Viewpoint: Yes, the Japanese-U.S. research team called the Super-Kamiokande Collaboration announced experiment results in 1998 that proved that neutrinos do indeed have mass.
Viewpoint: No, the experiments of earlier twentieth-century scientists repeatedly indicated that neutrinos did not have mass.
In 1931 Wolfgang Pauli first predicted the existence of a subatomic particle that Enrico Fermi would later name the neutrino. The neutrino must exist, Pauli reasoned, because otherwise the atomic process known as beta decay would violate the physical laws of conservation of energy and conservation of angular momentum. Neutrinos had never been detected, so Pauli concluded that they didn't interact with most other particles or forces. This implied they were extremely small particles with no charge or mass.
The only force that noticeably affects neutrinos is the "weak" force, a subatomic force that is not as strong as the force that holds the atomic nucleus together, but that likewise operates only at very short range. Because of their extremely limited interactions with other particles and forces, neutrinos can travel huge distances unimpeded by anything in their path. Neutrinos arising from the nuclear reactions in the Sun stream through Earth and all its inhabitants without having any effect whatsoever. For this reason they are extremely difficult to detect, and their existence was not confirmed until a quarter century after Pauli's prediction.
Today's neutrino detectors, kept deep underground to avoid stray particles on Earth's surface, may contain thousands of tons of fluid. While trillions of neutrinos pass through the fluid every day, only a few dozen are likely to be detected.
Scientists have discovered that there are three types of neutrinos, each associated with a different charged particle for which it is named. Thus they are called the electron neutrino, muon neutrino, and tau neutrino. The first type of neutrino to be discovered was the electron neutrino, in 1959. The muon neutrino was discovered in 1962. The tau neutrino has yet to be directly observed. It was inferred from the existence of the tau particle itself, which was discovered in 1978. The tau particle is involved in decay reactions with the same imbalance that Pauli solved for beta decay by postulating the electron neutrino.
One ongoing issue in neutrino research is called the "solar neutrino problem." This refers to the detection of fewer electron neutrinos than expected, given the known energy output of the Sun. One possible explanation for this phenomenon could be "oscillation" between the different neutrino types. That is, electron neutrinos could change into muon or tau neutrinos, which are even more difficult to detect. Similarly, scientists have observed a deficit in the number of muon neutrinos they would expect to see coming from cosmic rays.
Neutrino oscillations, if they exist, are a quantum mechanical phenomenon dependent on the difference in the masses of the two types of particles. That means that if researchers could prove that neutrino oscillation occurs, at least one of the neutrino types involved must have a non-zero mass. In 1998 a Japanese-U.S. research team called the Super-Kamiokande Collaboration announced that they had discovered evidence of oscillation between muon neutrinos and either the tau neutrino or a new, unknown type.
—SHERRI CHASIN CALVO
Wolfgang Pauli inferred the existence of the neutrino in 1930 from the discovery that a small amount of energy was missing in the decay products in certain types of radioactivity (beta decay). In this type of decay, a neutron in an atomic nucleus is converted into a proton, with emission of an electron and an antineutrino, a neutral particle that could not be detected directly. Pauli also suspected that the mass of this particle would be zero because it could not be detected in the decay experiments. The mass difference between the initial and final nuclei produced during this decay should correspond to the highest energy of the emitted particles. Any difference would then correspond to the mass of the neutrino itself, according to Einstein's mass-energy relationship. This energy difference, however, proved too small to be measured.
Neutrinos escaped direct detection until 1956 when Frederick Reines and Clyde Cowan Jr. captured neutrinos produced in a nuclear reactor. In subsequent experiments, the speed of neutrinos could not be distinguished from that of light. Also, there was no spread in the arrival time of neutrinos produced by the supernova SN observed in 1987. According to relativity, particles such as photons that travel at the speed of light have zero masses. This reinforced the idea that neutrinos might have zero mass.
The first hint that something might be wrong with this picture came in 1967 when scientists tried to detect neutrinos created by nuclear reactions in the sun's core. In this and subsequent experiments, scientists consistently observed a shortfall of about half of the neutrinos that were predicted by theory.
