When you spend years looking for ghosts, you might expect that what you find could be a little shocking.
Nevertheless, physicist Donna Naples and her colleagues were surprised when they realized what their experiment at the Fermi National Accelerator Laboratory in Illinois had revealed. For 15 months they had fired bursts of subatomic particles called neutrinos into a 700-ton detector the size of a school bus; they found that the resulting events varied just a bit -- by 1 percent -- from what they had predicted.
 |
 |
|
Physicist Donna Naples of Pitt was part of the team of scientists who conducted 15 months of experiments on neutrino behavior at Fermilab, with surprising results. (Darrell Sapp, Post-Gazette) |
"It was a big surprise to us," said Naples, a physicist at the University of Pittsburgh and one of 45 scientists on the experimental team. Expectations had been that this study of neutrinos -- a ghostly particle that is simultaneously plentiful and difficult to detect -- likely would provide confirmation of the existing understanding of how fundamental particles behave, the so-called "standard model."
Instead, their finding meant either that they had screwed up royally -- or that they had stumbled upon a new force of nature.
The result was revealed to fellow physicists during a meeting at Fermilab in late October and announced publicly two weeks later. Physicists are still scrambling to make sense of it.
"I have a lot of colleagues who are off trying to figure out what we did wrong," said Kevin McFarland, a University of Rochester physicist who headed the experiment, called Neutrinos at the Tevatron, or NuTeV. But he also anticipates that theoretical physicists will begin to weigh in within the next month or two on what sort of adjustments must be made to the standard model to explain the results.
What it could mean, suggested Paul Langacker, a neutrino expert and theoretist at the University of Pennsylvania, is the existence of a new force, one that would complement the other four known forces in the universe -- gravity, electromagnetism, the strong force and the weak force.
The standard model, he explained, does a good job of describing the behavior of all forces and particles, at least down to a scale that is one-thousandth the size of an atomic nucleus. But subtle hints in previous experiments suggest that the standard model might break down at these smaller scales, he added, and that probably means another force is at work.
Of all these suggestive deviations, the one turned up by NuTeV is the most statistically significant to date, Langacker said. Even so, the numbers are so small that scientists might easily be deceived.
"It's a very interesting result," Langacker said, "but it's not so compelling that it couldn't be a statistical fluctuation."
Neutrinos are similar to the negatively charged electron except that they are electrically neutral. They are produced in abundance by nuclear reactions, such as those that power the sun. But they don't readily interact with each other or with other types of matter. A man standing out in the sunlight might have a thousand trillion neutrinos passing harmlessly through his body every second.
"A lot of us are working to understand neutrinos," said Naples, a Pitt grad who earned her doctorate at the University of Maryland before joining the faculty in 1998. The recent discovery that neutrinos have mass -- it was long assumed they had none -- has spurred new studies. One is a $135 million project called MINOS that will shoot a beam of neutrinos from Fermilab outside of Chicago to an iron mine in Minnesota, in the hope of figuring out just how much mass a neutrino has.
While MINOS is a typical high-energy physics experiment with some 200 investigators (including Naples), NuTeV was a modest affair, with just 45. Naples signed on as a post-doctoral fellow at Fermilab, taking on such glamorous jobs as crawling through beam tubes to install calibration detectors.
The idea was to use the Tevatron accelerator's beam of protons to produce bursts of neutrinos that would be directed toward the massive, 700-ton detector. Most of the neutrinos would pass through, the investigators knew, but they calculated that a few would strike nuclei in the detector. After the collisions, the neutrinos would either remain neutrinos or turn into another particle called a muon.
The researchers found that the rate at which neutrinos remained neutrinos after the collisions was 1 percent less than expected.
That's not much, but the measurement was so precise that it was surprising. Naples explains it this way: Imagine that for 30 years there had been no rain and that the weather forecaster predicted a 99 percent chance of a sunny day with no rain the next day. And it rains the next day.
So is there a new force or not? Langacker said that an answer may be in hand within a couple of years. Fermilab's Tevatron was recently upgraded, he noted, and an experiment has already begun that could create the particle that would be the so-called "force carrier" for the mysterious new force.
Naples acknowledges that all of this is hard for most people to understand.
"It's always murky," she said. "It's murky even to us. But if we understood it, it wouldn't be research."
Thursday, November 29, 2001
