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The Neutrino in 1980

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The Neutrino in 1980 I SCIENCE IDEAS Neutrino experiments planned at the Los Alamos Meson Physics Facility and at the Nevada Test Site may hold the key to many advances in particle physics and cosmology. by Thomas J. Bowles and Margaret L. Silbar ifty years ago scientists postu- suits of a recent and controversial ex- laws, which have been so successful in lated the existence of the neu- periment, which indicate that neutrinos describing the phenomenology of weak F trino, a neutral, massless, spin- oscillate or spontaneously change their interactions, will no longer apply to all ning particle traveling at the speed of “flavor” as they travel through “empty” physical processes. light, passing unimpeded through the space—almost as if the apple falling Experiments are now being designed whole earth and filling the universe in from Newton’s tree transformed itself at Los Alamos and elsewhere to search copious numbers. After numerous ex- into an orange before it struck him on further for evidence of a finite neutrino periments and reformulations, the prop- the head. If this result is proved correct, mass. erties and behavior of the neutrino still it will mean that the neutrino, long The Los Alamos Meson Physics Fa- challenge our theories about the forces assumed to be a massless particle, does cility (LAMPF) provides the best testing and symmetries of nature. have a nonzero rest mass. It will also ground for certain classes of ideas about The present puzzle concerns the re- mean that some of the conservation neutrino oscillation, for it produces neu- LOS ALAMOS SCIENCE 93 SCIENCE IDEAS trino fluxes of uniquely high intensity electrons have a wide spectrum of miens. He assumed that the neutrino, and special composition and time struc- energies, as shown in Fig. 1. Pauli sug- like the other fermions, has a particle ture. At the Nevada Test Site, the ex- gested that the missing energy was car- and an antiparticle form, but since the tremely high instantaneous neutrino flux- ried away by a highly penetrating (and neutrino has no charge, and perhaps no es available when nuclear devices are hence, undetected) particle with no magnetic moment, the distinction be- detonated provide other unequalled charge and little or no rest mass. He also tween neutrino and antineutrino was not capabilities to test concepts of weak noted that, to conserve angular momen- then known. interactions, tum and to preserve the “law of spin and However, the mathematical form of The Laboratory has played a central statistics,” this neutral particle must be a the interaction dictated that fermion role in the development of neutrino phys- fermion, a particle with an intrinsic spin number (the number of fermions minus ics, for it was a Los Alamos the number of antifermions) be con- team—Frederick Reines and the late In 1934, two years after the discovery served in the reaction. Since the neutron, Clyde Cowan, Jr.,—who first observed of the neutron, Enrico Fermi used the the proton, and the electron are particles, the neutrino in reactor experiments. This neutrino hypothesis to formulate a and by convention are assigned a fer- role will continue as experiments on theory of beta decay. This theory cor- mion number of + 1, the neutral, un- neutrino oscillation and neutrino masses rectly predicted the shape of the electron detected particle emitted in beta decay help test grand unified theories of fun- spectrum and provided the central ideas must be the antineutrino with a fermion damental particles. New data from these for what was to become the theory of all number of —1. Then the total fermion experiments may indicate the existence weak interactions. number is + 1 before and after the reac- of totally new interactions in nature. The basic process in nuclear beta tion. (As we will see, this and other They may even suggest that our expand- decay is the change of a neutron (n) into number-conservation laws have played ing universe will come to a halt and a proton (p) with the emission of an an important role in understanding and finally contract. predicting weak-interaction processes.) But the question of whether or not the Is the Neutrino Massless? neutrino has a mass remained open. That same year Hans Bethe and The possibility of a nonzero rest mass Fermi postulated that this process takes Rudolf Peierls pointed out that Fermi’s for the neutrino has been considered place through the direct interaction of theory allowed the neutrino to induce since 1930 when Wolfgang Pauli postu- these four particles, which are all fer- inverse beta decay: lated its existence to save the laws of energy and momentum conservation in beta decay of radioactive nuclei. Ori- ginally beta decay was thought to be a This reaction provided a means for veri- two-body process in which a nucleus, fying the existence of the neutrino by such as RaE, changes to a unique final observing the emitted neutron and state, in this