Experimental results from particle physics seldom make the front pages of newspapers, or even the back pages. But last January a story about neutrinos made page one of the New York Times. Physicists at Los Alamos National Laboratory, the paper reported, had found strong evidence that neutrinos, which some researchers had long thought to be massless particles, might have some minuscule mass after all. If true, the result would have profound implications not only for physics but also for cosmology.
The news report stirred up considerable controversy among physicists. There was the usual indignation that the discovery had appeared in a newspaper before being published in a peer-reviewed physics journal. Even now, five months after the Times story, the details of the results have yet to be published. Until they are, many physicists feel they don’t yet know enough to judge the validity of the Los Alamos experiment.
But the main reason for the fuss was simply that the experiment was so difficult and the stakes so high. Neutrinos are notoriously elusive. They carry no electric charge, and if they do have mass, they are probably hundreds of thousands of times lighter than electrons, now the lightest known particle. These traits allow trillions of neutrinos to shoot unnoticed and harmlessly through our bodies every second--and through physicists’ detectors as well. Physicists have tried and failed for decades to determine whether neutrinos have mass.
Moreover, in recent years the issue has taken on a more cosmic significance. Even if neutrinos possess only a tiny amount of mass, it is now clear that since there are so many of them, their collective mass would be far greater than that of all the stars, planets, galaxies, and other ordinary matter in the universe. Massive neutrinos, if they exist, would constitute an important fraction--perhaps as much as one-fifth--of the mysterious dark matter that apparently makes up 90 percent or more of the universe. And whoever discovers the neutrino mass would probably have a better than 90 percent chance of winning the Nobel Prize.
The Los Alamos researchers say they weren’t expecting that type of discovery at all. I think most people on the experiment thought that we would not see evidence for neutrino mass, because many other experiments have searched, says William Louis, a physicist at Los Alamos. Nonetheless, he and his colleagues found evidence by observing a subtle and until now only theoretical process called neutrino oscillation.
According to the neutrino-oscillation theory, the three types of neutrinos--electron, muon, and tau, each named for the particle it produces when it does happen to interact with an atom--are distinguished as well by having different masses. But the mass of an individual neutrino is not immutable; it is quantum mechanical--that is, at any given instant there is a certain probability that the mass will oscillate from one state into a lighter or heavier one, thereby transforming the neutrino from one type into another. The neutrino-oscillation theory has become popular because it explains a long-standing puzzle about the sun: why it seems to emit far fewer electron neutrinos than physicists think it should. Neutrino- oscillation fans believe some of the sun’s electron neutrinos are changing on their way to Earth into a different, probably more massive, type that current experiments can’t detect.
Neutrinos can only change their mass, obviously, if they have mass to begin with. To find out if they do, the Los Alamos team built a neutrino source they could control better than the sun, one that emits a variety of neutrino types. Then they paired it with a detector and looked for one particular type of neutrino--the antielectron neutrino. (Each of the three neutrino types has an antimatter counterpart.) In principle, the physicists can calculate how many antielectron neutrinos to expect in their detector if neutrinos don’t oscillate. If their detector registers an excess of antielectron neutrinos, another type of neutrino--specifically, antimuon neutrinos--must have been transformed. And therefore neutrinos must have mass.
The Los Alamos experimental setup smacks of Rube Goldberg. To produce neutrinos, Louis and his colleagues use a particle accelerator at Los Alamos to shoot protons at a one-foot-long cylindrical container of water. The protons collide with water molecules, producing particles called pions, which in turn slam into a copper target generating a spray of neutrinos and other particles. The detector sits 90 feet from the copper target, behind a 60-foot-thick steel-and-earth wall. Only the neutrinos can penetrate the thick wall, though, because only they are so loath to interact with matter.
The detector is a tank filled with 51,000 gallons of mineral oil. All types of neutrinos enter the tank, but antielectron neutrinos leave a unique trail, revealing their presence when they collide with protons in the mineral oil. The collision converts the proton into a positron, the antimatter counterpart of an electron, and a recoiling neutron. The neutron itself then collides with another proton, emitting a signature burst of gamma rays that should be detected by at least one of 1,220 photomultiplier tubes lining the oil-filled tank.
