Science / Medicine : Puzzling...
In the offbeat world of particle physics, with its esoteric bestiary of quarks and leptons and muons, perhaps no particle is as unusual as the neutrino.
The “little neutral one” is unique in that it has no electrical charge, moves at the speed of light and has either an infinitesimal mass or no mass at all--scientists are not yet sure. It is ubiquitous, yet impossible--so far--to detect directly. Untold billions, produced in solar nuclear reactions, pour out of the sun every second, streaming through the Earth in an unseen sleet far more dense than a driving rainstorm.
Researchers so far have only been able to see the damage neutrinos leave behind. On those extremely rare occasions when they interact with the nucleus of an atom, neutrinos transmute one element into a different one.
In various regions of the world, researchers have set up elaborate devices to search for neutrinos. In one project, researchers studying the prodigious output of neutrinos from the sun using a swimming-pool sized detector buried deep underground in the Homestake Mine at Lead, S.D., are able to see the atomic “footprint” of only one neutrino every two to three days.
Physicists are extremely interested in neutrinos because the ghostlike particles play a central role in in understanding how the universe works. Astrophysicists are mystified, for example, because results from the first neutrino detectors indicated that the sun emits only about a third as many neutrinos as theory predicts.
Theoreticians scoffed at that conclusion, suggesting that the detectors were themselves deficient. But new results from a more sensitive detector buried deep beneath the Caucasus Mountains in the Soviet Union, reported today in Applied Physics Letters, confirm the insufficiency of neutrinos coming from the sun. These results suggest that deficiencies exist in either our knowledge of the sun or our knowledge of physics.
Recently, the mystery of the neutrino has taken an even more bizarre turn as researchers have begun to see hints of a new type of neutrino, the so-called heavy neutrino. Though still very small, with only about 4% of the mass of the electron, this hypothetical particle has theoretical physicists and cosmologists in a dither because it does not fit into the “standard” model of nuclear physics.
“It just messes everything up,” said Yale University astronophysicist Lawrence Kraus. The most extreme case, he noted, involves the Big Bang, the now widely accepted primordial explosion in which the universe was created. If heavy neutrinos exist in the quantities suggested by recent experiments, the initial mass of the universe would have been so large that it would have collapsed back into itself within a few billion years.
Obviously, it didn’t. So if the heavy neutrino is real--and new evidence presented last month at an American Physical Society meeting at Michigan State University suggests it is--it must have changed into something else before the universe could collapse. But the standard model contains no way that could have happened.
Hence, if heavy neutrinos are real, physicists and cosmologists are going to have to make substantial revisions in the standard model, even though that model explains most other aspects of the universe fairly well.
“This is the neutrino no one really wants,” said astrophysicist Michael S. Turner of the University of Chicago. “This is truly the neutrino from hell.”
To understand the dilemma, one must look at the history of the search for neutrinos. The existence of the neutrino was first postulated in 1931 by Viennese physicist Wolfgang Pauli. Despite Pauli’s reputation for brilliance, his proposal was discounted by other physicists, in large part because only two fundamental particles--the positively charged proton and the negatively charged electron--were then known. The “invention” of a new particle, especially one with no electrical charge, seemed a particularly audacious act at that time.
The existence of the neutrino was championed a year later by Italian physicist Enrico Fermi. By that time, the neutron (a particle the size of the proton, but with no electrical charge) had been discovered and scientists were more ready to accept the existence of other particles as well. But it was not until 1956 that experimentalists were able to confirm its existence by, ironically, studying a process that is the reverse of its formation.
In that year, physicists Frederick Reines and Clyde Cowan set up a tank containing 1,000 pounds of water alongside a nuclear reactor at the Atomic Energy Commission’s Savannah River facility, where theory said a massive flux of neutrinos should exist. They detected evidence of one or two positrons--the anti-particle counterpart of an electron--every hour with the apparatus, conclusive proof that the neutrino was real.
In subsequent decades, researchers have come to believe that neutrinos may not always travel at the speed of light and that some may have mass. In the arcane world of physics, mass and energy are equivalent, and the masses of fundamental particles are generally expressed as energy. The maximum mass of the neutrino, according to most conventional experiments, is perhaps 4,000 electron volts (abbreviated 4 keV), compared to about 425 keV for an electron and about 850,000 keV for a proton.
