Questions and Answers about Neutrinos
and the KamLAND Announcement of December 2002
University of Hawaii, Professor of Physics, KamLAND Collaborator
WHAT ARE NEUTRINOS?
Neutrinos are the least massive elementary particle in the set of building blocks of nature, which include six quarks (down, up, strange, charmed, bottom and top) and leptons. Neutrinos have no charge and are in the family of neutral leptons. They do not feel the strong force that binds quarks into protons and neutrons, and protons and neutrons into nuclei.
There are three kinds, or "flavors" as they are called, of
neutrinos electron, muon and tau. There are also three anti-neutrinos of
the same flavors. The neutrinos get their names from their charged lepton
brethren in order of increasing mass, the electron, muon and tauon. Many
theoreticians have thought the mass of neutrinos to be zero in years past.
The findings of such small values is both a mystery and undoubtedly a
WHERE DO NEUTRINOS COME FROM?
Neutrinos are produced in many circumstances in the earth, in the sun, stars and galaxies. The ones we are concerned with here are the result of radioactive decays in nuclear power reactors around Japan. These are electron type anti-neutrinos. This is in contrast to solar neutrino experiments which measure electron type neutrinos coming from the middle of the sun.
The KamLAND experiment also sees neutrinos from radioactive decays throughout the earth, mostly from Uranium and Thorium and their decay products. These neutrinos have energies at the low end of the KamLAND observation range, and do not constitute a problematic background to the detection of the nuclear power reactors. Indeed from present results it is clear that KamLAND will soon have enough statistics to measure a positive signal from the earth's total radioactivity; so far KamLAND can only claim an upper limit. This never unprecedented observation has implications for the heating of the earth internally by radioactivity and is of significance for geologists.
HOW ARE ELECTRON ANTI-NEUTRINOS DETECTED?
Neutrinos generally go right through the earth unscattered, but occasionally one interacts, sometimes in the KamLAND detector. Typically an electron antineutrino strikes a quark in the nucleus of an Hydrogen atom, which is a proton, within the paraffin oil. The neutrino snatches a plus charge to become a positron and turns the proton into a neutron. The positron annihilates with an electron almost immediately, producing two gamma rays. The gamma rays travel some tens of centimeters in the liquid and themselves further interact, leading to more particles. The energy is deposited in heat eventually, but also in exciting molecules into states from which they can decay with the emission of light. Scintillating material which has been added to the oil is very efficient at this. In net one gets about 3000 optical photons for every MeV of energy deposited... in other words roughly 5,000-10,000 photons for every neutrino interaction of this type. All this happens in a few nanoseconds (billionths of a second).
Meanwhile, the neutron created by the neutrino interaction starts to rattle around aimlessly, bumping into atoms and eventually getting captured by another Hydrogen nucleus... to become Deuterium. This occurs with the release of the binding energy of Deuterium of 2.2 MeV, and happens usually after about 200 microseconds (a long time on this scale of events). Hence the signature for anti-neutrino interactions is an initial burst of photons in the right energy range from the positron followed in several hundred microseconds by a second burst of light caused by the neutron.
These two bursts of light then must be in the right range of intensity and
sufficiently close in time. Moreover, using the time of receipt of the
light as sensed by the big photomultipliers (light detectors) one can
reconstruct the location of the origin of the light pulses and demand that
they are within a meter or two of each other. This set of requirements
serves then to make a very clean filter for background events. Data is
recorded from every tiny burst of light in the detector. The filtering
process to find antineutrino events is done off-line, which permits
careful study of the backgrounds, calibrations and such. In the end the
anti-neutrino events are essentially without background (about 1
background to 80 real anti-neutrino events).
WHAT IS THE SOLAR NEUTRINO PROBLEM?
The "solar neutrino problem" has persisted for many years, since 1968 when the first solar neutrino experiment, for which Ray Davis was awarded the Nobel Prize this year, made observations of electron neutrinos from the sun. This experiment measured the rate of transmutation of Chlorine into Argon in a tank of cleaning fluid in a deep mine in Homestake, North Dakota. Later similar radiochemical experiments in Russia and Italy with Gallium being transmuted to Germanium, showed a similar discrepancy (though of different magnitude). Then the Kamiokande detector in Japan detected the elastic scattering of neutrinos from electrons in water, and made the first on-line measurements of neutrinos from the sun. For this Masatoshi Koshiba was awarded the Nobel Prize in 2002. The final piece of the puzzle was produced in 2000 and with more convincing statistics in 2002 by the SNO collaboration. The latter measured not only a deficit in the electron neutrinos, but also found the predicted rate of solar neutrinos in a new measurement which senses total numbers of solar neutrinos.
