SUPER KAMIOKANDE DETECTOR resides in an active zinc mine inside Mount Ikenoyama. Its stainless-steel tank contains 50,000 tons of ultrapure water so transparent that light can pass through 70 meters of it before losing half its intensity (for a swimming pool that figure is a few meters). The water is monitored by 11,000 photomultiplier tubes that cover the walls, floor and ceiling. Each tube is a hand blown, evacuated glass bulb half a meter in diameter. The tubes register conical flashes of Cherenkov light, each of which signals a rare collision of a high-energy neutrino and an atomic nucleus in the water. Technicians in inflatable rafts clean the bulbs while the tank is filled [right). one man's trash is another man's treasure. For a physicist, the trash is background--some unwanted reaction, probably from a mundane and well-understood process. The treasure is signal--a reaction that we hope will reveal new knowledge about the way the universe works. Case in point: over the past two decades, several groups have been hunting for the radioactive decay of the proton, an exceedingly rare signal (if it occurs at all) buried in a background of reactions caused by elusive particles called neutrinos. The proton, one of the main constituents of the atom, seems to be immortal. Its decay would be a strong indication of processes described by the Grand Unified Theories that many believe lie beyond the extremely successful Standard Model of particle physics. Huge proton-decay detectors were placed deep underground, in mines or tunnels around the world, to escape the constant rain of particles called cosmic rays. But no matter how deep they went, these devices were still exposed to penetrating neutrinos produced by the cosmic rays.
The first generation of proton-decay detectors, operating from 1980 to 1995, saw no signal, no signs of proton decay--but along the way the researchers found that j the supposedly mundane CONES OF CHERENKOV LIGHT are emitted when high-energy neutrinos hit a nucleus and produce a charged particle. A muon-neutrino [top] creates a muon, which travels perhaps one meter and projects a sharp ring of light onto the detectors. An electron, produced by an electron-neutrino [bottom], generates a small shower of electrons and positrons, each with its own Cherenkov cone, resulting in a fuzzy ring of light. Green dots indicate light detected in the same narrow time interval. neutrino background was not so easy to understand.
One such experiment, Kamiokande, was located in Kamioka, Japan, a mining town about 250 kilometers (155 miles) from Tokyo (as the neutrino flies). Scientists there and at the 1MB experiment, located in a salt mine near Cleveland, Ohio, used sensitive detectors to peer into ultra-pure water, waiting for the telltale flash of a proton decaying.
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Such an event would have been hidden, like a needle in a small haystack, among about 1,000 similar flashes caused by neutrinos interacting with the water's atomic nuclei. Although no proton decay was seen, the analysis of those 1,000 reactions uncovered a real treasure--tantalizing evidence that the neutrinos were unexpectedly fickle, changing from one species to another in midflight. If true, that phenomenon was just as exciting and theory-bending as proton decay.
Neutrinos are amazing, ghostly particles. Every second, 60 billion of them, mostly from the sun, pass through each square centimeter of your body (and of everything else). But because they seldom interact with other particles, generally all 60 billion go through you without so much as nudging a single atom. In fact, you could send a beam of such neutrinos through a light-year-thick block of lead, and most of them would emerge unscathed at the end. A detector as large as Kamiokande catches only a tiny fraction of the neutrinos that pass through it every year.
Neutrinos come in three flavors, corresponding to their three charged partners in the Standard Model: the electron and its heavier relatives, the muon and the tau particle. An electron neutrino interacting with an atomic nucleus can produce an electron; a muon-neutrino makes a muon; a tau-neutrino, a tau. For most of the seven decades since neutrinos were first posited, physicists have assumed that they are massless. But if they can change from one flavor to another, quantum theory indicates that they most likely have mass. And in that case, these ethereal particles could collectively outweigh all the stars in the universe.