However, already in 1957, long before the detection of solar neutrinos proved to be a problem, the Italian physicist Bruno Pontecorvo, then working in the Soviet Union, proposed that neutrinos could change continuously from one type into another. Now we know that three types, or "flavors," of neutrinos exist: tau, muon, and electron neutrinos, named after the particles they are associated with during their creation. The continual change from one type into another type of neutrino is called "oscillation." This phenomenon is a quantum mechanical effect (superposition of several quantum states) and is also observed in other elementary particles, such as K0 mesons. For example, if we detect a muon neutrino, it is possible that it just oscillated from being an electron neutrino before arriving at the detector. But according to quantum mechanics, oscillations are only permitted if neutrinos have mass.
The Standard Model of particle physics, a global theory that describes elementary particles and their interactions, developed since the 1970s, predicted neutrinos with zero mass. Therefore, at first the fact that neutrinos may have mass was viewed as a possible crack in the Standard Model. But theorists are now extending the Standard Model so it can incorporate neutrino masses.
The neutrino deficit shown by the solar neutrino experiments was the first indication that the Standard Model did not describe these particles accurately: some changed flavor and thus escaped detection. Another so-called neutrino anomaly became apparent around the mid-1980s. Both the Kamiokande detector in the Japanese Alps and the Irvine-Michigan-Brookhaven (IMB) detector in the Morton salt mine in Ohio started observing neutrinos produced by cosmic-ray particles colliding with atomic nuclei in the earth's atmosphere. According to theory, these interactions should produce twice as many muon neutrinos as electron neutrinos, but the detectors discovered roughly equal amounts. Scientists explained this discrepancy by the possibility that muon neutrinos change into either electron or tau neutrinos before reaching the detectors.
The successor to the Kamiokande is the Super-Kamiokande, the world's largest neutrino detector. It consists of a huge tank containing 50,000 tons of purified water, and it is protected from cosmic rays by a 1,000-m (c. 1,094 yd) rock layer. Eleven thousand photomultiplier tubes line the tank and track the light flashes (Cerenkov radiation) caused by muons or electrons produced by neutrinos interacting with nuclei that travel through the water at velocities higher than the velocity of light in water.
The photodetectors also allow the tracking of the direction of the incoming neutrinos. In 1998 Super-Kamiokande results showed that about half of the muon neutrinos produced in the earth's atmosphere that had traveled through the earth over a distance of 13,000 km (c. 8,078 mi) had disappeared, as compared to those produced in the atmosphere just overhead of the detector.
One of the weaker points in the neutrino experiments with solar or atmospheric neutrinos is the impossibility of controlling the neutrino source: there was always the possibility that theory simply predicted wrong neutrino fluxes. Therefore researchers tried to use neutrinos created in nuclear reactors or particle accelerators. At CERN, the European Laboratory for Nuclear Physics, a proton beam from the Super Proton Synchrotron (SPS) was aimed at a beryllium target for the production of muon neutrinos.
These muon neutrinos were aimed at two detectors, NOMAD and CHORUS, placed at a 900-m (c. 984 yd) distance from the neutrino source that tried to pick up tau neutrinos that would have been formed by the muon to tau oscillation. This type of search is called an "appearance" search, and an appearance of a different type of neutrino than those that created in the accelerator would be a much stronger proof that neutrinos oscillate than a "disappearance" experiment. But data taken with both detectors have not revealed a measurable neutrino oscillation. Several similar experiments, using nuclear reactors for neutrino sources, such as CHOOZ in the French Ardennes, also could not confirm neutrino oscillations.
In 1996 researchers using the Liquid Scintillating Neutrino Detector (LSND) at Los Alamos announced the detection of electron antineutrinos produced by oscillating muon antineutrinos. The muon antineutrinos were produced by a proton beam hitting a target in an accelerator. However, some researchers doubt the results because the obtained oscillation rates differ from oscillation rates measured with other experiments. Others believe that a fourth kind of neutrino, a so-called sterile neutrino that only interacts very little with matter, may explain the discrepancy in results.
