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Neutrino Physics: Curiouser and Curiouser

by John G. Cramer

Alternate View Column AV-54
Keywords: solar neutrinos SAGE gallium homestake chlorine tritium endpoint imaginary mass

Published in the September-1992 issue of Analog Science Fiction & Fact Magazine;
This column was written and submitted 2/15/92 and is copyrighted ©1992 by John G. Cramer.
All rights reserved. No part may be reproduced in any form without
the explicit permission of the author.

    New data on the nature of neutrinos has been appearing recently which is very strange indeed. Second generation solar neutrino detectors and neutrino rest-mass measurements are both telling us that this elusive particle is even more peculiar than had been previously supposed.

    Wolfgang Pauli first suggested the existence of what we now call the neutrino in order to preserve the law of conservation of energy. Previously, in 1911, James Chadwick had demonstrated that in the radioactive process called beta decay the emitted "beta particle" (now known to be an electron) was emitted with some random amount of its kinetic energy missing. Instead of the expected sharp spike of well-defined kinetic energy, a sample of many such emitted electrons showed that their kinetic energies were distributed over a broad bump-like distribution.

    Following the discovery of the missing energy in beta decay many physicists, Niels Bohr among them, were ready to abandon the law of conservation of energy. But Pauli had a better idea: he guessed that the missing energy was being removed by a new particle that was invisible to Chadwick's detectors. We now know that he was correct.

    Like the electron the neutrino spins on its axis like a tiny top, but unlike the electron it has no electric charge, little or no mass, it interacts only very weakly with matter, and it always travels at or near the speed of light. A neutrino can pass through light-years of lead without absorption or scattering.

    We now know that neutrinos come in several distinct varieties or "flavors". For each of the three charged lepton flavors, electron (e), mu lepton (µ) and tau lepton (t), there is corresponding neutrino flavor: the electron neutrino (ne), mu neutrino (nµ), and tau neutrino (nt), and each flavor comes in matter and antimatter varieties.

    Neutrinos also play an important role in astrophysics. In stars the fusion reactions are fueled by a medium that is essentially all protons. During the fusion process about half of the proton participants are converted into neutrons through weak interaction processes that involve neutrinos. A proton in some nucleus is transformed to a neutron, and at the same time a positron (anti-matter electron) and a neutrino are emitted and share the available energy. The neutrinos produced in such fusion reactions exit the star at the speed of light, carrying their share of the energy away with them.

    The result is that all stars are bright sources of energetic neutrinos. About 610 trillion neutrinos produced about 8.3 minutes ago in fusion reactions at the center of our sun are passing through your body in the second it takes to read this line. If it is night outside as you read this, the solar neutrinos are passing through the earth to reach you. There are so many neutrinos streaming in our direction from the sun that it is possible to detect them, even though the chances of detecting any particular neutrino are extremely small.

    The first successful experiment to detect neutrinos from the sun was mounted in 1968 in the Homestake gold mine in Lead, South Dakota by Ray Davis and his group from Brookhaven National Laboratory. This experiment, conducted 850 feet below ground level in a 100,000 gallon tank filled with per-chloro-ethylene cleaning solvent, has been in continuous operation for well over two decades and has produced a famous result. Only about 1/3 of the expected number of solar neutrinos are detected. Either the sun is producing only 1/3 of the neutrinos that it should, or else the Homestake detector is somehow missing 2/3 of them. This neutrino deficiency was later confirmed by the Kamiokande II detector in Japan which, although it operates on a different principle, is sensitive to neutrinos in about the same energy range as the Homestake detector.

    This puzzling deficiency of solar neutrinos, usually referred to as "the solar neutrino problem", has prompted a second generation of solar neutrino experiments. The first second generation experiment to produce results is SAGE, an acronym for the "Soviet-American Gallium Experiment". The experiment was initiated as a joint venture, with the Soviet scientists providing the deep underground site and about $25 million worth of gallium, while the Americans provided the computers, detection electronics, and other hardware. The breakup of the Soviet Union has created a problem of nomenclature for the SAGE experiment, which is still in progress. The experiment is located within the territorial boundaries of Russia, and so it has been suggested that perhaps the acronym should be changed to "RAGE".

    In the SAGE detection system a large quantity of gallium (element 31) is purified and held in underground tanks, waiting for solar neutrinos to transmute the gallium-71 isotope in the tanks to radioactive germanium-71, which has a half life of 11.4 days. A chemical procedure separates the few radioactive germanium atoms from the gallium and transports them to a sensitive detector where their decays are counted.

    If the results of the Homestake solar neutrino experiment were puzzling, the SAGE results are shocking: in over a year of counting, the net number of solar neutrinos they have detected, after subtraction of a small background, is zero. In an operating period during which hundreds of neutrinos should have been detected, none are counted.

    This null result from SAGE is very difficult to explain. The system is supposed to be detect neutrinos in a lower range of energies that are not accessible for the Homestake and Kamiokande II detectors. The strong implication of the two results is that there is not only a suppression of solar neutrinos, but that it is greater at lower energies than at high.

    I will not, because of space limitations, discuss in detail theories that seek to explain these observations. The most plausible explanations use the concept of "neutrino oscillations", in which electron neutrinos are converted into mu neutrinos or tau neutrinos in flight, neutrino flavors that would be unable to transmute gallium to germanium in the SAGE detector.