A Bigger Neutrino Trap
AS IS SO OFTEN the case in particle physics, the way to make progress is to build a bigger machine. Super-Kamiokande, or Super-K for short, took the basic design of Kamiokande and scaled it up by about a factor of 10 [see illustration on preceding pages]. An array of light-sensitive detectors looks in toward the center of 50,000 tons of water whose protons may decay or get struck by a neutrino. In either case, the reaction creates particles that are spotted by means of a flash of blue light known as Cherenkov light, discovered by Pavel A. Cherenkov in 1934. Much as an aircraft flying faster than the speed of sound produces a shock wave of sound, an electrically charged particle (such as an electron or a muon) emits Cherenkov light when it exceeds the speed of light in the medium in which it is moving. This motion does not violate Einstein's theory of relativity, for which the crucial velocity is c, the speed of light in a vacuum. In water, light propagates 25 percent slower than c, but other highly energetic particles can still travel almost as fast as c itself. Cherenkov light is emitted in a cone along the flight path of such particles.
In Super-K, the charged particle generally travels just a few meters and the Cherenkov cone projects a ring of light onto the wall of photon detectors [see il-THE AUTHORSEDWARD KEARNS, TAKAAKI KAJITA and YOJITOTSUKA are members of the Super-Kamiokande Collaboration. Kearns, professor of physics at Boston University, and Kajita, professor of physics at the University of Tokyo, lead the analysis team that studies proton decay and atmospheric neutrinos in the Super-Kamiokande data. Totsuka recently became the director of KEK, Japan's national particle physics laboratory, after serving as spokesperson for Super-K since its inception. lustration on opposite page]. The size, shape and intensity of this ring reveal the properties of the charged particle, which in turn tell us about the neutrino that produced it. We can distinguish the Cherenkov patterns of electrons from those of muons: the electrons generate a shower of particles, leading to a fuzzy ring quite unlike the crisper circle from a muon. From the Cherenkov light we also measure the energy and direction of the electron or muon, which are decent approximations of the energy and direction of the neutrino.
Super-K cannot easily identify the third type of neutrino, the tauneutrino. Such a neutrino can interact with a nucleus and make a tau particle only if it has enough energy. A muon is about 200 times as heavy as an electron; the tau, about 3,500 times. The muon mass is well within the range of atmospheric neutrinos, but only a tiny fraction are at tau energies, so most tau-neutrinos in the mix will pass through Super-K undetected.
One of the most basic questions experimenters ask is How many? We have built a beautiful detector to study neutrinos, and the first task is simply to count how many we see. The related question is How many did we expect? To answer that, we must analyze how the neutrinos are produced.
Super-K monitors atmospheric neutrinos, which are born in the spray of particles when a cosmic ray strikes the top of our atmosphere. The incoming projectiles (called primary cosmic rays) are mostly protons, with a sprinkling of heavier nuclei such as helium or iron. Each collision generates a shower of secondary particles, mostly pions and muons, which decay during their short flight through the air, creating neutrinos [see illustration at right]. We know roughly how many cosmic rays hit the atmosphere each second and roughly how many pions and muons are made in each collision, so we can predict how many neutrinos to expect.
