Indiana University Research & Creative Activity

Mind/Brain

Volume 30 Number 2
Spring 2008

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neutrino illustration
A neutrino signal as observed by the MiniBooNE experiment.
Photo courtesy Fermilab

Ghost Particles and the Mystery of Mass

by Hal Kibbey

Three types of neutrinos are found in nature, yet scientists know very little about these "ghost particles." Neutrinos are the least understood of all the fundamental particles, despite being among the most abundant particles in the universe. The universe’s abundance of neutrinos may explain how galaxies formed and why antimatter has disappeared. Ultimately, these elusive particles may explain the origin of the neutrons, protons, and electrons that make up all the matter in the world around us.

Physicists from Indiana University Bloomington are part of an international collaboration of scientists at Fermi National Accelerator Laboratory in Batavia, Ill., that is gaining new insights into neutrino oscillation, in which neutrinos change from one kind to another. One project, called MINOS, has confirmed that the neutrino has a tiny mass--not zero mass, as was thought for many years.

"When we sent a beam of one kind of neutrinos from the Fermilab site straight through Earth to a particle detector 450 miles away, we detected fewer of these neutrinos than we expected," says Mark Messier, one of the IU physicists involved in MINOS. If neutrinos had no mass, the particles would not have changed as they passed through the Earth, but some of the neutrinos had changed. This was a clear observation of neutrino oscillation and hence neutrino mass.

The total mass of all neutrinos may equal the mass of all visible matter--such as the stars and planets--in the universe today, affecting the evolution of the largest structures of the universe.

"At the moment, a property as basic as the mass of neutrinos is unknown," Messier says. "The smallness of the neutrino mass raises the question of why it is so much smaller than its companion particles such as the electron. This may indicate that the mechanism that leads to neutrino mass fundamentally differs from the mechanism that leads to the masses of the other particles."

IU Bloomington professors Stuart Mufson, James Musser, and Jon Urheim also are heavily involved in the MINOS project, which includes about 150 scientists, engineers, technical specialists, and students from 32 institutions in Brazil, France, Greece, Russia, the United Kingdom, and the United States.

In another project at Fermilab called MiniBooNE, a team of scientists from 17 universities and laboratories recently tested the current structure of particle physics, known as the Standard Model of Particles and Forces. For the time being, at least, the Standard Model appears safe.

The results from the Fermilab experiment resolved questions that were raised by an experiment at Los Alamos National Laboratory in the 1990s, which appeared to contradict findings of other neutrino experiments worldwide. The goal of the Fermilab group was to either confirm or refute the Los Alamos observations, which have troubled the neutrino physics community for more than a decade.

Several experiments have shown that the three types of neutrinos can oscillate from one type to another and back. However, the Los Alamos observations implied the presence of a fourth type of neutrino. Finding evidence that the new type of neutrino really exists would have thrown serious doubt on the Standard Model. Instead, the Fermilab experiment decisively ruled out the interpretation of the Los Alamos lab as being due to neutrino oscillations with the standard description. The ultimate cause of the Los Alamos observations is being investigated with further running of the MiniBooNE experiment.

Rex Tayloe, a member of the IU team at Fermilab, says, "It has been established that neutrinos have mass, but they are much lighter than any other particle, such as the quarks inside a proton or electron. If we can understand the mass of neutrinos by studying neutrino oscillation, we can perhaps understand some of the mystery of mass in general."

Hal Kibbey, a longtime science writer for Indiana University, is now a freelance writer in Bloomington.