(For Educational Purposes Only) http://partners.nytimes.com/library/national/science/021900sci-dark-matter.html February 19, 2000

Evidence of Mystery Particles Stirring Excitement and Doubt

By JAMES GLANZ

A team of physicists based at the University of Rome has generated both intense excitement and profound skepticism among scientists around the world by presenting evidence that they may have detected a heavy particle that could solve a 70-year-old mystery in astronomy and lead to a conceptual breakthrough in physics.

The presumed particles would weigh at least 50 times as much as a proton and would almost always pass through other matter without a trace because of an extremely weak ability to interact with it. The new evidence, which so far has not been confirmed by other scientists, would suggest that space is swarming with enough of the particles to account for the long-sought "dark matter" that astronomers believe makes up some 80 percent of all the mass in the universe.

Though astronomers have been measuring the gravitational pull of the dark matter since the 1930's, they have never succeeded in detecting it directly.

A particle like the one that may have been found could also be part of an entire family of still-undiscovered particles predicted by an advanced theory of physics called supersymmetry. Many physicists regard supersymmetry as a possible first step toward an ultimate theory that would account for all the known forces and particle behaviors in nature -- marrying quantum theory and gravity, for example.

"If this is right, this is clearly one of the great discoveries of the last hundred years," said Dr. Michael Turner, a cosmologist at the University of Chicago. "To discover that most of the matter in the universe is not what we're made of -- that's pretty spectacular."

But a number of scientists, including Dr. Turner, said it was still unclear whether the finding was correct.

A member of the team, Dr. Pierluigi Belli, is scheduled to present the results next Friday in Marina del Rey, Calif., at the Fourth International Symposium on Sources and Detection of Dark Matter in the Universe. But this week, the team made a paper describing the possible detection of the particle, variously called a neutralino and a weakly interacting, massive particle, or WIMP, available on its public Web site (www.lngs.infn.it).

According to the paper, whose lead author is Dr. Rita Bernab "cumulative analysis" of data collected over three years in an underground experiment at the Gran Sasso National Laboratory east of Rome "favors the possible presence of a WIMP." The group came to its conclusion by noting seasonal variations in the counts registered on their detector, as expected if Earth is passing through a cloud of the particles in its orbit.

"They have, clearly, a seasonal variation," said Dr. Bernard Sadoulet, an astrophysicist at the University of California at Berkeley, who has read the team's paper.

Dr. Sadoulet, with Dr. Blas Cabrera of Stanford University and Dr. David O. Caldwell of the University of California at Santa Barbara, leads another group that will make the first public presentation of the results of its own search for dark matter at the same conference.

But Dr. Sadoulet added that "there is a lot of skepticism in the community" as to whether the Rome group has really detected the elusive WIMP. He said that unrelated seasonal effects, such as temperature variations around the sensitive detectors or changes in levels of natural radioactivity, could still be mimicking the expected signal.

In response to questions by e-mail, Dr. Bernabei wrote that "all known sources" of such effects had been thoroughly analyzed in the paper. She also pointed out that the new results showed "full compatibility" with the team's earlier, published results, based on less data, which suggested a WIMP of about the same size.

Dr. Frank Avignone, a physicist at the University of South Carolina who has visited the Gran Sasso laboratory, said that "the experiment, the apparatus, has been constructed very carefully." Dr. Avignone added, "I'm not willing to say, 'No, I don't believe it,' " although he also expressed doubt that the measurement could be considered definitive.

Dr. Avignone said that if correct, the finding "would definitely be at the level of a Nobel Prize."

Eight of the scientists listed as authors on the team's new paper, including Dr. Bernabei and Dr. Belli, are affiliated with both the University of Rome and the National Institute of Nuclear Physics in Italy. In addition, four authors are identified as members of the Institute of High Energy Physics of the Chinese Academy of Sciences in Beijing.

Based on decades of indirect evidence, astronomers are all but certain that dark matter makes up the vast majority of all matter in the universe. Without the gravitational pull of the dark matter, clusters of galaxies would fly apart, since the galaxies are generally orbiting around each other too quickly to be held by the gravity of observable matter.

The gravity of the dark matter is also thought to prevent individual galaxies from breaking to pieces. Moreover, astronomers have led the presence of the dark matter's powerful gravity by observing the way that it bends light rays, as it should according to Einstein's theory of relativity.

But because the matter seems utterly invisible, astronomers believe that the dark matter is very different from the ordinary stuff of which stars, planets and people are made. Cosmologists studying how all matter was presumably created in the Big Bang, the explosion in which the universe is thought to have been born, have come to similar conclusions about the dark matter.

Interacting only weakly among each other and with ordinary matter, dark matter WIMPs are thought to clump like an immense cloud or gas around the visible, starry parts of galaxies. Called a halo, this structure may extend for hundreds of thousands of light years into space beyond the visible disk of a galaxy like the Milky Way.