Tricks with Ratios
UNFORTUNATELY, this estimate is only accurate to 25 percent, so we take advantage of a common trick: often the ratio of two quantities can be better determined than either quantity alone. For HIGH-ENERGY COSMIC RAY striking a nucleus in the atmosphere [below] generates a shower of particles, mostly pions. The pion's sequence of decays produces two muon-neutrinos for every electron-neutrino. Equal neutrino rates should be seen from opposite directions [above] because both result from cosmic rays hitting the atmosphere at the same zenith angle, 6. Both these ratios are spoiled when muon-neutrinos traveling long distances have time to change flavor. HOW QUANTUM WAVES MAKE A NEUTRINO OSCILLATE WHEN A PION DECAYS [topleft), it produces a neutrino. Described quantum-mechanically, the neutrino is apparently a superposition of two wave packets of different mass [purple and green). The wave packets propagate at different speeds, with the lighter wave packet getting ahead of the heavier one. As this proceeds, the waves interfere, and the interference pattern controls what flavor neutrino--muon (red) or tau [blue)--is most likely to be detected at any point along the flight path [bottom). Like all quantum effects, this is a game of chance, with the chances heavily favoring a muon-neutrino close to where it was produced. But the probabilities oscillate back and forth, favoring the tau-neutrino at just the right distance and returning to favor the muon-neutrino farther on. When the neutrino finally interacts in the detector [top right), the quantum dice are rolled. If the outcome is muon-neutrino, a muon is produced. If chance favors the tau-neutrino, and the neutrino does not have enough energy to create a tau particle, Super-K detects nothing. ----E.K., T.K. and Y.T. Super-K, the key is the sequential decay of a pion to a muon and a muon-neutrino, followed by the muon's decay to an electron, an electron-neutrino and another muon-neutrino. No matter how many cosmic rays are falling on the earth's atmosphere, or how many pions they produce, there should be about two muon-neutrinos for every electron-neutrino. The calculation is more complicated than that, but the final predicted ratio is accurate to 5 percent, providing a much better benchmark.
After counting neutrinos for almost two years, the Super-K team has found that the ratio of muon-neutrinos to electron-neutrinos is about 1.3 to 1 instead of the expected 2 to 1. Even if we stretch our assumptions about the flux of neutrinos, how they interact with the nuclei and how our detector responds, we cannot explain such a low ratio--unless neutrinos are changing from one type into another.
We can play the ratio trick again to test this surprising conclusion. The clue to our second ratio is to ask how many neutrinos should arrive from each possible direction. Primary cosmic rays fall on the earth's atmosphere almost equally from all directions, with only two effects spoiling the uniformity. First, the earth's magnetic field deflects some cosmic rays, especially the lowenergy ones, skewing the pattern of arrival directions. Second, cosmic rays that skim the earth at a tangent make neutrino showers that do not descend deep into the atmosphere, and these can develop differently from those that plunge straight in from above.
But geometry saves us: if we look up into the sky at some angle from the vertical and then down into the ground at the same angle, we should see the same number of neutrinos coming from each direction. Both sets of neutrinos are produced by cosmic rays hitting the atmosphere at the same angle; it is just that in one case the collisions happen overhead and in the other they are partway around the world. To use this fact, we select neutrino events of sufficiently high energy (so their parent cosmic ray was not deflected by the earth's magnetic field) and then divide the number of neu- Wolfgang Pauli rescues conservation of energy by hypothesizing an unseen particle that takes away energy missing from some radioactive decays. Enrico Fermi formulates the theory of beta-decay incorporating Pauli's particle, now called the neutrino (little neutral one in Italian). Frederick Reines (center) and Clyde Cowen first detect the neutrino using the Savannah River nuclear reactor. At Brookhaven, the first accelerator beam of neutrinos proves the distinction between electron-neutrinos and muon-neutrinos. Raymond Davis, Jr., first measures neutrinos from the sun, using 600 tons of cleaning fluid in a mine in Homestake, S.D. trinos going up by the number going down. This ratio should be exactly 1 if none of the neutrinos are changing flavor.
We saw equal numbers of high-energy electron-neutrinos going up and going down, as expected, but only half as many upward muon-neutrinos as downward ones. This finding is the second indication that neutrinos are changing identity. Moreover, it provides a clue to the metamorphosis. The upward muon-neutrinos cannot be turning into electron-neutrinos, because there is no excess of upward electron-neutrinos. That leaves the tau-neutrino. The muon-neutrinos that become tau-neutrinos pass through Super-K without interaction, without detection.
