K2K -from KEK to Kamioka- Long Baseline Neutrino Oscillation Experiment


What are neutrinos?
What is the difference between electron neutrinos, muon neutrinos and tau neutrinos?
What are solar neutrinos?
What is the solar neutrino puzzle?
What are atmospheric neutrinos?
What is the atmospheric neutrino puzzle?
Why is it important to learn if neutrinos have mass?
What are "neutrino oscillations"?
What is Super-Kamiokande?
What is a water Cherenkov detector?
What is KEK?
What is K2K?
How do you make a neutrino beam?
What is a scintillating fiber (SciFi) tracking detector?
What is a Lead (Pb) Glass detector?
What is a muon detector?
What does GPS stand for?
How long does it take a neutrino to get from KEK to Super-K?
How can you tell that a neutrino detected at Super-Kamiokande came from KEK?
Can you tell a neutrino's mass by measuring its speed?
How can you determine neutrino mass from K2K measurements?

Neutrinos are subatomic particles which are stable, and very abundant in the Universe, since they are produced in radioactive decay processes. Billions of neutrinos occupy every cubic centimeter in the Universe, although most have such tiny energy that they could never be detected.

Neutrinos were postulated in the 1930s (see history of neutrinos) but not actually detected by experimenters until 1956. Until 1998, all observational evidence was consistent with neutrinos being massless, although theorists had speculated on the possibility of neutrinos having non-zero mass for many years.

Neutrinos are fundamental particles: part of the basic set of building blocks of nature. We identify 4 forces (gravity, the electromagnetic force, and the strong and weak nuclear forces), although we believe all these forces may be unified on extremely small distance scales (much smaller than the size of a proton).  There are 12 fundamental particles: 6 quarks (electrically charged particles which interact via the strong force) and 6 leptons (particles which interact via the weak force). Each fundamental particle also has a corresponding antiparticle. [return to top]

The leptons can be further divided: there are 3 electrically charged leptons (the electron, the muon and the tau particles) and 3 neutral leptons (the electron neutrino, muon neutrino and tau neutrino). Each variety, or "flavor" of neutrino is associated with the corresponding charged lepton. When a muon neutrino interacts with a nucleus (actually, with a quark in the nucleus) via the weak force, it produces a muon, for example. Thus we can identify neutrinos by the charged leptons they produce when they interact. [return to top]

The sun burns via nuclear fusion reactions, in which neutrinos are produced. However, the sun's nuclear processes can only produce electron neutrinos (and antineutrinos), and these solar neutrinos must have relatively low energy - below about 20 MeV. [return to top]

Since the late 1960s, experiments to detect solar neutrinos have consistently observed only about half as many solar neutrinos as expected from theoretical calculations. The sun's nuclear physics has been extensively studied since Hans Bethe first worked out the relevant theory in the early 1940s. Since we think we know how the sun works in great detail, the solar neutrino deficit is very difficult to understand, especially since nobody has been able to find significant error or bias in the various different experiments that have published these results. Theorists have shown that it is almost impossible to devise a model of the sun which can produce the solar neutrino deficit without major violence to accepted basic physics. [return to top]

Atmospheric neutrinos are neutrinos produced in the earth's atmosphere by the collisions of high energy cosmic ray particles with nuclei making up the air. These collisions produce subatomic particles which decay to neutrinos.

Cosmic rays are protons and nuclei which have been accelerated to high energy by shock waves from supernova explosions in our Galaxy. These particles arrive at the top of the earth's atmosphere uniformly from all directions, because their paths have been "well stirred" by our Galaxy's magnetic fields - the cosmic ray has no "memory" of its point of origin. Cosmic ray intensities and energy spectra have been extensively studied for decades in the relevant energy range.

As a result, we expect atmospheric neutrinos to arrive from all directions uniformly, and to have abundances and energy spectra which can be deduced from well-known cosmic ray spectra, taking into account all the details of particle-nucleus interactions which we have studied extensively in accelerator laboratory experiments. [return to top]

As with solar neutrinos, many experiments have studied atmospheric neutrinos, and they have consistently found an anomaly. Because of the characteristics of the particles which decay into neutrinos, we expect atmospheric neutrinos to contain about twice as many muon neutrinos as electron neutrinos. Instead, experiments find nearly equal numbers. Again, as in the case of solar neutrinos, we have detailed theoretical models combined with extensive experimental data to back up our predicted numbers for atmospheric neutrinos, and nobody can find serious problems with the experiments. [return to top]

If neutrinos are massless, only rather exotic theories could be used to explain the solar and atmospheric neutrino puzzles, and in most cases, a speculative model that explains the solar puzzle could not explain the atmospheric neutrino puzzle simultaneously.

On the other hand, the idea that neutrinos might not be precisely massless has been explored for years, and one of its consequences is "neutrino oscillations", which could explain both puzzles neatly. Neutrino mass does minimal violence to our fundamental picture of nature, the "Standard Model". Massive neutrinos would just be yet another broken symmetry in nature (along with the known facts that quarks and charged leptons all have different masses). While the Standard Model does not include neutrino masses, it could easily adapt to the news. Indeed, since publication of evidence for neutrino mass by Super-Kamiokande in 1998, most physicists now accept this view.