But in 1985, physicist John J. Simpson of the University of Guelph in Ontario reported that he had seen evidence for a 17-keV neutrino, one far heavier than any theoretician had ever suspected.
Simpson was studying tritium, a radioactive isotope of hydrogen whose nucleus contains one proton and two neutrons. He carefully measured the energy of the electrons emitted during beta-decay, a nuclear reaction known to produce neutrinos. In beta decay, a neutron becomes a proton by ejecting an electron. At the same time, a neutrino is emitted in the opposite direction, balancing the forces of the atom.
Theory said that all the energy of the nuclear reaction should be carried away by the electrons because the neutrinos have no mass. But he observed a “kink” in a plot of electron energies which suggested that about 1% of the neutrinos actually had a mass of 17 keV.
Although most physicists rejected the idea of a heavy neutrino, Simpson’s results prompted a fevered search for evidence for or against the particle. That evidence was slow in coming, but earlier this year, three other groups of researchers reported that they had also seen evidence of 17-keV neutrinos in beta-decay from a variety of radioisotopes.
Those three groups included physicist Eric Norman and his colleagues at the Lawrence Berkeley Laboratory; former Simpson student N. Andrew Hime, now at the Los Alamos National laboratory, who found confirmatory evidence while working with physicist Nicholas Jelley at Oxford University in England, and physicist Ante Ljubicic and his colleagues at the Ruder Boskovic Institute in Zagreb, Yugoslavia. Norman and Hime presented new data at the Michigan meeting in a small room packed with more than 300 researchers.
But many other researchers, such as Caltech physicist Felix Boehm, whose group also presented new data at the Michigan meeting, have found no evidence for the heavy neutrino. Perhaps significantly, these researchers have all used a different form of detector than that employed by Simpson and Norman. Those who have seen the 17-keV neutrino used electron detectors composed of silicon crystals. Those who have not seen it used magnetic spectrometers.
Each side is finding shortcomings in the other’s experimental technique. Spectrometer proponents argue that it is the crystal detectors themselves that are the source of the signal interpreted as a 17-keV neutrino. Crystal users say the spectrometer users have to use data correction techniques that are much larger than the effect they are searching for--thus inadvertently masking the signal from the 17-keV neutrino.
So who do people believe?
“If you took a poll and forced people to vote, I think more people would vote no,” the 17-keV neutrino is not real, said theoretician Boris Kayser of the National Science Foundation. “But quite a number of theoretical physicists have taken the time and energy to (develop new theories to) accommodate this if it is real. It’s worth their while to invest personal effort. That’s indicative of something.”
The strongest evidence in favor of the heavy neutrino, Simpson said, is the growing number of radioisotopes for which it has been observed. “I’m as cautious as the next scientist,” he said, “but it would be remarkable that these experiments with very different techniques in at least five isotopes show the same effect. It’s unlikely that it is some phenomenon of crystals. Nobody has thought of a phenomenon that could do it.”
Norman hopes to have an answer soon with a new type of experiment that uses neither spectrometers nor crystals. “If we don’t see it (in the new experiment), that’s fine,” he said. “ . . . I don’t care which way it turns out, however. I just want to know what the answer is.”
What is a Neutrino?
A neutrino is a tiny, nearly massless particle produced in certain subatomic reactions. Here, in a type of reaction called beta-decay, neutron becomes a proton by ejecting an electron. At the same time, a neutrino is emitted in the opposite direction, balancing the forces on the atom.
Neutron
Neutrino: 4 keV (mass)
Proton: 850,000 keV
Electron: (beta particle) 425 keV
Sources: Washington Post, Physics Today
Evidence for the ‘Heavy’ Neutrino?
Among evidence offered to support the idea of the heavy neutrino is a “kink” in this energy spectrum, which measures the energy of electrons emitted in the radioactive decay of sulfur-35.
Beta Spectrum (observed/expected)
This curve is expected if about 1% of the neutrinos have a mass of 17 keV.
If all neutrinos have a mass of 4 keV, the expected curve should be a straight line.
Electron Energy (keV)
