's are produced in a variety of ways. For experiment.13 (See figure 5.) What does this have to do example, there are the pp fusion reaction with leptons? Well, why couldn't the next lepton have 2 + p + p -> H + e + ve + 0.42 MeV fractional electric charge? Such a lepton could have been produced in the early universe and would probably be and the boron decay 8 8 + stable. Past searches in bulk matter for free particles B -> Be + e + ve+ 14.6 MeV. with fractional electric charge show that such particles are rarer than one for every 1020 nucleons; but the chemi- The ve's that reach Earth are detected using weak-inter- cal processes used to produce the samples have raised action reactions of the ve. But only about two-thirds of questions as to the strictness of this limit. the expected ve flux is observed, the expected flux having been calculated from models of the Sun's properties and energy production. The hypothesis is that in the course Do neutrinos change their lepton flavors? of traveling from the energy-producing inner core of the There has been some evidence and a great deal of theory Sun to Earth, some ve's have changed into the other neutrinos, vM or vT. These vM's or vT's would not have enough energy to produce fx,'s or Table 2. Properties of the charged leptons T'S, nor would they participate in the re- and their associated neutrinos actions used to detect the expected ve's. The general hypothesis is that the Generation 2 1 3 change from, say, ve to vM would not be Charged lepton Electron Muon Tau permanent, but would be part of an oscil- name lation between ve and vM. The oscillation Charged lepton would depend on a mixing angle, 9, and symbol an oscillation rate. And the oscillation rate would depend upon the masses of the Charged lepton 0.51 106 1777 neutrinos! Thus, if neutrinos oscillated mass in units of 2 between lepton flavor, not only would this MeV/c be a wonderful new phenomenon in ele- Charged lepton Stable 2.2 x 10" 2.9 x 10-13 mentary particle physics, but in addition lifetime in neutrino masses could be indirectly meas- seconds ured. (See box 2 on page 40.)
Associated VT This section is headed with a question neutrino symbol because the case is not yet proven. The Upper limit on about 1.5 x 10 0.17 about 20 most important open question is whether the calculations of the Sun's properties neutrino mass 15 in MeV/c2 and the predicted ve flux are correct. There are also open questions about the The magnitude of electric charge of the charged leptons is 1.6 x 10 " coloumbs, the particles being consistency of the various measurements arbitrarily defined to have negative charge and the antiparticles to have positive charge. The neutrinos, of of the v flux. Powerful new experiments course, have zero electric charge. The generation number 1, 2, 3 gives the historical order of discoverry oof e the charged leptons; that is also the order of the masses of the charged leptons. The first generation in no will certainly clarify the experimental con- 2 known way is more fundamental than the second or third generation. The MeV/c mass uniutt equalequals sistency issues (see PHYSICS TODAY, July 1.78 x 10 kg. For additional information on lepton properties, see reference 2. 1996, page 30).
OCTOBER 1997 PHYSICS TODAY 39 Box 2: Neutrino Flavor Oscillations
The possibility of lepton flavor nonconservation among the Because v2 and v3 have the same momentum p but different neutrinos is based on the hypothesis that the vc, vM and vT are masses, E2 and £3 are different. Therefore, with time, v2 and combinations of three other neutrinos called v,, v2 and v3. v3 move out of phase with each other. This is a well-known Then, for example, quantum mechanical phenomenon, and it means that v^t) acquires a vT component! vT = C/rfV! + Ur2 V2 + UTi vy At time t, the probability of the vM having oscillated into a It is assumed that each of the neutrinos v1; v2 and v3 has a unique vT is 2 mass, so that ve, vM and vT are each mixtures of masses. To sin 20 ( E2-Ei illustrate how this leads to oscillations, consider the v -v )i T P(vK -> vT) = —;— 1 - cos —-— t system and suppose that the only nonzero coefficients U are Z 1 ft Uel and those connecting v^ and vT with v2 and v3. Then, using In the approximation that p is much larger than me, the mixing angle 6 and setting U^2 = cos 6, 2 2 3 E2-E} = (m2 -mi )c /2p. v = U vj e ei Thus, the second term in this equation gives an oscillation v^ = v cos 6 + v sin 6 between v and vT, and the oscillation frequency is proportional 2 3 2 to the difference of the squared masses, m2 - w3 . Therefore, vt = -v2 sin 6 + v3 cos 6. if experiments find vT's in a beam that was initially all vM's, we learn not only that the mixing angle 8 is nonzero, but also There are several ongoing and planned high-energy experiments something about the size of neutrino masses. Past experiments that begin with a v^ beam entering the apparatus. As the v^ have not found such a signal; 8 for vM - vT mixing is less than moves downstream with momentum p, its time evolution is about 3 x 10"2 for m 2 - w 2 greater than about 10 eV2/c4. This given by 2 3 means that v^ is mostly v2 and vT is mostly v3 if the mass-squared v V cos 2 4 fi(t) — 2 0 exp(—iE2t/fi) + v3 sin 8 exp(—iE-^t/h). differences are above 10 eV /c .