Fickle Flavor
THE ABOVE TWO RATIOS are good evidence that muon-neutrinos are transforming into tau-neutrinos, but why should neutrinos switch flavor at all? Quantum physics describes a particle moving through space by a wave: in addition to properties such as mass and charge, the particle has a wavelength, can diffract, and so on. Furthermore, a particle can be the superposition of two waves. Now suppose that the two waves correspond to slightly different masses. Then, as the waves travel along, the lighter wave gets ahead of the heavier one, and the waves interfere in a way that fluctuates along the particle's trajectory [see box on opposite page]. This interference has a musical analogue: the beats that occur when two notes are almost but not exactly the same. NUMBER OF HIGH-ENERGY MUON-NEUTRINOS seen arriving on different trajectories at Super-K clearly matches a prediction incorporating neutrino oscillations [green] and does not match the no-oscillation prediction [blue]. Upward-going neutrinos [plotted toward left of graph] have traveled far enough for half of them to change flavor and escape detection.
In music this effect makes the volume oscillate; in quantum physics what oscillates is the probability of detecting one type of neutrino or another. At the outset the neutrino appears as a muon-neutrino with a probability of 100 percent. After traveling a certain distance, it looks like a tau-neutrino with 100 percent probability. At other positions, it could be either a muon-neutrino or a tauneutrino.
This oscillation sounds like bizarre behavior for a particle, but another familiar particle performs similar contortions: the photon, the particle of light. Light can occur in a variety of polarizations, including vertical, horizontal, left circular and right circular. These do not have different masses (all photons are massless), but in certain optically active materials, light with left circular polarization moves faster than right circular light. A photon with vertical polarization is actually a superposition of these two alternatives, and when it is traversing an optically active material its polarization will rotate (that is, oscillate) from vertical to horizontal and so on, as its two circular components go in and out of sync.
For neutrino oscillations of the type we see at Super-K, no optically active material is needed; a sufficient mass difference between the two neutrino components will cause flavor oscillations whether the neutrino is passing through air, solid rock or pure vacuum. When a neutrino arrives at Super-K, the amount it has oscillated depends on its energy and the distance it has traveled. For downward muon-neutrinos, which have traveled at most a few dozen kilometers, only a small fraction of an oscillation cycle has Neutrino astronomy: the 1MB and Kamiokande proton-decay experiments detect 19 neutrinos from Supernova 1987A in the Large Magellanic Cloud. The Z decay rate is precisely measured at SLAC and CERN, showing there are only three active neutrino generations. Super-K assembles evidence of neutrino oscillation using atmospheric neutrinos. Tau-neutrino events The solar neutrino puzzle is solved, are detected at Fermilab, with key evidence from the completing the distinction Sudbury Neutrino Observatory of neutrinos by flavor experiment in Ontario, begun in 1962 The solar neutrino puzzle is solved, with key evidence from the Sudbury Neutrino Observatory experiment in Ontario. Neutrinos could account for a mass nearly equal to that of all the stars combined. taken place, so the neutrinos' flavor is slightly shifted, and we are nearly certain to detect their original muon-neutrino flavor. The upward muon-neutrinos, produced thousands of kilometers away, have gone through so many oscillations that on average only half of them can be detected as muon-neutrinos. The other half pass through Super-K as undetectable tau-neutrinos.
This description is just a rough picture, but the arguments based on the ratio of flavors and the updown event rate are so compelling that neutrino oscillation is now widely accepted as the most likely explanation for our data. We have also done more detailed studies of how the number of muon-neutrinos varies according to the neutrino energy and the arrival angle. We compare the measured number against what is expected for a wide array of possible oscillation scenarios (including no oscillations). The data look quite unlike the no-oscillation expectation but match well with neutrino oscillation for certain values of the mass difference and other physical parameters [see illustration on preceding page].
With about 5,000 events from our first two years of experimentation, we were able to eliminate any speculation that the anomalous numbers of atmospheric neutrinos could be just a statistical fluke. But it is still important to confirm the effect by looking for the same muon-neutrino oscillation with other experiments or techniques. Detectors in Minnesota and Italy have provided some verification, but because they have measured fewer events they do not offer the same certainty.
