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Physicists play key role in confirming existence of elusive neutrino

The Tufts' High Energy Physics Group: Professors Jacob Schneps, William Oliver and Tomas Kafka.

Four Tufts physicists played an important role in the recent discovery of the tau neutrino, the last piece in the puzzle referred to by physicists as the Standard Model of elementary particles. The Tufts researchers were members of an international team of 54 scientists who performed this crucial experiment at Fermilab, a laboratory near Chicago operated by the U.S. Department of Energy. The accelerator at Fermilab provided the experiment with a beam of the highest-energy protons available in the world.

The findings represent the culmination of six years of work constructing the apparatus, performing the experiment and analyzing the data. The discovery brings to a close a 25-year quest to validate the long-accepted hypothesis of the existence of the tau neutrino.

The Tufts team consisted of Professors Jacob Schneps, William Oliver, Tomas Kafka and Thomas Patzak. They are members of the Tufts High Energy Physics Group, which was founded by Professor Schneps in 1957.

"Particle physics investigates the most fundamental structures in the universe, out of which everything, including ourselves, is made," said Schneps. "It is gratifying to play a part in this important milestone."

Tufts had the responsibility of designing and building an array of several hundred particle detectors to form the muon identifier, one of the essential subsystems of the experiment. The researchers fabricated their detectors in the shop at the Tufts Science and Technology Center over a period of one year. The Tufts team loaded their completed array of detectors into a moving van, then flew to Chicago to greet their van on its arrival at Fermilab, where they assembled the detectors into six walls, each approximately the size of a two-car garage door.

The researchers benefited from the work of then Tufts undergraduate Emanuel Hemsi, A97, who contributed key elements to the design and construction of the detector walls.

To capture the tau neutrinos in action, the experimental team used the Fermilab high-energy protons to create a beam of neutral particles that they directed to a target formed by a three-foot-long sandwich of iron and photographic plates. The tau neutrinos were discovered by observing their characteristic interaction with atomic nuclei in the target to produce the particle called the tau lepton. Computer analysis of the data confirmed four tau lepton sightings out of the millions of events generated by the interactions of other beam particles that entered the target.

"The experiment was extremely difficult because the tau lepton lives only a tenth of a trillionth of a second," Schneps said. "It travels only a fraction of a millimeter before it disintegrates. And the events themselves are incredibly rare. Even after three years of analysis we found only four of them--but these four were very convincing."

Essential for the success of the experiment was the identification and measurement of the extremely short tracks left by the tau leptons in the photographic plates. This measurement was made possible by the recent development by the Japanese collaborators of technology to operate microscopes at high speed under computer control.

"The tau neurino was the missing piece of the puzzle, so it was clear what had to be done to discover it," said Oliver. "The problem was that what had to be done was very difficult. The difficulty may be appreciated by the analogy of looking for needles in a haystack, except that the needles are too small to be seen without using a microscope."

Although the tau neutrino discovery has as yet no practical applications, another experimental neutrino project is already under way. In this project the Tufts team is again working with an international collaboration to use the unique capabilities of Fermilab.

The new experiment is designed to measure the neu-trino mass by firing a neutrino beam from Fermilab to a detector now under construction in a mine in northern Minnesota. Because of the curvature of the earth, the beam neutrinos pass deep under Wisconsin en route to Minnesota. Neutrinos are a major component of the matter present in the universe, so evidence of a measurable neutrino mass would have profound implications on the fields of high-energy physics and astronomy.

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