Neutrino mass: a positive view.

Neutrino mass: A positive view

Consideration of the evidence fromsupernova 1987A now leads a number of physicists to conclude that neutrinos probably do have a rest mass, in contrast to previous work that interprets the evidence negatively. One group--Hong-Yee Chiu, Yoji Kondo and Kwing L. Chan of the NASA Goddard Space Flight Center in Greenbelt, Md.--finds a value of about 3.6 electron volts (eV) for a possible neutrino mass. Ramanath Cowsik of Washington University in St. Louis and a group at the Max Planck Institute in West Germany both say they find two values, 4 eV and 20 eV.

Neutrinos were originally thought tohave zero rest mass. Recent theories of particle physics and cosmology would like them to have a small rest mass nevertheless. If neutrinos have a rest mass, those that leave the supernova with higher energy come to earth sooner than those with lower energy. If two neutrinos with different energies leave the supernova at the same time, the rest mass can be calculated from the difference in their arrival times.

Finding the proper neutrino pairs inthe data is the trick: Nobody knows when a given neutrino left the supernova. Chiu's group paired each neutrino with every other and calculated trial masses. In this procedure the true pairs should show up by giving the same number every time; the false ones give a scatter of numbers. Such a repeated number--3.6 eV--appeared. Kondo says there is a 3 percent chance this result is accidental, giving 97 percent confidence in the result. Chiu stresses, however, that there is "a 3 percent chance we are wrong.'

Cowsik plotted energies and arrivaltimes of the neutrinos on a logarithmic graph and used a pattern-recognition machine--"the best pattern-recognition machine in the world,' he says--to find the correlations. He believes his two values, 4 and 20 eV, come from a mixture of two kinds of neutrino with two different rest masses being involved. Neutrinos come in three varieties or "flavors': electron neutrinos, muon neutrinos and tau neutrinos. The latest theories say that as a neutrino flies along, it can change from one flavor to another. Cowsik suggests that electron and tau neutrinos may be mixing together in this case.

The cosmological significance of allthis, as Cowsik stresses, is that even with these tiny masses--for comparison, a proton's mass is almost a million eV-- neutrinos would be the dominant form of matter in the universe, far outweighing anything we can see. They could make the universe close on itself, a condition that many cosmological theories favor.