If neutrinos have mass, their mass hierarchy (difference in masses between flavors) would be of great interest in comparison with the approximately-known mass hierarchy of quarks, for example. [return to top]

Elementary quantum theory tells us that if neutrinos have masses, and the fundamental mass states do not have exactly equal masses, the flavor states we observe would have to be mixtures of the underlying (and not directly observable) fundamental mass states. Thus a given neutrino would act like a mixture of electron, muon and tau neutrinos, and the flavor we happen to detect would depend upon where we observe the neutrino. In other words, we could prepare a beam of neutrinos that seems to be 100% muon neutrinos, but after travelling some distance (depending upon the neutrino energy) the neutrinos would appear to be electron or tau neutrinos. This phenomenon is called neutrino flavor oscillations. It occurs because the neutrinos are "really" mixtures of mass states, not pure flavor states. It is similar to the fact that you can polarize a light beam in a certain direction but still observe some amount of light polarized in a different direction.

Neutrino oscillations will occur even in a vacuum if neutrinos have mass. However, when neutrinos pass through the earth or other dense matter, they find themselves in an environment rich in electrons. Electron neutrinos thus have their oscillation behavior altered, while muon and tau neutrinos are unaffected. This effect is called "matter-enhanced" oscillations, or "MSW" oscillations, after the initials of theorists who described the phenomenon. [return to top]

Super-Kamiokande is the world's largest neutrino detector. It consists of a tank containing 50,000 tons of highly purified water. Such a large mass is needed because neutrinos interact very weakly, and only a tiny percentage of neutrinos passing through the tank will interact. Water is an ideal material for this purpose, since it is cheap and can be used as both target and detector medium in a water Cherenkov particle detector.

Super-Kamiokande is located in a mine in north-central Japan, near the town of Kamioka, about 250 km away from Tokyo. The detector is located underground to shield it from charged cosmic ray particles which would otherwise overwhelm the data acquisition electronics. The same mine housed the Kamiokande experiment in the 1980s. Kamiokande stands for Kamioka Neutrino Detector Experiment (originally it was Kamioka Nucleon Decay Experiment - the project was first intended to look for proton decay).   Super-Kamiokande was so named because it is an order of magnitude more massive than Kamiokande, which is no longer in operation.

In June, 1998, Super-Kamiokande announced  evidence for neutrino oscillations based on studies of atmospheric neutrinos, exciting considerable media interest. These results have since been published in Physical Review Letters. [return to top]

When a subatomic particle moves at relativistic speed, near c, the speed of light in a vacuum, it may be going faster than the speed of light in a material medium. The speed of light in a medium is c/n, where n is its index of refraction. For example, pure water has an index of refraction of 1.33, meaning the speed of light in water is 3/4 the speed of light in a vacuum. Any charged particle moving faster than c/n will emit Cherenkov light in a cone-shaped pattern with a characteristic Cherenkov angle. Cherenkov light may be described as the electromagnetic analogue of a sonic boom, a sort of optical shock wave.

If we line the walls of a water tank with light detectors, we can observe the characteristic ring shaped light patterns produced by particles emitting Cherenkov light. Super-Kamikokande's water tank, 40m across and 40m tall, is lined with over 11,000 photomultiplier tubes, each of which can respond to a single photon of Cherenkov light. [return to top]

KEK is the Japanese national high energy physics laboratory, located in Tsukuba Science City, about 40 km northeast of Tokyo. KEK's facilities include a proton synchrotron accelerator which can produce an intense beam of 12 GeV protons. Last year, KEK completed construction of a neutrino beam line aimed toward Super-Kamiokande. [return to top]

K2K stands for KEK to Kamiokande. It is a Japan-Korea-US collaborative research project in which the KEK neutrino beam is directed through the earth to Super-Kamiokande, providing a controlled "laboratory" source of neutrinos, allowing us to check Super-K results obtained from atmospheric neutrinos. In order to characterize the neutrino beam before it enters the earth, a "near detector" has been built on the KEK site.

At KEK, the 12 GeV proton beam is directed onto an aluminum target which is part of a "two-horn" magnet system. Proton interactions in the target produce intense jets of secondary subatomic particles. The horn magnets are large aluminum structures which can carry intense pulses of electrical current, producing concentrated magnetic fields. These fields, plus the horn structures themselves, are designed to focus just those secondary particles which decay into muon neutrinos into a 200m long decay pipe, and defocus and absorb other particles. At the end of the decay pipe is a block of steel absorber and an additional 100m of earth absorber to stop any particles which have not decayed in the decay pipe. The result is a beam consisting almost entirely of muon neutrinos, aimed about 1 degree downward into the earth.

