World of Physics Jolted by Finding on Neutrinos
In a finding that is being called the physics discovery of the decade, an international team of researchers reports today that the elusive, ghost-like subatomic particle known as the neutrino has mass. This suggests, among other things, that it represents at least part of the so-called missing or dark matter necessary to explain how large-scale structures in the universe came into being.
The discovery will reverberate through the physics community for years, experts said, not only because it touches on the fate of the universe but also because it will increase our fundamental understanding of matter.
“This is something that physicists have hoped for and eagerly sought for decades,” said astrophysicist John Bahcall of Princeton University’s Institute for Advanced Studies. “It is a huge step forward.”
Neutrinos--formed primarily in the big bang that created the universe and in the fiery nuclear furnaces of stars--are by far the most common elementary particles. The discovery that they have mass, just like the more substantial particles known as electrons and protons, is being announced today at a Takayama, Japan, meeting called Neutrino ’98.
The finding may mean that the universe contains so much mass that it will eventually collapse upon itself in a reversal of the big bang, rather than continuing to expand indefinitely as some cosmologists have predicted.
The discovery also will force revisions in what physicists call the Standard Model--the set of esoteric rules that describe how particles interact with each other to form larger particles, atoms and everything else in the universe.
Mass is the property that gives an object weight in a gravitational field. The Standard Model says at this point that neutrinos have no mass, just like the photons that carry light energy. The finding that they do have mass--a minimum of about one ten-millionth that of an electron--should help resolve some inconsistencies in the theory.
‘Holy Grail of Physics’
“These new results could prove to be the key to finding the Holy Grail of physics, the unified theory,” better known as the theory of everything, said University of Hawaii physicist John Learned. Scientists are trying to devise such a theory to account for all interactions of energy and matter.
“One only gets such great data once or twice in a professional lifetime--and maybe never,” he added.
Neutrinos are one of the most unusual beasts in the exotic menagerie of subatomic particles. Other such particles include leptons, fermions, bosons, gluons and gravitons, as well as their antimatter analogs. A neutrino carries no charge--hence the name, “little neutral one,” conferred on it in 1931 by Italian physicist Enrico Fermi.
A neutrino is not affected by the “strong force” that holds electrons, protons and neutrons together in atoms. As a result, it passes easily through matter--trillions pass through our bodies each minute without doing any damage to our own atoms.
Physicists have never actually “seen” a neutrino. Instead, they have observed the debris that results on the exceedingly rare occasions when a neutrino collides with an atom, typically an atom of water.
Researchers have concluded that there are three types of the particle: the electron neutrino, the muon neutrino and the tau neutrino, each named for the particle that is produced in the collision. There is also an antimatter counterpart of each particle, bringing the grand total to six. Antimatter particles have an equal but opposite electrical charge to the corresponding regular particles.
For two decades, physicists have been puzzled by results of experiments to detect neutrinos. Large detectors built to measure the particles produced by the sun and those produced when cosmic rays strike atoms in the upper atmosphere of the Earth have found far fewer neutrinos than theory predicted.
That failure was initially attributed to defects in the detectors, but the new results provide “very strong evidence” that the problem lies in the nature of neutrinos themselves, said physicist William Louis of the Los Alamos National Laboratory in New Mexico.
The key is showing that neutrinos can repeatedly transform themselves--like Jekyll and Hyde--from one type to another. For example, an electron neutrino can become a tau neutrino and then go back again. Physicists call this process oscillation.
In the abstruse world of quantum mechanics, that means that at any given time, there is a certain probability that a neutrino has one mass and another probability that it has a slightly different mass.
The key point, said physicist Janet Conrad of Columbia University, is that theory firmly states that neutrinos can oscillate only if they have mass in the first place and if different types of neutrinos have different masses.
The new results show that neutrinos do oscillate, thereby proving that they do have mass.
But to show that, researchers had to construct a $100-million detector, called Super-Kamiokande, buried in an old zinc mine 3,250 feet under Mt. Ikena in the Japanese Alps. The massive cylindrical detector contains 12.5 million gallons of ultra-pure water and is lined with an acre of photomultiplier tubes, which detect light and convert it into an electrical signal.
The observatory was 90% funded by the Japanese government and 10% by the United States. Researchers at the site include teams from UC Irvine, the University of Wisconsin, the University of Hawaii and Boston University.
The photomultipliers observe those rare occasions when a neutrino strikes an atom of water, producing either an electron or a muon. When the particle passes through the water it leaves a streak of light several meters long. The computers controlling the detector determine the streak’s orientation. On average, the researchers observe 5.5 such events daily.
The detector does not record tau neutrinos, however. So if a muon neutrino is oscillating with a tau neutrino, the detector will register fewer neutrinos than expected.
In the experiment being described today, the researchers monitored both neutrinos produced from cosmic rays striking the atmosphere directly above the detector and those produced in the same fashion on the opposite side of the Earth, about 8,000 miles away.
When the researchers observed a critical difference in the numbers of neutrinos detected from the two directions, that was the evidence needed for their historic finding.
Because neutrinos passing through the Earth have more time to oscillate, the detector, theoretically, should see fewer of them. Past detectors have not been big enough to observe enough neutrinos to confirm that they are oscillating.
But Super-Kamiokande was big enough: The team observed about half as many neutrinos passing through the Earth as were coming from the atmosphere above them.
“That’s really strong evidence that oscillations are actually happening,” Conrad said.
The experiment does not reveal the mass of a neutrino, however. Instead, it shows the difference in mass between the two forms of the oscillating neutrino. That represents the minimum mass of a neutrino.
With about one ten-millionth of the mass of an electron, the neutrino is a very small particle indeed. But because there are 50 billion neutrinos in the universe for every electron, there is much more total mass in neutrinos than in electrons in the universe, and maybe even more than in all normal matter, the researchers said.
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Detecting Neutrino Mass
An international team of physicists has refuted the idea that subatomic particles called neutrinos have no mass. The experiments are taking place in a detector tank located 3,250 feet below a mountain in Japan. Here is an overview of the detection process:
1) Cosmic ray hits Earth’s atmosphere, creating a spray of secondary particles, some of which decay into neutrinos.
2) Neutrinos begin to oscillate as they pass through Earth.
3) Neutrino strikes another elementary particle in detector tank. This interaction is analyzed to determine the type of neutrino and its flight path.
The farther neutrinos travel, the more time they have to oscillate. By comparing neutrinos coming “up” through the Earth to those coming from overhead, physicists determined that they were different and that neutrinos from below oscillate, which they could do only if they have mass.
Source: University of Hawaii