A new generation of long baseline experiments will allow physicists to pin down neutrino oscillations much more accurately. Because the neutrinos travel over much longer distances, it is more likely they would undergo oscillations. A first successful experiment, dubbed K2K, was announced in Japan in July 2000. Neutrinos produced in an accelerator at the Japanese National Accelerator Facility (KEK) in Tsukuba were aimed at the Super-Kamiokande. The neutrinos traveled over a distance of 250 km (155 mi). During an experimental run that lasted from June 1999 to June 2000, the Super-Kamiokande only detected 27 muon neutrinos from the 40 muon neutrinos it would have detected if neutrinos would not oscillate. In July 2001 K2K researchers announced that a total of 44 muon neutrinos have been detected from 64 that should have arrived if neutrinos did not oscillate.
The U.S. Department of Energy's Fermilab near Chicago is now planning to set up a long baseline terrestrial experiment called MINOS (Main Injector Neutrino Oscillation). Muon neutrinos, produced by a proton accelerator at Fermilab, will be aimed at a detector placed 800 m (c. 875 yd) underground in the Soudan iron mine in Minnesota at a distance of 730 km (438 mi) from Fermilab. Neutrinos will be detected by electronic particle detectors placed in between stacks of steel plates. The energy range of the neutrinos will be similar to the atmospheric neutrinos detected by Super-Kamiokande, and therefore MINOS would be an independent check on the Super-Kamiokande results.
In Europe, a long baseline experiment is planned for 2005, when CERN will aim pulses of neutrinos at the Gran Sasso National Laboratory, the world's largest underground laboratory excavated deep under a mountain in the Appenines in central Italy. At CERN, a proton beam from the SPS (Super Proton Synchrotron) will hit a graphite target and produce muon neutrinos. While traveling over a distance of 730 km (438 mi) to Gran Sasso, some of the muon neutrinos will change into tau neutrinos. Two detectors, OPERA and ICANOE, will look for these tau neutrinos. Unlike the other long baseline experiments, which look for the disappearance of certain types of neutrinos, the CERN-Gran Sasso experiment will look for the appearance of neutrinos that have undergone oscillations, and therefore the results will be much more reliable.
Longer baselines will also allow a higher accuracy, and scientists are already studying the possibilities of building "neutrino factories," muon storage rings that would produce very intense beams of neutrinos that could be aimed at detectors several thousands of kilometers away.
From stone circles arranged by the ancient Celts to space-based telescopes, astronomers have always kept their eyes on the skies—unless they are looking for neutrinos, not galaxies or planets, but subatomic particles that have had scientists pounding the theoretical pavement since 1931.
It all started around 1900 when a series of experiments and some good luck showed physicists that the atom was not a featureless ball. Rather, it had an internal structure. The atomic age was born. It was radiation that got their attention—atoms of elements like uranium emitted some kind of ray.
By the 1920s they knew the atom had a core, which they named a nucleus, with electrons moving around it. Inside the nucleus were protons and neutrons. Some of the nuclear radiation turned out to be beta particles (speeding electrons) that the nucleus emitted when heavier elements decayed into lighter ones. But atoms that emitted beta particles did so with less energy than expected. Somehow energy was being destroyed, and it was starting to look like physicists might have to abandon the law of conservation of energy.
Then, in 1931, physicist Wolfgang Pauli suggested that when an atom emits a beta particle it also emits another small particle—one without a charge and maybe without mass—that carries off the missing energy.
In 1932 physicist Enrico Fermi developed a comprehensive theory of radioactive decays that included Pauli's hypothetical particle. Fermi called it a neutrino, Italian for "little neutral one." With the neutrino, Fermi's theory explained a number of experimentally observed results. But it took another 25 years or so to prove neutrinos existed.
American physicists Frederick Reines and Clyde Cowan Jr. conducted an elaborate experiment in 1956 at the Savannah River nuclear reactor. They set up a detection system that focused on one reaction a neutrino might cause and detected the resulting gamma rays produced at just the right energies and time intervals. In 1959 they announced their results, and they later shared the 1995 Nobel Prize in physics for their contribution to the discovery. This neutrino was later determined to be an electron neutrino. Their findings confirmed Pauli's theory, but that was not the end of questions about neutrinos.