    Other second generation solar neutrino detectors in Italy and Canada are about to go into operation. We can expect new data from these experiments which should provide new insights on the solar neutrino problem.

    An even more puzzling result seems to be coming from several recent attempts to measure the rest mass of the electron neutrino. Why do the three neutrinos species, unlike their charged lepton brothers and their quark cousins, have rest masses that are nearly (or exactly) zero? The standard model of particle physics is silent on this question. Unlike the photon, which must have zero mass because it is the mediating particle of the infinite-range electromagnetic force, there is no fundamental reason why neutrinos should be massless. They just are, as nearly as we can tell from measurement.

    The best technique for measuring the neutrino rest mass does so indirectly by examining the energy spectrum of electrons produced in a low-energy nuclear beta decay. The "end-point" or region of the electron energy spectrum where the highest energy electrons are found is most sensitive to the mass of the e-neutrino (or e-anti-neutrino, which should have identical mass). If the neutrino has zero mass, the distribution near the end-point smoothly merges into the baseline. But if the neutrino has a small mass, the distribution at the end-point is chopped off early, producing a "nose" with an abrupt edge at the end of the electron energy distribution.

    Measurements performed in this way have indicated that the rest mass-energy of the electron neutrino must be less than about 15 electron-volts. A number of second-generation experiments have recently been initiated to improve this limit by high-precision measurements of the end-point region of the beta decay of tritium, the mass-3 isotope of hydrogen, which because of its 18.6 keV transition energy is the lowest energy beta decay known. The very low energy of the transition enhances the "nose" effect produced by the neutrino mass at the end-point and makes for the most sensitive measurements.

    It is not widely appreciated that the end-point technique does not actually measure the mass of the neutrino. Because of the way that the neutrino mass affects the electron energy spectrum, the measured quantity is the square of the neutrino mass.

    And this is where the interesting, although statistically shaky, results appear: of the six most recent experimental determinations of neutrino mass, all have given negative values of the mass-squared to within the statics of the measurements. The experimental observation is that in the vicinity of the end point the yield of electrons lies above the zero-mass line, while for neutrinos with non-zero real mass, the electron yield should lie below this line. The measured mass-squared values are negative to an accuracy of several standard deviations in the most recent of these experiments.

    These experimenters have been strangely quiet about mass-squared measurements with negative values. If the results had been positive by the same amount, the literature would be filled with claims that a non-zero value for the neutrino mass had been established. But a negative mass-squared is not something that can be easily publicized.

    You obtain the measured mass value from a mass-squared measurement by taking the square root of the measured value. However, the square root of a negative number is an imaginary number. Thus the measurements could, in principle, be taken as an indication that the electron neutrino has an imaginary mass.

    What are the physical implications of a particle with an imaginary rest mass? Gerald Feinberg of Columbia University has suggested hypothetical imaginary-mass particles which he has christened "tachyons". Tachyons are particles that always travel at velocities greater than the speed of light. Instead of speeding up when they are given more kinetic energy, they slow down so that their speed moves closer to the velocity of light from the high side as they become more energetic. Feinberg argued that since there are no physical laws forbidding the existence of tachyons, they may well exist and should be looked for. This has prompted a number of experimental searches for tachyons which, up to now, have produced no convincing evidence for their existence.

    Some theoretical support for the existence of tachyons, however, has come from superstring theories. These "theories of everything" can predict the masses and other properties of fundamental particles. It has been found that some superstring theories predict a family of particles with a lowest-mass member that is "tachyonic", in that it has a negative mass-squared. I should add that such predictions normally lead to the rejection of the theory as "unphysical".

    So, are neutrinos tachyons? Probably not. It is far more likely that the negative values found in the neutrino mass-squared measurements originate in some unsuspected experimental effect. Nevertheless, it is interesting to contemplate the possibility that the electron neutrino is a tachyon and to ask whether it is possible in the light of available data.

    Supernova 1987A, for example, might be taken as a "test bed" for the tachyonic neutrino hypothesis because both the light and the neutrinos from the explosion had to cover 160,000 light years to travel from the Large Magellanic Cloud to our detectors on earth. We could view SN-1987A as a 160,000 year race between photons and neutrinos, with the fastest particles reaching the finish line first.

    In fact, the neutrinos were observed to arrive 18 hours before the photons. However, this is attributed to stellar dynamics rather than FTL neutrinos. The neutrinos can leave the exploding star at once, while the photons must wait until the explosive shock wave travels from the core of the collapsing star to its surface. The more important fact is that there was a 12 second time spread between the arrival of the first detected neutrinos and the last and the apparent grouping of the arriving neutrinos in "clumps" (possibly the result of poor statistics). This could be (but has not yet been) used to place an upper limit on how "tachyonic" the electron neutrino could be.

    And so, in summary, the neutrino mysteries continue. Is the electron neutrino a tachyon? Does it change its flavor in transit from the sun to the earth? Watch this column for future late-breaking developments in neutrino physics. The only thing that is clear at the moment is that we do not have the final word on this most peculiar and enigmatic of fundamental particles.


SF Novels by John Cramer:  my two hard SF novels, Twistor and Einstein's Bridge, are newly released as eBooks by Book View Cafe and are available at : http://bookviewcafe.com/bookstore/?s=Cramer .

AV Columns Online: Electronic reprints of about 177 "The Alternate View" columns by John G. Cramer, previously published in Analog , are available online at: http://www.npl.washington.edu/av.


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This page was created by John G. Cramer on 7/12/96.