On the subatomic trail of the elusive neutrino
After five years of planning and construction, an American research team is checking out a $3.5 million experiment to try to pin down the most elusive form of matter - the neutrino. Neutrinos are subatomic particles that play key roles in certain types of radioactivity and in the nuclear fusion processes that power the sun and stars.
Yet they interact so weakly with other forms of matter that they can zip through the solid Earth as though it were not there. In fact, more than a billion of them passed harmlessly and unnoticed through your body in the time it took to read this sentence.
This ability to pass undetected makes the neutrino a difficult object for physicists to study. It's the least understood of the fundamental material particles, even though it's the most abundant particle in the universe.
Physicists have worked with it for more than half a century. Yet they don't know its intrinsic mass or even whether it has any intrinsic mass at all.
Physicists now realize that they need to know the neutrino better. The question of neutrino mass, for example, is bound up with the fate of the universe.
Neutrinos are so abundant that, if they have any mass at all, their combined gravitational force could be enough to eventually halt - or even reverse - the expansion of the universe.
Since the beginning of the decade, experimenters in several countries have been looking for neutrino mass with inconclusive results.
The new American search is being conducted by Wolfgang Stoeffl and colleagues Daniel J. Decman and Jon Engelage at the Lawrence Livermore National Laboratory in Livermore, Calif. It's one of 15 neutrino mass studies - including a second Livermore experiment - under way around the world. It also is one of the most sensitive searches yet made, according to Dr. Stoeffl.
He explains: ``We believe our experiment will be the most accurate ever conducted. We worked very hard to identify and fix all of the problems others have faced in making this very difficult measurement.''
Physicists first suspected the neutrino's existence when they puzzled over a type of radioactivity called beta decay. In this process, an unstable atom of certain elements sometimes transforms spontaneously into a slightly less massive atom, emitting an electron in the process.
Physicists know how much energy the transformation releases. They expected the emitted electron would carry away that energy. Instead, they found that these electrons had a range of energies.
Austrian physicist Wolfgang Pauli suggested that the electron was sharing the energy with an unseen partner. Following up this suggestion in 1933, Italian physicist Enrico Fermi published a theory of beta decay that included an unseen particle that had no electric charge and no intrinsic mass.
He called it the neutrino (Italian for ``little neutral one''). Two decades later, experimenters demonstrated its existence by managing to detect a few of the millions of neutrinos pouring out of a nuclear reactor.
Physicists know the neutrino has no electric charge because it leaves no trail in detectors that track charged particles. But there has never been any proof that it has no mass. This has just been taken for granted.
Then, in 1980, Valentin Lubimov and his team at the Institute for Theoretical and Experimental Physics in Moscow challenged that assumption. They had taken a detailed look at the way the energy released in beta decay is shared between electrons and neutrinos.
They claimed to have found that a small amount of the energy is converted into neutrino mass.
It was a startling claim, which many physicists disbelieved because of large uncertainties in the experiment.
Walter Kundig at the Swiss Institute for Nuclear Research in Zurich and John Wilkerson at the Los Alamos National Laboratory in New Mexico have led experiments to try to confirm the Moscow results. But, so far, they have been unable to do so.
Physicists measure particle energies in terms of electron volts. One electron volt (eV) is the energy an electron gains when it is accelerated by a voltage difference of one volt.
The Moscow group last year claimed that about 30 eV is converted into neutrino mass in beta decay. That's quite small - only about 1/10000th the mass of an electron. But it's large for a particle that's supposed to have no mass at all. The Swiss group, however, found that the neutrino mass was less than 18 eV and probably was zero. Dr. Wilkerson's team at Los Alamos has found that the mass must at least be under 27 eV. Improvements in their experiment should enable them to search for a neutrino mass as low as 10 eV.
The Stoeffl team at the Livermore laboratory expects to do better than that. ``We're the third generation'' of this type of experiment, team member Engelage notes.
He says that, with the many subtle adjustments his team's equipment can make, it should be able to measure a neutrino mass as low as 5 eV. With the equipment now entering the final check-out stage, preliminary results could be available by the summer of 1989.