In 1961 physical chemist Ray Davis and theoretical physicist John Bahcall started wondering if there was a direct way to test the theory of how stars shine. They wanted to find a way to observe neutrinos that were supposed to be produced deep inside the Sun as hydrogen burned to helium. They knew it was a long shot, because anything that could escape from the center of the Sun would be very hard to detect with a reasonable-sized experiment on Earth.
The next year, in 1962, experiments at Brookhaven National Laboratory and CERN, the European Laboratory for Nuclear Physics, discovered that neutrinos produced in association with particles called muons did not behave like those produced in association with electrons. They had discovered a second neutrino flavor, the muon neutrino.
Two years later, in 1964, Bahcall and Davis proposed the feasibility of measuring neutrinos from the Sun, and the next year—in a gold mine in South Africa—Reines and colleagues observed the first natural neutrinos, and so did researchers Menon and colleagues in India.
In 1968 Ray Davis and colleagues started the first radiochemical solar neutrino experiment using 100,000 gallons of cleaning fluid—carbon tetrachloride—a mile underground in the Homestake gold mine in South Dakota. The chlorine in the cleaning fluid absorbed neutrinos and changed chlorine atoms into detectable radioactive argon. But the Homestake experiment captured two or three times fewer solar neutrino interactions than Bahcall calculated on the basis of standard particle and solar physics. Physicists called it the solar neutrino problem.
Several theorists suggested the missing electron neutrinos had oscillated—turned into another kind of neutrino that the Homestake experiment could not detect—but most physicists thought it was more likely that the solar model used to calculate the expected number of neutrinos was flawed. Besides, neutrino oscillation would be possible only if neutrinos had mass, and current theories said neutrinos had no mass.
But the discrepancy between standard calculation and experimental detection was telling physicists about something new in particle physics—that something unexpected happened to neutrinos after they were created deep inside the Sun.
In 1985 a Russian team reported measuring neutrino mass—10,000 times lighter than an electron's mass—but no one else managed to reproduce the measurement.
The Kamiokande detector went online in 1986 in the Kamioka Mozumi mine, 186 miles northwest of Tokyo in the Japanese Alps. For 1,200 years the mine had given up silver, then lead and zinc. Now, down among its 620 miles of tunnels, was a plastic tank, bigger than an Olympic swimming pool, full of ultraclean water and thousands of photomultiplier tubes that detected tiny flashes of light in the dark mine water—spontaneous proton decay. When a tube detected a flash it sent an electronic signal to a computer that recorded time and location.
There were no flashes at Kamiokande until February 23, 1987, at 7:35 A . M ., Greenwich mean time. That morning, the Kamioka detector recorded 11 events, each made up of 30 photo-multiplier flashes in a certain time sequence and pattern. Something had entered the detector and interacted. It all took 10 seconds.
When a massive star explodes as a supernova, the blast unleashes a thousand times more neutrinos than the Sun will produce in its 10-billion-year lifetime. More than 20 years earlier, theoretical astrophysicists said supernovas should release huge numbers of neutrinos. They had come from the core of the exploding star, escaping into space. Hours or days later, a shock wave from the main explosion would reach the surface, producing a blast of light as the star blew apart. And that is what happened at Kamiokande.
Several hours after the neutrinos hit the Kamioka mine, Supernova 1987A became the first exploding star in 384 years to be seen by the naked eye. Two years later, in 1989, Kamiokande became the second experiment to detect neutrinos from the Sun and confirmed the long-standing solar neutrino problem—finding about a third of the expected neutrinos.
For years there were experimental hints for neutrino oscillation, mainly from the smaller than expected number of solar electron neutrinos. Other experiments hinted at oscillations by muon neutrinos produced in the upper atmosphere, in a decay chain that yielded two muon neutrinos for every electron neutrino.
But early experiments at Kamiokande and at the Irvine-Michigan-Brookhaven detector near Cleveland suggested the muon-to-electron-neutrino ratio was one, not two. If that was true, half the muon neutrinos were missing. Physicists needed proof to show the cause was neutrino oscillation. To answer that question, in 1996 another detector went online at the Kamioka mine. Super-Kamiokande was a $130 million neutrino detector built to find out whether neutrinos had mass.
