The Mass of the Neutrino

A billion of them pass through your body every microsecond, but until recently almost nothing was known about the particles called neutrinos. Produced as a side effect of the nuclear reactions that power our sun and other stars, these ghostlike bits of matter are believed to be the most numerous particles in the universe. But they interact so weakly with ordinary matter that nearly all the neutrinos that enter the earth on one side will emerge from the other side of our planet without even slowing down.

Our first real peek at the properties of the elusive neutrino has come from a huge detector in a played out Japanese zinc mine. An international team of physicists outfitted the mineshaft with wall-to-wall light sensors, and then filled the whole thing with water so pure that you can see through it for a hundred meters, compared to only a few meters for typical tap water. Neutrinos stream through the 50 million liters of water continually, just as they flood everything else around us, and the vast majority never interact with a water molecule. A very small percentage, however, do annihilate themselves in the water, and the tiny flashes of light they produce can be detected by the beachball-sized vacuum tubes that line the darkened mineshaft. Most of the neutrinos around us come from the sun, but for technical reasons this type of water-based detector is more sensitive to the less common but more energetic neutrinos produced when cosmic ray particles strike the earth's atmosphere.

Neutrinos were already known to come in three "flavors," which can be distinguished from each other by the particles created when they collide with matter. An "electron-flavored neutrino" creates an ordinary electron when it is annihilated, while the two other types create more exotic particles called mu and tau particles. Think of the three types of neutrinos as chocolate, vanilla, and strawberry. When you buy a chocolate ice cream cone, you expect that it will keep being chocolate as you eat it. The unexpected finding from the Japanese experiment is that some of the neutrinos are changing flavor between the time when they are produced by a cosmic ray and the moment when they wink out of existence in the water. It's as though your chocolate ice cream cone transformed itself magically into strawberry while your back was turned.

Here's how it worked. The experiment detects some neutrinos originating in the atmosphere above Japan,and also many neutrinos coming from distant parts ofthe earth. A neutrino created above the Atlantic Ocean arrives in Japan from underneath, and the experiment can distinguish these upward-traveling neutrinos from the downward-moving local variety. They found that the mixture of neutrinos coming from below was different from the mixture arriving from above, with some of the electron-flavored and tau-flavored neutrinos having apparently changed into mu-flavored neutrinos during their voyage through the earth. The ones coming from above didn't have time to change flavors on their much shorter journey.

This is interpreted as evidence that the neutrinos are vibrating back and forth among the three flavors, like arope vibrating back and forth as a wave passes through it. On theoretical grounds, it is believed that such a vibration can only occur if neutrinos have mass. Only arough estimate of the mass is possible at this point: it appears that neutrinos have a mass somewhere in the neighborhood of one billionth of the mass of an electron, or about 10^-39 kg.

If the neutrino's mass is so tiny, does it even matter? The answer from cosmologists is a resounding yes. Although a single neutrino's mass may not amount to much, they are so numerous that they may have had adecisive effect on the gravitational forces that have molded the evolution of the universe from the big bang to the present time.