Matter’s Missing Piece Shows Up

Physicists have found the first direct evidence for the last of the 12 subatomic particles considered the  fundamental building blocks of matter. A research team from the United States, Japan, Korea, and Greece has unveiled four sets of particle tracks that it attributes to the long-sought tau neutrino.

One particle per trillion in a suspected tau neutrino beam (from left) strikes an iron nucleus, spraying particles through layers of steel, plastic, and charge-detecting emulsion (narrow bars). A telltale tau lepton disintegrates (arrow), emitting a daughter particle at a new angle. Line segments in emulsion enable researchers to reconstruct particle tracks. Illustrations adapted from Fermilab

Newfound tau neutrino (third row, right) completes chart of fundamental matter particles. Rows contain similar particles. Columns show groupings (I, II, III) that grow more massive from left to right.

Elusive bits of matter with no charge and little or no mass, neutrinos interact extremely rarely with other matter, making them very difficult to detect. Until now, experimenters could find only hints of tau neutrinos’ presence, such as energy and momentum missing from decays of the particle called a tau lepton. Nonetheless, scientists haven’t doubted the existence of the tau neutrino, also known as nu tau. It’s part of the framework of the so-called standard model of particle physics.

“This [new finding] makes us feel sure that we do have the basic-matter particles,” comments Leon M. Lederman of Fermi National Accelerator Laboratory (Fermilab) in Batavia, Ill., one of the discoverers of the muon neutrino in 1961.

“It’s a very important step, maybe a dramatic step, because it gives us the confirmation that we’ve been looking for a long time,” adds John N. Bahcall of the Institute for Advanced Study in Princeton, N.J. The fundamental particles of matter are in two families, leptons and quarks. Physicists found the last of the six quarks 5 years ago (SN: 7/1/95, p. 10).

By the early 1960s, researchers had already discovered four leptons: the electron, its more massive and negatively charged cousin the muon, and a neutrino associated with each of them. With the 1975 discovery of the tau lepton—yet more massive than the muon and likewise negatively charged—physicists surmised that it must have a companion neutrino, too. Until now, however, experiments seeking direct evidence have come up empty-handed.

In principle, at least, each of the neutrino types can betray its presence by a reaction that takes place when the particle strikes a nucleus. These rare collisions create the neutrino’s electrically charged, massive sister lepton, which comes flying out with other debris.

If the sister particle is an electron or muon, scientists spot it relatively easily. However, being the most massive and shortest-lived lepton, the tau lepton travels only about a millimeter before breaking apart. Detectors that can pick up other neutrinos miss the fleeting object.

Byron G. Lundberg of Fermilab and his colleagues presented the new evidence at a seminar on July 21 at the lab. To capture submicrometer detail, Fermilab’s Direct Observation of Nu Tau, or DONUT, experiment recorded charged-particle tracks in an emulsion akin to that of black-and-white photographic film but finer grained Electronic detectors common in high-energy physics experiments can’t resolve such tiny traces, Lundberg says.

During 1996 and 1997, the team bombarded a tungsten target with protons from Fermilab’s powerful Tevatron accelerator. The scientists then filtered out unwanted particles to create what they presumed was an electrically neutral tau neutrino beam. They directed the beam at a stack of millimeter-thick steel layers interleaved with sheets of emulsion-coated plastic.

Since then, the researchers have been studying the tracks produced. A group at Nagoya University in Japan developed an automated system to examine millions of stubby traces in the thin emulsion layers. From those short lines, researchers reconstructed particle paths running through the plastic and steel.

In a minuscule fraction of DONUT traces, tracks spring from a point with no incoming particle trail. That’s to be expected when a neutral, invisible missile triggers the burst, researchers say. In each of four cases, or events, one track caroms off at a new angle after about a millimeter—the giveaway that a tau lepton from the tau neutrino-nucleus impact decayed into another charged particle plus neutral companions.

“The signature is a kink,” Lundberg notes. The probability that particles other than tau neutrinos produced the four events is less than a tenth of a percent, says DONUT’s Jacob Schneps of Tufts University in Medford, Mass.

Physicists say the new findings bolster their confidence as they embark on a new round of experiments to explore whether neutrinos have mass (SN: 1/30/99, p. 76). Researchers at the Super-Kamiokande detector in Japan, west of Tokyo, offered striking evidence 2 years ago that neutrino types may morph, or “oscillate,” into each other—a trick possible only if the particles weigh something (SN: 6/13/98, p. 374). Scientists also hope to test speculation, stemming from some oscillation experiments, that there is an even less interactive, or sterile, neutrino not anticipated by the standard model.

Researchers on the proposed European-based OPERA experiment intend to use a much larger emulsion stack to spot tau neutrinos produced from muon neutrinos during oscillations.

Fermilab’s success buoys that plan, DONUT members say. In DONUT’s wake, researchers also foresee developing tau neutrino beams that are more intense to enable in-depth studies of the particle’s properties.

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