The creator of the neutrino is testing and teasing us. Moshe Gai The Oscillating Neutrino We do not know . [if] neutrinos are massive or massless. We do not know if the potentially massive neutrinos are Majorana or Dirac, and we do not know if these neutrinos can oscillate among flavours. An introduction to neutrino . In short, there is a great deal we do not know about neutrinos. Jeremy Bernstein, 1984. masses and mixings Richard Slansky, Stuart Raby, Terry Goldman, and Gerry Garvey as told to Necia Grant Cooper C Looks left-handed. No—right-handed?!* he neutrino, the theoretical construct The primer that follows explains why this of sixty years ago, has acquired a strange behavior would fit in with theoretical Tpresence in both physics and cosmology. expectations and how oscillations could reveal It is both actor and probe. It explains numerous neutrino masses no matter how small. It also mysteries of the observable world. Yet every new introduces questions that will become relevant. characteristic it reveals opens up more questions Why are neutrino masses so small? Do the very about its true nature. light neutrinos have very heavy relatives that For decades, these bits of matter have been make their masses small and give us hints of the described as massless, left-handed particles: new physics predicted by the Grand Unified left-handed because they were always “spinning” Theories? Are neutrinos their own antiparticles? counterclockwise in the manner of a left-handed Do neutrinos have very light sterile relatives corkscrew. But new evidence implies that neutrinos that provide a hiding place from all interactions? have very tiny masses and can spin in either Physicists continue to chase after neutrinos, direction. Remarkably, the new data also suggest and every time these ghostly particles are caught, that neutrinos might oscillate, or periodically they seem to point toward new challenges present themselves as one of several different types. and new possibilities. *This neutrino must have mass. 8 Los Alamos Science Number 25 1997 Number 25 1997 Los Alamos Science he Oscillating Neutrino The Oscillating Neutrino After Reines and Cowan detected Neutrinos in the strong decays are 10–16 second and First Family Second Family Third Family he neutrino in the late 1950s, particle Standard Model 10–23 second, respectively. hysics went through a spectacular It is precisely this lack of interaction Up u Charm c Top t flowering that culminated in the formu- The Standard Model identifies strength that makes the neutrino so Electric charge 5 12/3. Electric charge 5 12/3. Electric charge 5 12/3. ation of the Standard Model. This twelve building blocks of matter (see elusive. For not only does the weak Protons have two up quarks; Is heavier than the u. Is heavier than the c. model incorporates all that is known Figure 1), six quarks and six leptons force create neutrinos, often through neutrons have one. bout the subatomic world. It identifies (and their respective antiparticles). The beta decay, but it also mediates the Mass 3 MeV/c2. Mass < 1,500 MeV/c2. Mass 175,000 MeV/c2. he most elementary constituents of quarks are the building blocks that have only processes that can absorb them. < < matter, the elusive neutrino being fractional electric charge and interact The intimate connection between Down d Strange s Bottom b mong them, and then describes all primarily through the strong nuclear the weak force and the neutrino has Quarks he ways in which these elementary force, also called the color force. Color sometimes made their separate proper- Electric charge 5 21/3. Electric charge 5 21/3. Electric charge 5 21/3. Protons have one down quark; Is heavier than the d. Is heavier than the s. onstituents can interact with and binds quarks together to form the ties difficult to sort out. In fact, the neutrons have two. ansform among each other. Awesome proton, the neutron, all nuclei, and all theory that the neutrino is massless and n scope, this theory provides a consis- the other hadrons (strongly interacting left-handed (and the antineutrino right- Mass < 6 MeV/c2. Mass < 170 MeV/c2. Mass < 4,500 MeV/c2. ent picture of every realm of the particles). The charged leptons are the handed) was invented to explain why hysical world: from the hot, dense building blocks that interact through the weak force violates the symmetry arly universe resulting from the the other three forces of nature (weak, known as parity, also called right-left, Electron e Muon m Tau t ig Bang to the thermonuclear furnace electromagnetic, and gravity) but never or mirror, symmetry. If the weak force Electric charge 5 21. Electric charge 5 21. Electric charge 5 21. t the center of the Sun, from through the strong force. As a result, conserved parity, any weak process and Is responsible for electrical Is heavier than the e. Is heavier than the m. henomena