This would seem to vindicate the people who have been calculating solar neutrino fluxes for many years, and who have often been accused of getting it wrong and causing the solar neutrino problem (and hence wasting much time and money of experimentalists). Now we know they indeed got it right, and it appears that the neutrinos are getting out of the sun but something is happening to them to change their mix of flavors. Simple oscillations between the three known types of neutrinos seems the best bet, but some ambiguities remain. Thus physicists have been awaiting the results from the KamLAND experiment which allows for testing this conclusion in a way totally independent of the problems of the sun.
WHAT IS DIFFERENT ABOUT KAMLAND RESULTS?
The KamLAND experiment is the latest in a long line of experiments measuring neutrinos from reactors. In fact in 1955 Fred Reines and Clyde Cowan made the very first observations of neutrinos using exactly the same mechanism as in KamLAND. This was an incredible step forward since many had thought neutrinos undetectable.
In the interem there have been nearly a dozen experiments near reactors, getting larger and farther away. The latest two experiments in the 1990's at CHOOZ in France and Palo Verde in the US, operated with several ton detectors at distances of about a kilometer. So, KamLAND represented a large jump in range of about a factor of one hundred, sensing reactors at a typical distance of 180 km. These previous experiments had served very well to study the spectrum of neutrinos from reactors, neutrino cross sections, etc... all this permit the rate of events expected (if no oscillations are taking place) in KamLAND to be known to a couple of percent. Previous experiments saw no sign at all of neutrino disappearance, as we now know, because they were too close. (There were several false alarms from earlier experiments however, each laid to rest by further work).
The major difference between the solar neutrino experiments and KamLAND is that the KamLAND experiment depends totally upon local, heavily studied, man-made sources of anti-neutrinos. And of course the solar source is neutrinos, not anti-neutrinos. That we now find neutrinos and anti-neutrinos to behave in the same way with respect to oscillations is an important test of something the physicists call CPT invariance. In practice in this case it means that neutrinos and anti-neutrinos behave in the same way (same mass and oscillations). This symmetry, while greatly trusted by theorists, has been called into question in this instance, as an escape for another set of experiments which have seemingly incompatible results with all others (LSND). It is also true that up to now CPT had not been heavily constrained for neutrinos. It seems that the speculative theories are tellingly denied by the KamLAND results and CPT is a safe symmetry to some fair level of precision.
WHAT ARE OSCILLATIONS?
Neutrino oscillations are a peculiar quantum mechanical effect. It's hard to find a good macroscopic analogy as it has to do with the particle-wave duality of fundamental matter.
We only know what a particle is by the way it is produced or
interacts; that is how we name it. When a pion decays, it results in a
muon and a muon (anti)neutrino; when a neutron decays, it results in a
proton, an electron and an electron (anti)neutrino. When a muon is
produced by a neutrino we know it was produced by a muon neutrino. And so
Another way to know a particle is by weight, as expressed by speed given a certain amount of energy and also as it is attracted by gravity. Usually these identifications are the same for each particle, but muon neutrinos appear to be very mixed up.
If we create a muon neutrino beam at an accelerator and pass it through a kilometer of earth and iron shielding to eliminate all the charged particles, we see muons occasionally produced in a detector, in the right direction and just after the particle beam pulse strikes the production target. Neutrinos are well known particles in this sense, real and stable particles, and their interactions have been studied at the particle accelerators, underground and at reactors for more than 30 years.
The strange situation for neutrinos, different from all the other elementary particles, is that the state of the particle which we call the electron or muon or tauon neutrino may not be the same as the particle mass state. Neutrinos are a Dr. Jekyll and Mr. Hyde sort of affair. Even more strange, the neutrinos are apparently composed of at least three different masses,
The muon neutrino may be effectively composed of half each of two states of slightly different mass that oscillate in and out of phase with each other as they travel along, alternately interacting as a muon neutrino and then making a tau neutrino. Which is observed depends on where the detector intercepts the beam.