case RaF, and emits an positron. However, the probability for electron (e-): inverse beta decay is so low (the cross section is about 10–44 cm2) that neutrino detection was not pursued until almost 20 years later when an intense source of Conservation of energy and momentum antineutrinos became available from fis- implies that all electrons in such a sion reactors. two-body decay reaction come out with Fig. 1. Typical energy spectrum of elec- In 1953. Frederick Reines and Clyde the same energy. Instead, the emitted trons emitted in beta decay. Cowan, Jr., reported results consistent 94 LOS ALAMOS SCIENCE SCIENCE IDEAS Fig. 2. Neutrinos can be detected by observing the events nucleus with a high neutron-capture cross section, such as associated with inverse beta decay, the interaction of an cadmium, to produce capture gamma rays. The positrons antineutrino with a proton to produce a neutron and a and/or the gamma rays are observed by their interactions with positron. The positron annihilates with an electron to produce a detecting medium, such as a liquid scintillator. two gamma rays. The neutron eventually interacts with a with the occurrence of inverse beta de- fermion that does not participate in the cay in an experiment at the Hanford reactor. Incontrovertible proof of the and v) is assigned a lepton number of+ 1 neutrinos existence came three years later when Reines and Cowan. together and the decay of the muon, lepton number of –1. With these as- with F. B. Harrison, Herald W. Kruse, signed numbers, the law of lep- and Austin D. McGuire, repeated the ton-number conservation says that total experiment at the Savannah River reac- lepton number remains constant in weak tor. Figure 2 shows how neu- Although the neutrinos were not ob- processes. This law, which emerges natu- trino-induced inverse beta decay can be served, their existence was assumed, this rally from the assumed mathematical observed. time to preserve lepton-number con- form of weak interactions, is consistent By this time other weak processes servation, a special case of fer- with all observed weak processes and were known, in particular the decay of mien-number conservation. A lepton is a explains the nonoccurrence of processes LOS ALAMOS SCIENCE 95 SCIENCE IDEAS that would violate it. Then, in 1957, the startling discovery that parity is not conserved in beta decay led to a major breakthrough in our conception of the neutrino and of weak interactions in general, At the suggestion of T. D. Lee and C. N. Yang, C. S. Wu and her collaborators meas- ured the direction of electrons emitted in the beta decay of polarized Co60. They found that 40% more electrons were emitted along the Co60 spin axis than opposite it. This near-maximum vio- lation of parity, or right-left symmetry, could be explained if the antineutrinos emitted in beta decay exist only in a longitudinally polarized form, that is. with spin vector pointing either along or opposite the direction of motion (right- or left-polarized, respectively), but not both. The property of longitudinal polariza- tion led to a new and very appealing theory of the neutrino. Unlike other known fermions, whose wave functions have four components corresponding to particle and antiparticle in both right- and left-polarized states, the neutrino has only two components: the neutrino is Fig. 3. In the standard two-component neutrino theory, the neutrino is a massless particle with intrinsic spin, or angular momentum, of 1/ h. The neutrino is always always left-polarized, or more precisely. 2 left-handed, and the antineutrino is left-handed, that is, tile direction of its spin vector is opposite that of its momentum; always right-handed.* The other two the antineutrino is always right-handed with the direction of its spin vector the same components (right-handed neutrino and as that of its momentum. This distinction between particle and antiparticle impossible left-handed antineutrino) are missing. only if the neutrino is traveling at the speed of light (and is therefore massless). Such a two-component theory in which Otherwise, transformation to a reference frame moving faster than the neutrino would neutrino and antineutrino are distin- reverse the momentum and cause the neutrino to appear to be an antineutrino. guished by their handedness implies that The massless two-component neutrino weak interactions. Second, the the neutrino travels at the speed of light theory had two important consequences left-handed character of the neutrino and is therefore a massless particle (see for weak interactions. First, since the helped to establish a universal Fig. 3). neutrino and its antiparticle were not the left-handed mathematical form for all weak-interaction processes, thus explain- *Right- or left-handed is not exactly the same as lepton number assignments of + 1 and ing why even massive particles emerging right- or left-polarized except for massless parti- cles, but for this discussion we will ignore the –1, respectively.
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