Miércoles 2 de Mayo de 2007, Ip nº 189

How Did the Universe Survive the Big Bang? In This Experiment, Clues Remain Elusive
Por Kenneth Chang

An experiment that some hoped would reveal a new class of subatomic particles, and perhaps even point to clues about why the universe exists at all, has instead produced a first round of results that are mysteriously inconclusive.

“It’s intellectually interesting what we got,” said Janet M. Conrad, professor of physics at Columbia University and a spokeswoman for a collaboration that involves 77 scientists at 17 institutions. “We have to figure out what it is.”

Dr. Conrad and William C. Louis, a physicist at Los Alamos National Laboratory, presented their initial findings in a talk yesterday at the Fermi National Accelerator Laboratory, outside Chicago, where the experiment is being performed.

The goal was to confirm or refute observations made in the 1990s in a Los Alamos experiment that observed transformations in the evanescent but bountiful particles known as neutrinos. Neutrinos have no electrical charge and almost no mass, but there are so many of them that they could collectively outweigh all the stars in the universe.

Although many physicists remain skeptical about the Los Alamos findings, the new experiment has attracted wide interest. The Fermilab auditorium was filled with about 800 people, and talks were given at the 16 additional institutions by other collaborating scientists. That reflected in part the hope of finding cracks in the Standard Model, which encapsulates physicists’ current knowledge about fundamental particles and forces.

The Standard Model has proved remarkably effective and accurate, but it cannot answer some fundamental questions, like why the universe did not completely annihilate itself an instant after the Big Bang.

The birth of the universe 13.7 billion years ago created equal amounts of matter and antimatter. Since matter and antimatter annihilate each other when they come in contact, that would have left nothing to coalesce into stars and galaxies. There must be some imbalance in the laws of physics that led to a slight preponderance of matter over antimatter, and that extra bit of matter formed everything in the visible universe.

The imbalance, some physicists believe, may be hiding in the dynamics of neutrinos.

Neutrinos come in three known types, or flavors. And they can change flavor as they travel, a process that can occur only because of the smidgen of mass they carry. But the neutrino transformations reported in the Los Alamos data do not fit the three-flavor model, suggesting four flavors of neutrinos, if not more. Other data, from experiments elsewhere, have said the additional neutrinos would have to be “sterile” — completely oblivious to the rest of the universe except for gravity.

The new experiment is called MiniBooNE. (BooNE, pronounced boon, is a contraction of Booster Neutrino Experiment. “Booster” refers to a Fermilab booster ring that accelerates protons, and “mini” was added because of plans for a second, larger stage to the research.)

MiniBooNE sought to count the number of times one flavor of neutrino, called a muon, turned into another flavor, an electron neutrino. The experiment slams a beam of protons into a piece of beryllium, and the cascade of particles from the subatomic wreckage includes muon neutrinos that fly about 1,650 feet to a detection chamber, a tank 40 feet in diameter that contains 250,000 gallons of mineral oil.

Most of the neutrinos fly through unscathed, but occasionally a neutrino crashes into a carbon atom in the mineral oil. That sets off another cascade of particles, which is detected by 1,280 light detectors mounted on the inside of the tank.

From the pattern of the cascades, the physicists could distinguish whether the incoming neutrino was of muon flavor or electron. To minimize the chances of fooling themselves, they deliberately did not look at any of the electron neutrino events until they felt they had adequately understood the much more common muon neutrino events. They finally “opened the box” on their electron neutrino data on March 26 and began the analysis leading to their announcement yesterday.

For most of the neutrino energy range they looked at, they did not see any more electron neutrinos than would be predicted by the Standard Model. That ruled out the simplest ways of interpreting the Los Alamos neutrino data, Dr. Conrad and Dr. Louis said.

But at the lower energies, the scientists did see more electron neutrinos than predicted: 369, rather than the predicted 273. That may simply mean that some calculations are off. Or it could point to a subtler interplay of particles, known and unknown.

“It’s tantalizing,” said Boris Kayser, a Fermilab physicist not on the MiniBooNE project. “It could be real. But this remains to be established.”

Dr. Louis said he was surprised by the results. “I was sort of expecting a clear excess or no excess,” he said. “In a sense, we got both.”

  12/04/2007. The New York Times.