This detector is a big tank of clean water 1 km (0.6 mi) underground—a 50,000-ton cylinder, 132 ft around and high, with 11,146 photomultiplier tubes lining the tank. The tubes are each sensitive to illumination by a single photon of light—a level about equal to the light visible on Earth from a candle at the distance of the Moon.
Any charged particle moving near the speed of light in water produces a blue Cerenkov light, sort of a cone-shaped optical shock wave. When an incoming neutrino collides with an electron, blue light hits the detector wall as a ring of light. A photomultiplier tube sees the light and amplifies it, measuring how much arrived and when, and the computer registers a neutrino hit.
The tube array also samples the projection of the distinctive ring pattern to determine a particle's direction. Details of the ring pattern—especially whether it has a muon's sharp edges or an electron's fuzzy, blurred edges—is used to identify muon-neutrino and electron-neutrino interactions.
In 1997 Super-Kamiokande reported a deficit of cosmic-ray muon neutrinos and solar electron neutrinos at rates that agreed with measurements by earlier experiments. And in 1998, after analyzing 535 days of data, the Super-Kamiokande team reported finding oscillations—and so mass—in muon neutrinos.
This was strong evidence that electron neutrinos turned into muon and tau neutrinos as they streamed away from the Sun, but astrophysicists needed something more. The Sudbury Neutrino Observatory (SNO), 2 km (1.2 mi) underground in INCO's Creighton mine near Sud-bury, Ontario, went online in November 1999 to determine whether solar neutrinos oscillate on their trip from the Sun's core to Earth and to answer other questions about neutrino properties and solar energy generation.
The SNO detector is the size of a 10-story building. Its 12-m-diameter (c. 13.1 yd) plastic tank contains 1,000 tons of ultrapure heavy water and is surrounded by ultrapure ordinary water in a giant 22-m-diameter by 34-m-high cavity. Outside the tank is a 17-m-diameter geodesic sphere that holds nearly 9,500 photomultiplier tubes that detect light flashes emitted as neutrinos stop or scatter in the heavy water.
The detector measures neutrinos from the Sun in two ways—one spots a neutrino as it bounces off an electron (any of the three neutrino flavors cause a recoil and are detected); the other detects an electron neutrino when it hits a neutron in SNO's 1000-ton sphere of heavy water. Only an electron neutrino can make the neutron emit an electron and trigger the detector. The two methods, along with results from Super-Kamiokande, were designed to show how many neutrinos come from the Sun and what proportion are muon or tau neutrinos.
On June 18, 2001, SNO's Canadian, American, and British scientific team announced they had spotted neutrinos that had been missing for 30 years. SNO confirmed what several experiments, especially Super-Kamiokande in Japan, had already shown—the missing electron neutrinos from the Sun had changed to muon and tau neutrinos and escaped detection.
The transformation also confirmed earlier observations that neutrinos had mass, and the SNO measurements agreed with first-principles calculations of the number of solar neutrinos created by the Sun. The solar neutrino problem was solved, according to a team member, with a 99% confidence level. The answer is oscillations.
But does the solution to the solar neutrino problem create more problems for the Standard Model of particle physics? Neutrinos are massless in the Standard Model, but the model could be extended to include massive neutrinos through the Higgs mechanism—a phenomenon that physicists believe gives other particles mass.
But most particle theorists do not want to extend the model. They prefer using another version of the Higgs mechanism called the seesaw mechanism. This includes neutrino interactions with a very massive hypothetical particle. For the range of parameters indicated by data from Super-Kamiokande, the heavy mass would be within a few orders of magnitude of the scale where physicists believe strong and electroweak forces unify.
Massive neutrinos also could contribute to dark matter. Dark matter, like missing mass, is a concept used to explain puzzling astronomical observations. In observing far-off galaxies, astrophysicists see more gravitational attraction between nearby galaxies and between inner and outer parts of individual galaxies than visible objects—like stars that make up the galaxies—should account for.
Because gravity comes from the attraction between masses, it seems like some unseen, or missing, mass is adding to the gravitational force. The mass emits no light, so it is also called dark matter.