The electron neutrino (or antineutrino) appears, in contrast, to be some mixture of all three neutrino masses. We have yet to get straight the total mix, but it appears that the mixture of electron neutrinos with some combination of the tau and muon neutrinos is as large as it might be. This peculiar result was not anticipated by theory and we do not understand why it is so.
WHAT DO OTHER STUDIES SHOW ABOUT NEUTRINO MASS AND
The mass of neutrinos and the possibility of their oscillations has eluded researchers for many years. Accelerator-based experiments and others using reactors and radioactive sources have so far only yielded upper limits on neutrino masses. No oscillations had been observed convincingly before the Super-Kamiokande results announced in 1998, which showed "the smoking gun" that muon neutrinos disappeared while traversing the earth. Subsequent studies (by Super-Kamiokande, K2K, MACRO and Soudan II) have yielded the information that muon neutrinos do indeed oscillate and that their partners are the tau neutrinos.
The solar neutrino situation is described above, but the essence is that from the SNO and Super-Kamiokande results on solar neutrino measurements, taken with earlier results, we can rather confidently conclude that the deficit of neutrinos compared to calculations, from the sun, probably indicates that electron neutrinos oscillate. Because of the distance to the sun, the fact that the solar model is very complicated, and that there exist some models (though not much in vogue) other than oscillations, makes people slightly uncomfortable with the solar results. The new KamLAND results, having to do with only a terrestrial experiment in relatively controlled conditions, wipe away many uncertainties and escape clauses: neutrinos do have mass and they all oscillate!
Many experiments have sought to directly measure absolute neutrino mass, which is very difficult. We know neutrinos are light, far less than the mass of the electron. Indeed, many theoreticians have thought neutrino mass would prove to be zero. The oscillations experiments measure only mass squared differences, so cannot tell us the absolute neutrino mass. Indeed neutrino masses may be nearly as small as the differences, or they may be all near to 1 eV, with small differences. It is possible that the total neutrino mass will be measured by astrophysical experiments in progress, if the total mass is at least around 0.1 eV. Because neutrinos from the Big Bang greatly outnumber protons in the universe (by two billion to one, or so), even a tiny neutrino mass has cosmological significance. The summed mass of all the neutrinos in the universe is close to the mass of all the stars one sees in the sky.
WHAT DOES THE KAMLAND DATA SHOW?
The KamLAND data indicates that the electron neutrinos and anti-neutrinos most probably oscillate with a mass squared difference of about 0.00007 eV^2 and with as large a mixing as it can be, or close to that. The mass difference is very small... about 0.008 eV. The mass difference between the two heavier neutrino mass states seems to be about 0.05 eV, roughly six times more. These should be compared with the mass of the electron, which is 511,000 eV. We have no theoretical model of the spread of these masses or their absolute total.
As stated above, the KamLAND data also shows complete consistency between solar data and a terrestrial experiment, and between neutrinos and anti-neutrinos.
WHAT DO THE ELECTRON NEUTRINOS OSCILLATE WITH?
Our data plus the solar data tell us unambiguously that electron neutrinos are oscillating, though we cannot be sure with what other neutrino state. Various tests now indicate that the oscillating partner of the electron neutrino is NOT a new sterile neutrino, but is entirely consistent with being some as yet uncertain combination of muon and tau neutrino.
ARE THE NEW FINDINGS CONSISTENT WITH OTHER DATA?
As said above, the KamLAND results are completely consistent with earlier experiments at smaller distances from reactors and with the ensemble of solar neutrino experiments. The KamLAND results are not in conflict with any other results.
KamLAND research was mostly funded by Japan's Mombusho (Ministry of Education, Science, Sports and Culture) and the U.S. Department of Energy, Division of High Energy Physics. The project was made possible by significant support from the Kamioka Mining and Smelting Company and many other corporations and individuals.
Locally, we particularly acknowledge significant support from the University of Hawaii, particularly the Department of Physics and Astronomy and the Dean of the School of Natural Sciences. Many
colleagues in the department, the Institute for Astronomy and the School of Ocean and Earth Sciences at UH have contributed to this work in various supportive ways over many years.
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last updated 12/5/02 jgl