Corroborating Evidence
FURTHER SUPPORT comes from studies of a different atmospheric neutrino interaction: collisions with nuclei in the rock around our detector. Electron-neutrinos again produce electrons and subsequent showers of particles, but these are absorbed in the rock and never reach Su-per-K's cavern. High-energy muonneu trinos make high-energy muons, which can travel through many meters of rock and enter our detector. We count such muons from upward-traveling neutrinos--downward muons are masked by the background of cosmic-ray muons that penetrate Mount Ikenoyama.
We can count upward-traveling muons arriving on trajectories that range from directly up to nearly horizontal. These paths correspond to neutrino travel distances (from production in the atmosphere to the creation of a muon near Super-K) as short as 500 kilometers (the distance to the edge of the atmosphere when looking horizontally) and as long as 13,000 kilometers (the diameter of the earth, looking straight down). We find that the numbers of muon-neutrinos of lower energy that travel a long distance are more depleted than higher-energy muon-neutrinos that travel a short distance. This behavior is exactly what we expect from oscillations, and careful analysis produces neutrino parameters similar to those from our first study.
If we consider just the three known neutrinos, our data tell us that muon-neutrinos are changing into tau-neutrinos. Quantum theory says that the underlying cause of the oscillation is almost certainly that these neutrinos have mass--although it has been assumed for 70 years that they do not. (The box on the opposite page mentions some other scenarios.)
Unfortunately, quantum theory also limits our experiment to measuring only the difference in mass-squared between the two neutrino components, because that is what determines the oscillation wavelength. It is not sensitive to the mass of either one alone. Super-K's data give a mass-squared difference somewhere between 0.001 and 0.01 electron volt (eV) squared. Given the pattern of masses of other known particles, it is likely that one neutrino is much lighter than the other, which would mean that the mass of the heavier neutrino is in the range of 0.03 to 0.1 eV. What are the implications?
First, giving neutrinos a mass does not wreck the Standard Model. The mismatch between the mass states that make up each neutrino requires the introduction of a set of so-called mixing parameters. A small amount of such mixing has long been observed among quarks, but our data imply that neutrinos need a much greater degree of mixing--an important piece of information that any successful new theory must accommodate.
Second, 0.05 eV is still very close to LONG-BASELINE neutrino oscillation experiments are planned in Japan and the U.S. Beams of neutrinos from accelerators will be detected hundreds of kilometers away. The experiments should confirm the oscillation phenomenon and precisely measure the constants of nature that control it. OTHER PUZZLES, OTHER POSSIBILITIES PARTICLE PHYSICISTS have been busy sorting out other indications of neutrino mass. For more than 30 years, scientists have been capturing electron-neutrinos generated by nuclear fusion processes in the sun. These experiments have always counted fewer neutrinos than the best models predict. Super-K has also counted solar neutrinos and finds only about 50 percent of the number expected. If solar neutrinos are changing flavor, this deficit is understandable, because at solar energies Super-K responds to the electron flavor and mostly ignores those transformed into the muon or tau flavors. The Sudbury Neutrino Observatory [SNO) in Ontario, however, which uses 1,000 tons of heavy water, has recently achieved a breakthrough in proving this change. The heavy water allows SNO to measure the total number of neutrinos (electron, muon and tau) as well as the number of electron-neutrinos alone, and it shows that the total is much greater. The accounting seems to balance. It appears that the mass splitting associated with solar neutrinos is much smaller than that for atmospheric neutrinos. This fits a picture in which the three flavors of neutrinos are spread over three distinct neutrino masses. But the picture doesn't allow for the hint of neutrino oscillation, suggesting much larger masses, detected at Los Alamos National Laboratory. Some exotic explanations are waiting in the wings while Fermilab checks this signature. Physicists are also checking the theory that transforms solar neutrinos. In a cavern in the same zinc mine as Super-K, a detector has been built