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April 28, 2002

Cosmic Weight Gain: A Wispy Particle Bulks Up

By GEORGE JOHNSON

Lawrence Berkeley National Laboratory
At the center of the Sudbury Neutrino Observatory's detector is an acrylic plastic vessel about 13 yards across and filled with a liquid called heavy water. Surrounding it is a geodesic sphere holding 10,000 light sensors tuned to watch for the tiny flashes created by neutrinos passing through.


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IT is not often that a correction must be made to a poem. But when the subject is the mercurial science of particle physics, that is the risk a writer takes.

In 1960, when he was 28, John Updike published a literary invention that is still read fondly today not "Rabbit, Run," which also came out that year, but 19 lines of verse, quietly unleashed in The New Yorker, called "Cosmic Gall."

Neutrinos, they are very small.

They have no charge and have no mass

And do not interact at all.

The earth is just a silly ball

To them, through which they simply pass. . . .

There, in the second line of the first verse, is the error as laid bare last weekend in a joint meeting in Albuquerque of the American Physical Society and the American Astronomical Society. The ghostly particles are still understood to carry no charge, but experimental results announced at the conference seem to have clinched the case, building for years, that they are not entirely without heft.

It was a sad day for poetry. Somehow, "They have no charge and little mass" just doesn't have the same ring.

In fact the next line of the poem had long ago been edited (to read "scarcely interact at all") by the Nobel laureate Murray Gell-Mann when he borrowed "Cosmic Gall" for a lecture. As insouciant as neutrinos are, they do occasionally consort with their neighbors.

Every particle electrons, protons, photons, quarks has a unique personality. The neutrino, which began in 1930 as a half-baked idea, has simply matured. When heavy particles called neutrons decay into protons and electrons, the energy going into the process doesn't equal the amount coming out an impossibility. Hence the Austrian-born theorist Wolfgang Pauli was moved to propose that the reaction also emits evanescent little things that conveniently make up the difference. The neutrino ("little neutral one") was born, and christened by Enrico Fermi.

In later years, physicists went on to adopt a more whimsical style of nomenclature (quarks, gluons). Had the particles' debut been postponed a while, they might be known as wisps. They cannot carry any electrical charge, for that would mess up the equations. And until recently there was no reason to think they had any mass at all.

These qualities made the elusive particles "Pauli's poltergeists," they were sometimes called almost impossible to detect. ("They snub the most exquisite gas,/Ignore the most substantial wall.") If the earth is just a silly ball to them, how do you catch one in a trap?

It wasn't until 1956 that two experimenters, Frederick Reines and Clyde L. Cowan Jr., figured out how. Perhaps that is when Mr. Updike first heard of the things. Neutrinos, it appeared, were real after all. Soon they became a favorite among science writers, who came to the subject with a roster of adjectives so obligatory it sometimes seemed that each had been assigned its own word-processor key. (Four of them ghostly, insouciant, evanescent and elusive have already been used up in this article.)

Pouring from the sun, the equations predicted, trillions upon trillions of the fleeting particles should slice through the earth ("Like tall/And painless guillotines") every second. Hoping to snag a few with their detectors, experimenters lurked inside mine shafts and tunnels, places only neutrinos could penetrate.

When the results came in, theorists were astounded to learn that the sun was emanating only about a third of the neutrinos it was supposed to. Scrambling for an explanation, some ventured that there must be a black hole in the middle of the sun, gobbling most of them up. Or perhaps, another proposed, the sun had already burned out, stanching the neutrino flow. So dense is the solar core that photons, the carriers of light, take millions of years to find their way to the surface. Evidence of their decline would be delayed for eons. But once the nuclear furnace began to cool, the flux of unstoppable neutrinos would drop off immediately a warning of darker times ahead.

What has become the favored explanation was clinched last week at the meeting in Albuquerque. Over the years physicists have determined that there are actually three "flavors" of neutrinos. Maybe on their way to the earth, most of the plain vanilla ones the kind the detectors can most easily spot turn into chocolate or raspberry. That could only happen, the theorists say, if the particles have mass.

Last summer, the Sudbury Neutrino Observatory, located at the bottom of a Canadian nickel mine, 6,800-feet deep, reported that its new detector designed to register all three flavors seemed to have accounted for the missing particles. The latest, more refined results, raise the scientists' certainty to 99.999 percent. (Had they not stolen their own thunder with the earlier announcement, this would have been even bigger news.)

EXPERIMENTS like these, done purely for the intellectual thrill, have a beauty all their own. Donning helmets with Cyclopean lights, physicists crowd onto a clattering elevator for the daily commute down the mine shaft. Exiting at the bottom, they trudge through the rocky corridor leading to the detector.

The brightly lit subterranean lab looks like something from "The Andromeda Strain." Suspended inside an artificial lake 10 stories deep, a large acrylic sphere holds a thousand tons of a substance called heavy water (the hydrogen atoms have been beefed up with an extra neutron in their cores). Surrounding it is a Fulleresque scaffolding supporting 10,000 squinting electronic eyes. Day by day they wait for a neutrino speeding through the earth to collide with a heavy hydrogen nucleus.

The result is a tiny flash called Cherenkov radiation, a wink of light sparked by a traveler from the center of the sun.

Someone should write a poem about it.



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