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.