What is needed is a terrestrial experiment in which than the physicists whose work led to the discovery of the neutrino oscillations are detected directly. There have electron—Brookes, Goldsmith, Hertz, Kaufman, Lenard, been many attempts to do this. Except for one experi- Thomson, Weichert—but we are just as persistent, and ment, there has been no such detection of neutrino oscil- there are more of us. lations, and the results of that one experiment are yet to My work is supported by the Department of Energy. be confirmed. But particle physics experimenters are persistent. Experiments to detect neutrino oscillations References are going on now, and more are planned. 1. D. Griffiths, Introduction to Elementary Particles, Wiley, New York(1987). B. P. Roe, Particle Physics at the New Millennium, Practical uses for the other leptons Springer, New York (1996). There is no end to the practical uses of the electron. What 2. R. M. Barnett et al., Phys. Rev. D 54, 1 (1996). about the other leptons? Will they ever be used in the 3. M. L. Perl, in Les Prix Nobel, T. Frangsmyr, B. Lundeberg, technology of daily life or even for exotic engineering? eds., Almqvist & Wirksell, Stockholm (1996). There is no active use at present for the muon, the tau 4. A. Pais, Inward Bound, Clarendon Press, Oxford, England or the neutrinos, but there have been suggestions: (1986). \> The ability of muons to catalyze fusion was shown 5. J. J. Thomson, Philos. Mag. 44, 293 (1897). experimentally in 1957, and there has been substantial 6. CD. Anderson, H. L. Anderson, in The Birth of Particle Phys- experimental and theoretical research on muon catalysis ics, L. M. Brown, L. Hoddeson, eds., Cambridge University of fusion since then.16 In the simplest case, a fi~ forms a Press, Cambridge, England (1983). See also articles by G. molecule, p/ud with a proton p and a deuteron d. The p Bernardini, O. Piccioni and B. B. Rossi in same volume. 7. F. Reines, C. L. Cowan Jr, Phys. Rev. 92, 830 (1953). F. Reines, and d are sufficiently close so that a fusion reaction occurs: C. L. Cowan Jr, F. B. Harrison, A. D. McGuire, H. W. Kruse, 3He + ixr + 5.5 MeV Phys. Rev. 117, 159 (1960). -» 8. G. Danby, J. M. Gaillard, K. Goulianos, L. M. Lederman, N. We don't have muon catalyzed fusion reactors today, because Mistry, M. Schwartz, J. Steinberger, Phys. Rev. Lett. 9,36 (1962). with existing technology the energy required to produce the 9. M. L. Perl et al., Phys. Rev. Lett. 35, 1489 (1975). 10. P. F. Smith, J. R. J. Bennett, G. J. Homer, J. D. Lewin, H. E. muons is greater than the energy from the fusion. Walford, W. A. Smith, Nucl. Phys. B 206, 333 (1982). T. K I> Cosmic-ray muons were used once to look for hidden Hemmick et al., Phys. Rev. D 41, 2074 (1990). chambers in the pyramids, but no new chambers were 17 11. T. Yamagata, Y. Takamori, H. Utsunomiya, Phys. Rev. D 47, found. I have not heard of any other engineering use 1231 (1993). of the penetrating power of muons. 12. M. Marinelli, G. Morpurgo, Phys. Rep. 85, 161 (1982). P. F. D> There have been suggestions that muon neutrino beams Smith, Annu. Rev. Nucl. Part. Sci. 39, 73 (1989). could be used for geological research and prospecting deep 13. N. M. Mar et al., Phys. Rev. D 53, 6017 (1996). in the Earth.18 The rate of interaction of the neutrinos would 14. C. W. Kim, A. Pevsner, Neutrinos in Physics and Astrophysics, be proportional to the density of matter, with the interaction Harwood, Langhome, Pennsylvania (1993). F. Boehm, P. Vo- rate being measured by the muons so produced. Certainly gel, Physics of Massive Neutrinos, Cambridge University that would be a grand engineering project. Press, Cambridge, England (1987). \> As for the tau, to my knowledge, there has yet to be 15. V. Castellani, S. Degl'innocenti, G. Fiorentini, M. Lissia, B. Ricci, Phys. Rep. 281, 309 (1997). even one speculation as to its use. 16. C. Petitjean, Nucl. Phys. A 543, 79c (1992). 17. L. W. Alvarez et al., Science 167, 832 (1970). The next hundred years 18. A. De Rujula, S. L. Glashow, R. R. Wilson, G. Charpak, Phys. And so we enter the second hundred years of lepton research Rep. 99, 341 (1983). A. B. Borisov, B. A. Dolgoshein Phys. with mysteries and puzzles to solve. We are no smarter At. Nucl. 56,755 (1993). •
40 OCTOBER 1997 PHYSICS TODAY