that uses 1,000 tons of mineral oil doped with a chemical that emits light in response to the neutrino reaction. This detector counts electron-neutrinos from more than two dozen Japanese nuclear power reactors, from 80 to 400 kilometers away. The results are being compared with a precise model of how many neutrinos are expected from each reactor. This experiment should pin down the detailed particle physics revealed by solar neutrinos. Overall, our picture of neutrinos is just coming into focus. More clarity will rely on more ambitious projects. Later in this decade Super-K will be exposed to a beam of neutrinos from a much more intense accelerator being built near Japan's Pacific coast. The goal is to verify that muon-neutrinos change flavor to tau- and electron-neutrinos in a proportion that fits our new found expectations. Follow-up measurements may reveal the role of neutrinos in the matter-antimatter imbalance of the universe. Or we may be presented with new puzzles to solve. --E.K., T.K. and Y.T. zero, compared with other particles. (The lightest of those is the electron, with a mass of 511,000 eV. ) So the long-held belief that neutrinos have zero mass is understandable. But theoreticians who wish to build a Grand Unified Theory, which would elegantly combine all the forces except gravity at enormously high energies, also take note of this relative lightness of neutrinos. They often employ a mathematical device called the seesaw mechanism, which actually predicts that such a small but nonzero neutrino mass is natural. Here the mass of some extremely heavy particle, perhaps at the Grand Unified mass scale, provides the leverage to separate the very light neutrinos from the quarks and leptons that are a billion to a trillion times heavier.
Another implication is that the neutrino should be considered in the bookkeeping of the mass of the universe. For some time, astronomers have been trying to tabulate how much mass is found in luminous matter, such as stars, and in ordinary matter that is difficult to see, such as brown dwarfs or diffuse gas. The mass can also be measured indirectly from the orbital motion of galaxies and the rate of expansion of the universe. The direct accounting falls short of these indirect measures by a factor of 20. The neutrino mass suggested by our result is too small to resolve this mystery by itself. Nevertheless, neutrinos created during the big bang permeate space and could account for a mass nearly equal to the combined mass of all the stars. They could have affected the formation of large astronomical structures, such as galaxy clusters.
Finally, our data have an immediate implication for two new experiments. Based on the earlier hints from smaller detectors, many physicists have decided to stop relying on the free but uncontrollable neutrinos from cosmic rays and instead are creating them with high-energy accelerators. Even so, the neutrinos must travel a long distance for the oscillation effect to be observed. So the neutrino beams are aimed at a detector hundreds of kilometers away. One detector, MINOS, is being built in a mine in Soudan, Minn., to study neutrinos sent from the Fermilab accelerator near Batavia, 111., 730 kilometers away on the outskirts of Chicago.
Of course, a good atmospheric neutrino detector is also a good accelerator neutrino detector, so in Japan we are using Super-K to monitor a beam of neutrinos created at the KEK accelerator laboratory 250 kilometers away. Unlike atmospheric neutrinos, the beam can be turned on and off and has a well-defined energy and direction. Most important, we have placed a detector similar to Super-IC near the origin of the beam to characterize the muon-neutrinos before they oscillate. We are essentially using the ratio (again) of the counts near the source to those far away to cancel uncertainty and verify the effect. Since 1999, neutrinos from the first long-distance artificial neutrino beam have passed under the mountains of Japan, with 50,000 tons of Super-IC capturing a small handful. Exactly how many are being captured will be the next chapter in this story.
SA MORE TO EXPLORE The Search for Proton Decay. J. M. LoSecco, Frederick Reines and Daniel Sinclair in Scientific American, Vol. 252, No. 6, pages 54-62; June 1985. The Elusive Neutrino: A Subatomic Detective Story. Nickolas Solomey. Scientific American Library, W. H. Freeman and Company, 1997. Official Super-Kamiokande Web site: www-sk.icrr.utokyo.ac.jpdocsk K2K Long Baseline Neutrino Oscillation Experiment Web site: neutrino.kek.jp Super-Kamiokande at Boston University Web site: hep.bu.edu~superk
