NATURE'S most elusive form of matter is leading physicists on a merry chase as they try to pin down its properties. Its latest tantalizing "come on" has arisen from the study of its involvement in the radioactive decay of an old batch of uranium salt that the University of Chicago has stored for 33 years.This will-o'-the-wisp is a fundamental particle called the neutrino. It has no electric charge and zips along at - or nearly at - the speed of light. It interacts so weakly with other particles that it passes through Earth as though the planet were not there. In fact, more than a billion neutrinos passed harmlessly and unnoticed through your body while you read this sentence. The question that exercises physicists is whether or not neutrinos have intrinsic mass. They have worked with neutrinos for decades under the assumption that they have no such mass. This assumption is so firmly built into present physical theory that the discovery that at least some forms of neutrinos do have intrinsic mass would render this theory obsolete. This is the possibility that the uranium salt study now raises. Other lines of research over the past decade have also suggested neutrino mass. But physicists debate their validity. The new results reinforce the conclusion of these other studies that neutrinos can have mass, even though there is no agreement as to the exact amount of mass that may be involved. Anthony Turkevich and Thanasis Economou at the University of Chicago, and George Cowan at Los Alamos National Laboratory published the new findings this week in Physical Review Letters. Briefly put, they found that the radioactive decay of uranium atoms in their old sample proceeded 100 times faster than theory predicts. That theory assumes neutrinos have no mass. This faster decay rate could be explained if neutrinos can have at least a little mass, after all. As Dr. Turkevich notes, "Either the Standard Model [theory of matter] is wrong and neutrinos have mass, or we don't know how to calculate the decay rates of such heavy nuclei [as uranium]." And calculating such decay rates is something physicists have thought they knew very well. EUTRINOS do not respond to the electromagnetic force or to the strong force that holds atomic nuclei together. They interact with other particles only through gravity and through the so-called weak force. This weak interaction is involved in some forms of radioactive decay. That is why laboratory experiments looking for a possible neutrino mass generally have centered on studies of radioactive decay. According to Einstein's special theory of relativity, mass and energy are equivalent. One quantity can be stated in terms of the other. Thus physicists usually cite the mass of a basic particle in terms of its equivalent energy. They specify that energy in units of the electron volt (eV), which is the energy an electron gains when accelerated by a voltage difference of one volt. Possible values for neutrino mass range as high as 17,000 eV in some experiments. Others suggest a mass no higher than 1.6 eV. The Chicago/Los Alamos study points to a value of 14 eV. Even 17,000 eV is small compared with the 511,000 eV mass of the electron, the lightest particle definitely known to have intrinsic mass. Yet even a tiny neutrino mass, if confirmed, would be startling for physicists. In fact, the origin of the property called mass is, itself, such a mystery that physicists are looking for any new insight they can get. As British theorist Graham Ross of Oxford University has explained, "It may be hoped that the discovery of neutrino masses will require an extension of the [standard] theory that will shed light on this fundamental question."