November 7, 2001

Matter Isn't What It Used To Be: Neutrino Study Finds Flaw in Standard Model of Matter

Physicists appear to have uncovered a flaw in the model that for the last 30 years has successfully explained the workings of the universe. Their measurements indicate a surprising one percent discrepancy between predictions for the behavior of neutrinos and the way the elusive subatomic particles actually behave. The findings, announced at Fermilab, the world's highest energy accelerator, could mean that an unknown force or undiscovered particle is influencing the neutrinos.

"One percent may not seem a big difference," says Kevin McFarland, assistant professor of physics and astronomy at the University of Rochester and team leader of the project, "but the measurement is so precise that the probability that the predictions are right, given our result, is only about one in 400."

McFarland and the members of his team slammed a 100 million watt beam of protons into a target to produce tens of billions of neutrinos. Working at Fermilab over the course of eight years, they observed millions of interactions of the highest energy, highest intensity neutrinos ever produced. Extremely precise experiments on other particles have spelled out exactly how neutrinos should behave, but to the experimenters' surprise, when they looked at neutrinos with comparable precision, neutrinos did not appear to fall into line with expectations.

"It might not sound like much, but the room full of physicists fell silent when we first revealed the result," said physicist Sam Zeller, a graduate student from Northwestern University and collaborator on the experiment.

The neutrino is one of the fundamental particles that make up our universe. Neutrinos carry no charge, unlike positively charged protons or negatively charged electrons, so the only thing that effects them is the "weak force"-a force whose effects are usually only seen inside the nucleus of an atom. As a result, neutrinos rarely interact with anything, making them extremely difficult to detect. The sun emits incomprehensibly vast numbers of neutrinos-about 1,000 trillion pass through your body every second.

Physicists designed the NuTeV (Neutrinos at the Tevatron) experiment in order to observe the interactions of millions of the highest-energy, highest-intensity neutrinos ever produced. Starting with a proton beam from Fermilab's Tevatron, the world's highest-energy particle accelerator, experimenters created a beam of neutrinos directed at a giant particle detector. The detector itself was a 700-ton sandwich of alternating slices of steel and detector. As the beam passed from the first to the last slice, one in a billion neutrinos collided with a target nucleus, breaking it apart. After the collision with a nucleus, the neutrino could either remain a neutrino or turn into a muon, a particle that is a heavier cousin of the electron. When experimenters saw a nucleus break up, they knew a neutrino had interacted. If they saw a particle leaving the scene of the collision, they knew it was a muon. If they saw nothing leaving, they knew a neutrino (invisible to the detector's "eye") had come and gone. The scientists measured the ratio of muons to neutrinos and compared it with the predicted values, which other experiments have verified to a part per thousand accuracy for other particles. A painstaking years-long analysis of the data revealed the unexpected discrepancy.

Neutrinos have surprised particle physicists before, but the new data have left the experimenters wondering if their neutrinos have felt a new force previously unobserved in nature, or if there is some hitherto undiscovered particle influencing neutrino interactions.

Physicists in the United States, Japan, and Europe are planning a next generation of neutrino experiments which may solve this newly uncovered puzzle-or which may find even more puzzles. Other physicists, working at Fermilab or at CERN accelerators, could be observing previously unknown particles that may influence the neutrino.

McFarland has given a presentation on the measurement at Fermilab, and a paper describing the result has been submitted to Physical Review Letters. In addition to the University of Rochester, the 45-member collaboration included physicists from the University of Cincinnati, Columbia University, Fermilab, Kansas State University, Northwestern University, the University of Oregon, and the University of Pittsburgh. The research was supported by the National Science Foundation, the U.S. Department of Energy, and the Alfred P. Sloan Foundation.

Fermilab is operated by Universities Research Association, Inc. under a contract with the U.S. Department of Energy.