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Physicists knew exactly what they were looking for. Over the last 40 years they had assembled the puzzle called the Standard Model of elementary particles. One crucial piece, however, remained undetected: the tau neutrino.

“It’s simply been accepted that this guy exists,” said Regina Rameika, a senior scientist on the Direct Observation of the Nu Tau (DONUT) experiment, an international collaboration of 54 scientists from Japan, Greece, Korea and the United States.

After sifting through more than two years of millions of particle events, the DONUT collaboration finally found the missing puzzle piece. On July 21, they announced the first direct evidence for the tau neutrino, the third kind of neutrino known to particle physicists. They reported four instances of a neutrino interacting with an atomic nucleus to produce a charged particle called a tau lepton, the signature of a tau neutrino.

Although earlier experiments had produced convincing indirect evidence for the particle’s existence, no experiment had ever directly observed the tau neutrino, a massless or almost massless particle carrying no electric charge and barely interacting with surrounding matter.

“It is one thing to think that there are tau neutrinos out there,” said Byron Lundberg, spokesman of the DONUT experiment. “But to really look for the rare incidence of a tau neutrino hitting a nucleus and transforming into a tau lepton is a hard experiment to do.”

The tau neutrino is the most elusive neutrino of the Standard Model, the theoretical description that groups all particles into three generations. First-generation electron neutrinos and their second-generation cousins, muon neutrinos, are easier to produce and detect than tau neutrinos. Experimenters identified them in 1956 and 1962 by recording neutrino interactions creating either electrons or muons. More than 30 years of technological advancement have now allowed physicists to observe the third-generation tau neutrino producing a tau lepton.

“Fermilab has once more shown its capability for research at the frontiers of particle physics,” said Secretary of Energy Bill Richardson. “The Department of Energy continues to make critical contributions to today’s understanding of the fundamental structure of matter, including pioneering experiments with all three neutrinos.” Forty-four years ago, scientists discovered the first neutrino at the Savannah River Plant of the Atomic Energy Commission. In 1962, scientists found the second, the muon neutrino, at Brookhaven National Laboratory.

In 1997, using Fermilab’s Tevatron accelerator, scientists produced an intense neutrino beam, which they expected to contain tau neutrinos. The neutrino beam crossed the three-foot-long DONUT target of iron plates sandwiched with layers of emulsion, which recorded the particle interactions. In the target, one out of one million million tau neutrinos interacted with an iron nucleus and produced a tau lepton, which left its one-millimeter-long tell-tale track in the emulsion. Physicists needed about three years of painstaking work to identify the tracks revealing a tau lepton and its decay, the key to exposing the tau neutrino’s secret existence.

“The tau lepton leaves a track in the layers of emulsion, just as light leaves a mark on photographic film, but in three dimensions,” explained Vittorio Paolone from the University of Pittsburgh, also a spokesman of the collaboration and a senior scientist at one of the six U.S. and six foreign universities involved in the experiment. “The main signature of a tau lepton is a track with a kink, indicating the decay of the tau lepton shortly after its creation.”

DONUT collaborator Koyu Niwa of Nagoya University, Japan, where the crucial emulsion analysis was centered, cited the experiment’s distinctive technology.

“The tau neutrino results confirm the value of advanced emulsion technology for today’s particle physics experiments and point to its future development for next-generation experiments,” Niwa said. Using special scanning stations with computer-controlled video cameras, scientists at Nagoya created 3-D images of the particle tracks recorded in the emulsion layers.

“It was the proverbial needle in a haystack,” said Lundberg. The DONUT experiment recorded a total of six million potential interactions. By analyzing signals from various components of the 50-foot-long detector, they winnowed out all but 1000 candidate events. Of these, four events provided evidence for the tau neutrino.

Physicist Leon Lederman, who, along with Jack Steinberger and Melvin Schwartz, received the Nobel Prize in 1988 for the discovery of the second-generation muon neutrino, commented on the DONUT results.

“Having participated in the research that established that there are two neutrinos, it would seem to be disconcerting to now learn that there are three. I can hear people complaining: ‘Can’t these guys make up their minds?’” Lederman said with a twinkle. “But seriously: The direct confirmation of the tau neutrino is an important and long-awaited result. Important because there is a huge effort underway to study the connections among neutrinos, and long awaited because the tau lepton was discovered 25 years ago, and it is high time the other shoe was dropped.”

Stanford University physicist Martin Perl, winner of the 1995 Nobel Prize for discovering the tau lepton, the first indicator for a third generation of particles, congratulated the DONUT experimenters.

“Finding the tau neutrino is very important and very exciting,” said Perl. “DONUT was not an easy experiment, and now it opens a whole new world. There is the possibility of the tau neutrino interacting somewhat differently from the other neutrinos. We might have a chance of learning more about all other particles.”

After scientists at SLAC discovered the tau lepton, physicists realized that there should be a corresponding neutrino—an hypothesis further supported by E531, a 1982 experiment at Fermilab. In 1989, experimenters at CERN and SLAC found evidence that the tau neutrino is the third light neutrino of the Standard Model, but a direct observation was not yet feasible.

The new direct evidence for the tau neutrino is far from closing the chapter on neutrino physics. Scientists are eager to learn whether neutrinos have mass, a result that would put a crack in the Standard Model, leading to major changes in our picture of the evolution of the universe. Experiments to answer these questions are underway in Japan, under construction at Fermilab and planned at CERN. The ability to detect tau neutrinos is an important step toward identifying non-zero neutrino masses.

Physicists packed a lecture hall at Fermilab on July 21 to hear DONUT spokesman Byron Lundberg announce the first direct evidence for the tau neutrino during a scientific colloquium. Following the discoveries of the bottom and top quark at the laboratory, scientists at Fermilab have now lifted the veil on a third elementary particle.

“I wish there would be more elementary particles so that we could do this more often,” Rameika said in her opening remarks.

The tau neutrino just happened to be a puzzle piece that physicists knew all about—they just hadn’t located it yet. Scientists have strong evidence that there must be other puzzle pieces of unexpected shape and structure. The Higgs particle is a prime candidate for such a piece, and experimenters seem to be zooming in on it. Further (supersymmetric) pieces may also appear at future experiments, challenging scientists to connect them with pieces found so far.

The DONUT collaborators, from Fermilab and the universities of Minnesota, Pittsburgh, Kansas State, Tufts, California at Davis and South Carolina; Nagoya, Kobe and Aichi (Japan); Gyeonsang and Konkuk (Korea); and Athens (Greece), will submit a scientific publication of the tau neutrino results to a major physics journal in the near future.

And in August, Carolyn Ericson and Jason Sielaff from the University of Minnesota, two of DONUT’s six graduate students, will present the tau neutrino results at a meeting of the American Physical Society.

“Let’s hope the graduate students each get an event,” Lundberg jokingly remarked, referring to the fact that the collaboration has only identified four events so far. The DONUT analysis is not yet complete. A second method of analyzing the data is still to be completed, and scientists hope to double the number of tau neutrino events.

As Lundberg reassured the audience, “There are still plenty of puzzles left in the universe to solve.”