Scientists have known for a long time that visible matter is only a small fraction of the mass of the universe; the rest is a kind of matter that does not radiate light. Neutrinos with the kind of mass difference measured by Super-Kamiokande could make up a big part of that dark matter if their mass is much larger than the tiny splitting between flavors.
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According to current theory, the universe is "flat"; in other words, it contains just the right amount of matter so that it is at a point exactly in between collapse and infinite expansion. However, astronomers are only able to observe a fraction of the matter that is required to make the universe flat and therefore believe that most of the matter in the universe is unobservable. In fact, the edges of many galaxies rotate faster than they should if they contained only the visible matter. The fast motion of galaxies in clusters indicates that they are gravitationally held together by the presence of mass that cannot be observed.
According to estimates, about 90% of the mass of the universe consists of dark matter—matter that does not radiate light and therefore is invisible to us. A part of the dark matter consists of "normal," or "baryonic" matter, matter made up of electrons, protons, and neutrons, the constituents of atoms with which we are familiar. This matter would be found in the invisible "brown dwarfs," stars that are very cool and radiate little, cool intergalactic gas, and the so-called MACHOs (massive compact halo objects).
A large part of dark matter should also consist of non-baryonic particles—exotic particles that do not make up normal matter. Several of these that still would have to be discovered have been proposed over the years: WIMPs—weakly interacting massive particles, such as gravitinos, axions, and neutralinos. Up until now none of these particles has been detected. However, astrophysicists know that huge quantities of neutrinos were produced during the Big Bang. The discovery that neutrinos have mass has far-reaching implications for cosmology. The density of neutrinos is so high—more than 300 neutrinos per cubic centimeter—that even if the mass of neutrinos is very tiny (millions of times less than that of electrons) they should account for a sizeable portion of dark matter, perhaps up to 20%. Because neutrinos move at velocities close to that of light, they would make up the "hot" dark matter in the universe. Because of their speed they would "erase" smaller structures, such as galaxies. The existence of these galaxies is viewed as an argument that "cold" dark matter, in the form of normal matter or the slow-moving massive WIMPs, must coexist with hot dark matter.
Smallest unit of matter that can take part in a chemical reaction and cannot be broken down chemically into anything simpler.
Transformation of a radioactive nucleus whereby its number of protons is increased by one through the conversion of an neutron into a proton by the emission of an electron (and an antineutrino).
Named for Soviet physicist Pavel Cherenkov, who discovered this effect in 1934. It occurs as a bluish light when charged atomic particles pass through water or other media at a speed greater than the speed of light.
Highly energetic particles, mainly electrons and protons, that reach the earth from all directions.
Any matter in the universe that gives off no light of its own and does not interact with light the way typical matter does.
Principle stating that the total energy of a body is equal to its rest mass times the square of the speed of light.
Stable, negatively charged elementary particle that is a constituent of all atoms and a member of the class of particles called leptons.
Property that distinguishes different types of particles. Three flavors of neutrinos exist: tau, muon, and electron neutrinos.
Law that says energy can be converted from one form to another, but the total quantity of energy stays the same.
Fundamental charged particle (lepton) comparable to the electron in that it is not constituted of smaller particles, but with a mass approximately 200 times that of the electron.
One of the three main subatomic particles. A composite particle made up of three quarks. Neutrons have about the same mass as protons, but no electric charge, and occur in the nuclei of all atoms except hydrogen.
Positively charged elementary particle and a constituent of the nucleus of all atoms. It belongs to the baryon group of hadrons. Its mass is almost 1,836 times that of an electron.
Process of continuous disintegration by nuclei of radioactive elements like radium and isotopes of uranium. This changes an element's atomic number, turning one element into another, and is accompanied by emission of radiation.
(not to be confused with the "Standard Model" of the Sun) : General theory unifying the electric, weak, and strong force that predicts the fundamental particles. All predicted have been observed except for the Higgs particle.
The explosive death of a star, which temporarily becomes as bright as 100 million or more suns for a few days or weeks. The name "supernova" was coined by U.S. astronomers Fritz Zwicky and Walter Baade in 1934.
Circular particle accelerator in which the applied magnetic field increases to keep the orbit radius of the particles constant when their speed increases.
Fundamental charged particle comparable to the electron with a mass about 3500 times that of the electron.