On the morning of April 4, a dozen or so graduate students and postdoctoral fellows gathered in the offices of Elena Aprile, a physics professor at Columbia University, to get their first look at the data from an experiment on the other side of the world. In a tunnel deep under Gran Sasso, Italy, Dr. Aprile and an international team of scientists had wired a vat containing 134 pounds of liquid xenon to record the pit-pat of invisible particles, the so-called dark matter that astronomers say constitutes a quarter of the universe.
Photographers were on hand to record the action — after all, you never know — although theoretical calculations suggested that with only 100 days of observation, the xenon experiment was probably still shy of the time necessary to see dark matter. “We will not discover dark matter today,” Dr. Aprile said. “We will be doing this again and again.”
Dark matter has teased and tantalized physicists since the 1970s when it was demonstrated that some invisible material must be providing the gravitational glue to hold galaxies together. Knowing what it is would provide a roadmap to new particles and forces, a new view of what happened in the Big Bang, and more Nobel Prizes than you can count. Failure to find it would mean that Einstein did not get the laws of gravity quite right.
The best guess is that this dark matter consists of clouds of exotic subatomic particles left over from the Big Bang and known generically as wimps, for weakly interacting massive particles, which can pass through the Earth like smoke through a screen door.
Some particle physicists hope to produce them in the Large Hadron Collider outside Geneva or to read their signature in cosmic rays from outer space. An experiment to do just that, the Alpha Magnetic Spectrometer, is scheduled to be launched into space and installed on the International Space Station at the end of this month. Other physicists, including Dr. Aprile’s team, have been trying to catch the putative particles in detectors set far underground to guard against contamination from cosmic rays.
For the last year the eyes of the physics world have been on Dr. Aprile’s experiment in the Gran Sasso National Laboratory, part of Italy’s National Institute of Nuclear Physics, which is widely acknowledged as the biggest and most sensitive detector out there. She hopes to record the characteristic signal — a bump and a flash — of the rare collision of a wimp with a xenon nucleus. The experiment began last year and ran for 100 days.
At the push of a button the data, unseen until now to guard against unconscious bias, would begin flowing through an analysis pipeline and show up as red dots on a big computer screen.
On a table in the corner was a stack of folded yellow notepapers, on which collaboration members had written their bets on how many events — putative dark matter detections — would be recorded. They ranged from 20, by an optimistic graduate student, to 2 from a skeptical astrophysicist. The tension and giddiness in the room rose as the 10:30 deadline came and went, due to computer glitches.
Finally, the promised graph appeared on the screen, showing the first of 91 batches of data. A red dot appeared, the first event signal. It was rapidly joined by another, and then another, each accompanied by a sharp intake of breath in the room.
“Oh, God,” Dr. Aprile said as the count rose to four. “I can’t sit anymore.” She got up from her chair.
There were more oohs and ahs as the count climbed to six, more than would be expected from background radioactivity in the detector, and finally stopped.
Everybody clapped, and Dr. Aprile went around the room offering hugs and kissing cheeks. But the results, she admitted, were ambiguous.
“Six points mean nothing until they have been analyzed,” she said. “I feel optimistic about the future. We have a lot more to do.”
Indeed, the collaborators soon threw out three of those points, concluding that they had been caused by noise in the electronics.
“We knew within 10 minutes,” said Rafael Lang of Columbia. “It was totally obvious.”
That left them with three events, compared with two expected from background, not a large enough disparity to claim evidence of a wimp. On Wednesday evening Dr. Aprile’s group posted a paper on the physics Web site www.arXiv.com and on Physical Review Letters, saying they had not detected any wimps yet.
But the group refused to be disappointed. The results, members said, had set new and stringent limits on the nature of the putative dark matter particles, eliminating some theoretical models, as well as showing that their detector was performing up to snuff. Dr. Aprile called it “a spectacular result.”
Neal Weiner, a particle theorist at New York University, agreed, noting that these were only the first results from an experiment that will go on for years and get more sensitive. If there is any dark matter in their data set, they will not have to wait years to find out, he said, “we just to have to wait for later this year.”
Dr. Lang said: “It’s the feeling of the community that something new and big is just around the corner. We are not there just yet but maybe we are not far from it, and this is very exciting.”
Dr. Aprile said they would definitely be doing this again.
In an e-mail from Italy, she wrote, “I know there is nothing more exciting than a signal, but when we are searching for the unknown, the more we probe the closer we get to truth.”