At this point the neutrino beam enters the "near detector" experimental hall, which contains a 1000-ton water Cherenkov detector, a miniature Super-Kamiokande, plus an array of other detectors called the "Fine Grained Detector"(FGD). The FGD consists of a scintillating fiber detector, a lead (Pb) glass detector, and a muon detector. The near detector array is used to sample and define the characteristics of the beam. After passing through the near detectors, the neutrino beam enters the earth and emerges at Super-Kamiokande, 250 km away. [return to top]

Acrylic plastic can be doped with fluorescent material so that it emits ultra-violet light when a charged particle pass through it. Such "scintillator" material can be formed into thin optical fibers, which can in turn be fabricated into fiber sheets. By observing an array of scintillating fiber (sci-fi) sheets with a sensitive camera system,   the location where a charged particle passed can be determined by identifying which fibers lit up. The K2K Sci-Fi detector has 20 fiber modules, each with 4 layers of sci-fi sheets. The layers are bundled and viewed by 24 video cameras, which view the bundles through image intensifiers.

The sci-fi modules are placed between rows of tanks containing water, which serves as target material for neutrino interactions. Water is used so that interactions observed in the sci-fi detector will be identical to those observed in the 1-kT water Cherenkov detector. [return to top]

Glass made with a high lead (Pb) content has very high density and high index of refraction. Thus electrons entering such glass will produce a "cascade" and deposit their energy in a short distance, producing a substantial amount of light via Cherenkov radiation. The K2K sci-fi detector is followed by a Pb-glass detector array salvaged from a previous experiment at KEK. [return to top]

Muons are called penetrating particles because they pass through large amounts of material, unlike other charged subatomic particles. Thus it is rather easy to identify a muon: put a lot of material in the beam and see what emerges. This is the principle of the K2K muon detector, which also makes use of components salvaged from previous KEK experiments. 20 layers of iron are interleaved with particle detector modules called proportional counters. The muon detector helps positively identify muon neutrino events. [return to top]

The Global Positioning System (GPS) was developed by the US government, originally for military navigation purposes. However, now millions of GPS receivers are used by civilians around the world. International air transportation, ground transportation, surveying, geophysical and oceanographic research and other applications rely on GPS.

GPS is based on a set of 27 satellites that continuously broadcast coded timing and other information. Timing is derived from a large set of precision atomic clocks at the GPS ground station, and satellite clocks are continually corrected. By detecting message streams from 4 satellites simultaneously, a user's receiver can compute its location in space and time, ie its geographical coordinates and the current UTC (Universal, or Greenwich) time. Once the location is known, only one satellite needs to be viewable to track accurate time information.

K2K needs to know the precise time at 2 widely separated locations: KEK and Super-Kamiokande. Before GPS, it would have been necessary to operate a set of atomic clocks and shuttle them between sites to perform the necessary time synchronization. GPS provides high quality absolute time data free of charge - all we need is a relatively cheap GPS receiver and some interface electronics. The K2K time synchronization system provides 100 nsec timing accuracy. [return to top]

Travelling at c (since the neutrino mass is certainly very small, at most a few eV, and 1 GeV neutrinos are therefore highly relativistic), it takes about 825 microseconds to go from KEK to Super-K. If we know when a proton beam pulse (or "spill") strikes the target at KEK, we can predict when to expect neutrinos at Kamioka. [return to top]

KEK neutrinos will have rather high energy compared to most cosmic ray neutrinos. We expect only a few events per second in the appropriate energy range, and most of these can be clearly identified as background noise, not neutrino interactions.

At these energies, we expect the charged lepton produced in an interaction (remember, we do not see the neutrino itself, only its products) to be within about 10 degrees of the neutrino direction. Thus the muon or electron direction observed at Super-K should be within about 10 degrees of the direction to KEK.

Finally, as noted, we can restrict our search for KEK neutrino events to a time window of a few microseconds around the time of arrival predicted by our GPS time synchronization system. Each beam spill at KEK has its time measured by a GPS clock at KEK, and each event at Super-K is similarly timestamped. The two GPS clock systems (each with two clocks, for comparison and backup) are automatically synchronized by the GPS system. [return to top]

Although massive neutrinos must travel at less than the speed of light, due to the fundamental requirements of relativity, the difference is far too tiny for K2K to measure. Our time synchronization is good only to about 100 nanoseconds. This resolution is much better than we need for our particle identification requirements, however. [return to top]

K2K will measure neutrino flavor and energy at each site, and compare the ratio of muon neutrinos to electron neutrinos at the near and far detectors. If these ratios differ, that will provide evidence of neutrino oscillation. However, as with Super-Kamiokande's measurements of atmospheric neutrinos, such measurements can only  tell us the mass difference between neutrino flavors. This type of experiment cannot measure the absolute neutrino mass.

Also, both near and far detectors will not be able to identify tau neutrinos. Thus only muon and electron neutrinos can be identified. Oscillation into tau neutrinos can be observed by neutrino disappearance, however, as in Super-K atmospheric neutrino data. [return to top]

For further information, see also these sites:

K2K official home page at KEK

Super-Kamiokande official home page at Kamioka Observatory

US Super-K home page at U. of Washington

K2K US home page at U. of Washington

K2K/Super-K home page at SUNY/Stony Brook

The following sites also have excellent web pages intended for non-specialists:

K2K/Super-K home page at Boston University

Super-K home page at UC/Irvine

Super-K home page at U. of Hawaii

Maintained by J. Wilkes, updated 07/18/2002