About one WIMP of the kind possibly found by the Rome group would exist in a volume of space the size of a coffee cup. But because the sun is orbiting around the center of the Milky Way at a speed of about 140 miles per second, through the clouds of WIMPs, "a billion of them would be passing through your body every second," said Dr. Leszek Roszkowski, a particle physicist at Lancaster University in England. "And yet you would not be able to detect them."

Rarely, however, a WIMP should interact with ordinary matter in a collision. The Rome group set about trying to detect the WIMPs with a material called sodium iodide, which scintillates, or emits tiny flashes of light, when particles collide with it. The detector is underground in order to shield it from showers ordinary particles from space called cosmic rays.

And to be sure that they were seeing the WIMPs, the group looked for a rise and fall of the rate of detections over the course of the seasons. Because of the way in which Earth is orbiting the sun, which is itself in orbit around the galactic center, the detectors should sense a "wind" of WIMPs that is slightly stronger in the summer than the winter.

It is as if a child on a merry-go-round were whirling a ball on a string. The wind resistance on the ball would be greatest when it was moving in the same direction as the merry-go-round, and the least when moving in the opposite direction. In the WIMP experiments, the expected difference over a year would amount to "roughly 5 percent in the detection rate," Dr. Roszkowski said.

It is an observation of this seasonal variation, or modulation, that the Rome group is reporting in its paper, he said. The most likely WIMP mass indicated by the results is about equal to that of 60 protons, or an entire atom of nickel.

But Dr. Roszkowski added that because many processes vary with the seasons on Earth, and because the expected modulation is so slight, minor errors in the analysis could produce a false signal. "They are probing uncharted waters," he said. "Skepticism is the most healthy attitude in this respect."

He and other scientists pointed out that several other dark matter experiments, including those of the Berkeley group, have begun collecting data and the correctness of the result could be tested quickly.

But there is no questioning the impact of the discovery if it holds water, said Dr. David B. Cline, the physicist at the University of California at Los Angeles who is organizing the conference in Marina del Rey.

"The Copernican revolution told us we're not the center of the universe," Dr. Cline said. "This tells us we're not the matter of the universe." Intellectually, he said, the development "is just the tip of an incredible iceberg, if this is right."

-- Spoonfed (Spoonfed@spoonfeddd.xcom), February 20, 2000

Answers

I love that we don't know everything yet. Discovery is fascinating! I would hate for us to ever get to the point where we could explain EVERYTHING in the universe. Where is the fun in that? Speculating and learning...is part of what makes life so joyous.

-- kritter (kritter@adelpia.net), February 20, 2000.

Speculating and learning ... that's what this Forum has been about! Thanks kritter & spoonfed :-)

-- Ashton & Leska in Cascadia (allaha@earthlink.net), February 20, 2000.

Take with thimble full of ultra-dense, neutron star sodium chloride.

-- number six (#@#.com), February 20, 2000.

Interesting read, though blemished by misuse of the term gravity for gravitation (the former being specific to Earth).

-- David L (bumpkin@dnet.net), February 20, 2000.

And then there was that other discovery of dense matter:

RESEARCHERS DISCOVER NEW ELEMENT

The heaviest element known to science was recently discovered by materials researchers. The new element, tentatively named Administratium, has no protons or electrons, and thus has an atomic weight of 0. However, it does have one neuron, 125 assistant neutrons, 75 vice neutrons, and 111 assistant vice neutrons. This gives it an atomic mass of 312. These 312 particles are held together in a nucleus by a force that involves the continuous exchange of particles called morons.

Since it has no electrons, Administratium is totally inert. However, it can be detected chemically, since it impedes every reaction it comes into contact with. According to its discovers, a tiny amount of Administratium caused on reaction to take over four days to complete; the normal reaction time is less than one second.

Administratium has a normal half life of approximately three years, at which time it does not actually decay, but instead undergoes a reorganization in which neutrons, vice neutrons, and assistant vice neutrons exchange places. Studies have shown that the atomic mass usually increases after each reorganization.

Research at other laboratories indicates that Administratium occurs naturally in the atmosphere. It tends to concentrate at certain points, such as governmental agencies, large corporations, and universities. It is always found in the newest, best appointed and best maintained buildings.

Scientists point out that Administratium is known to be toxic at any level of concentration and can easily destroy any productive reactions where it is allowed to accumulate. Attempts are being made to determine how Administratium can be controlled to prevent irreversible damage, but results to date are not promising.

:-)

-- Jerry B (skeptic76@erols.com), February 20, 2000.

Jerry:

ROTFLMAO-best laugh I have had all week

Thanks.

-- Futureshock (gray@matter.think), February 20, 2000.

They are definitely going to have to rename this. I can not, in my lifetime, conceive of buying an intergalactic roadster with "WIMP" drive. I'm sorry, but that is just NOT going to happen.

-- ..- (dit@dot.dash), February 20, 2000.