at the smallest, subatomic leptons are never bound inside the its mirror image would be equally and chemical reactions. istance scales accessible at particle nucleus by the strong force. The leptons likely. Instead, in 1956, C. S. Wu and Mass 5 0.511 MeV/c2. Mass 5 105 MeV/c2. Mass 5 1,782 MeV/c2. ccelerators to those at the farthest include the electron, the heavier coworkers observed a striking asymme- eaches visible through the Hubble “electron-like” muon and tau, and these try in the beta decay of cobalt-60 (see n n Leptons Electron Neutrino ne Muon Neutrino m Tau Neutrino t elescope. The same forces and sym- three particles’ neutral partners: the box “Parity Nonconservation and metries and the same set of elementary the electron neutrino, the muon the Two-Component Neutrino” on page Electric charge 5 0. Electric charge 5 0. Electric charge 5 0. Is paired with electrons by the Is paired with muons by the Not yet seen directly. uilding blocks seem sufficient to neutrino, and the tau neutrino. Among 32.) The asymmetry suggested that all weak force. Billions fly weak force. Assumed to be paired with escribe the underlying physics of these twelve constituents, only the neu- the antineutrinos emitted in the decay through us every second. the tau by the weak force. ll phenomena observed so far. trinos are nearly or exactly massless. had right helicity,1 that is, they were Mass 5 0 (assumed). Mass 5 0 (assumed). Mass 5 0 (assumed). But for over two decades, ever Dubbed “the little neutral ones” “spinning” like right-handed corkscrews ince the Standard Model was initially because they have no electric charge, (rotating clockwise around their direc- ormulated, expectations of “physics neutrinos interact with matter only tion of motion). But in a universe with Anti-up u Anticharm c Antitop t w w w Antidown d Antistrange s Antibottom b eyond the Standard Model” have been through the weak force and gravity. right-left symmetry, an equal number w w w lmost palpable among those familiar Recall that the weak force creates of antineutrinos should have been with the model’s details: The theory neutrinos through beta decay (see the spinning counterclockwise, like left- Positron e1 Antimuon m1 Antitau t1 ust has far too many arbitrary parame- box “Beta Decay and the Missing Ener- handed corkscrews. The fact that only Antiparticles Electron antineutrino ne Muon antineutrino nm Tau antineutrino nt ers and mysterious relationships to be gy” on page 7). In that particular weak right helicity was observed is an w w w he final one. Now, after many years of decay process, a neutron, either free or example of “maximal” parity violation. earching, the first hard evidence for in a nucleus, transforms into a proton, ew physics may be at hand. The new and two leptons are created: an electron Figure 1. Building Blocks of Matter in the Standard Model hysics—nonzero neutrino masses and (or “beta” particle) and an electron 1Helicity is identical to handedness (or chirality) The elementary building blocks of lepton pair. Apart from the neutrinos, cosmic rays (mostly protons) with mixing” among the neutrinos from antineutrino. More generally, the weak for massless neutrinos and nearly identical to matter in the Standard Model are six which are massless, the particles in the molecules in the earth’s atmosphere. handedness for particles traveling near the speed ifferent families—has long been force is the force of transmutation, able of light. For that reason, helicity is sometimes quarks and six leptons, each carrying an second and third families are more The first hint that there are particles nticipated because it parallels the to transform one type, or “flavor,” of loosely referred to as “handedness.” For massive intrinsic spin of 1/2. The first family con- massive than their counterparts in the beyond the first family came in 1937 ehavior seen among quarks. But still, quark into another or one flavor of lep- particles, however, the two quantities are quite tains one quark pair—the up and first family. They are also unstable and with the discovery of the muon. The top different. Massive particles must exist in right- is quite exciting because it both ton into another. It is also the “weak- and left-helicity and in right- and left-handed the down—and one lepton pair—the only stick around for tiny fractions of a quark, the heaviest member of the third ffirms the central concepts of the est” known force (apart from gravity), states. As illustrated in the box on page 32 and electron and the electron neutrino. second because the weak force allows family, was not seen until 1995. And so tandard Model and appears to point about a hundred million times weaker the cartoon on page 29, helicity is the projection These four particles make up the them to decay into less massive far, the tau neutrino has not been of the spin along the direction of motion.