For five months Louis and his colleagues fired their proton beams and looked for gamma-ray flashes. They observed a dozen flashes more than they expected and concluded that those events were probably caused by antimuon neutrinos metamorphosing into less massive antielectron neutrinos on their way to the detector. The experiment was not designed to measure neutrino mass precisely; but based on the rate at which the oscillations occurred in their detector, the researchers estimate that the antielectron neutrino has a mass of between one-millionth and one one-hundred-thousandth that of an electron.
Elated by the results, the Los Alamos team presented their data at an astrophysics conference. Word got out, the Times ran the story, and soon physicists were getting their first glimpse of a potentially revolutionary discovery by reading their morning paper.
Although most physicists still haven’t been able to look at a definitive paper on the Los Alamos results, those who are familiar with the pitfalls of neutrino research have expressed skepticism. Alfred Mann, a physicist at the University of Pennsylvania, was a member of the Los Alamos team until last summer. We did not see completely eye to eye on how the experiment ought to be analyzed, says Mann, explaining why he left the team. It seemed best to me, instead of standing around being the resident itch, to just leave.
Mann emphasizes that he respects the work of his erstwhile colleagues, and that disagreement over interpretation is a normal, and crucial, part of the scientific process. This is a reasonable scientific difference of opinion, he says. One would like to say that it’s all objective and cold-blooded, but it isn’t. It depends on assumptions and one’s judgments and experience and so forth.
Mann’s judgment is that the Los Alamos group has probably not observed neutrino oscillations. Before he left the collaboration, he guided the research of a Ph.D. candidate, James Hill. Hill analyzed the Los Alamos neutrino data. In particular, he studied how normal background processes, such as cosmic rays, might trigger gamma rays and other events in the detector that mimic the appearance of massive neutrinos. The Los Alamos team also tried to take this into account, but Hill and Mann think Louis and his colleagues may have underestimated the effect. The surface layers of the oil-filled detector, says Mann, are more susceptible to background events than its depths, and this may affect the reliability of the experiment’s statistics: what looks like an excess of antielectron neutrinos becomes more doubtful if surface events are excluded.
Events around the periphery of the detector should not be taken into the analysis, says Mann, largely because of background. Well, then the whole question is, how deep within the detector do you go? My student has strong evidence, he believes and I believe, for digging a little deeper, going more into the central region of the detector than Bill Louis has. As a result, he would keep certain events that we would say don’t belong. One cannot say for certain that they are background, but one can say for certain that they are questionable as real events, and as a consequence one should not include them in an estimate of a positive result in an issue that’s as important as this one.
Mann isn’t the only physicist reacting cautiously to the Los Alamos experiment. The Los Alamos group’s main competitor in the search for massive neutrinos is a team of German and British physicists working at the Rutherford Appleton Laboratory near Oxford, England.
Los Alamos has been taking data for five months, says Jonny Kleinfeller, a spokesperson for the Rutherford experiment. We have been taking data since 1989, and we don’t see any evidence for neutrino oscillations. Still, Kleinfeller doesn’t flatly deny that the Los Alamos experiment may have turned up massive neutrinos; with a more intense proton beam, Los Alamos churns out data more rapidly than Rutherford does. We cannot exclude the possibility that they have seen something, says Kleinfeller. We will be able to do that in two years’ time.
Louis agrees that he and his colleagues need to take more data to really nail down the issue of neutrino mass, but he believes the additional work will confirm their current results. Physicists around the world-- especially those working on the solar neutrino problem--would like to see more evidence. If the Los Alamos group is right, the solar neutrino people still have a problem on their hands. The oscillation rate observed at Los Alamos is not enough to explain the solar neutrino shortfall.
Unfortunately, the physics community may be in for a long wait. The Los Alamos team has to deal not only with skeptical colleagues but also with a budget-conscious federal government. This fall the Department of Energy will be taking over the Los Alamos accelerator for defense-related work. After that, Louis and his colleagues will have only limited access to the accelerator.
Says Louis: We’re just hoping that we will somehow be able to get another ten months or more of running, which will triple our present data sample.