Fundamental theory under question

Fermilab's control room: The result was a surprise

By BBC News Online science editor Dr David Whitehouse

Physicists may have found a flaw in the theory that for the last 30 years has successfully explained the behaviour of the fundamental building blocks of matter.

New measurements of neutrinos, ghostly sub-atomic particles that hardly interact with anything, indicate a surprising 1% discrepancy between predictions of their behaviour and the way they actually behave.

"One percent may not seem a big difference," said Professor Kevin McFarland, of the University of Rochester, New York US, "but the measurement is so precise that the probability that the predictions are right, given our result, is only about one in 400."

The findings, announced to puzzled scientists at the US Fermilab, the world's highest-energy accelerator, could mean that an unknown force or undiscovered particle is influencing the neutrinos.

One in a billion

Physicists designed the NuTeV (Neutrinos at the Tevatron) experiment to observe the interactions of millions of the highest-energy neutrinos ever produced.

The Fermilab experiment threw up more questions

Starting with a proton beam from Fermilab's Tevatron, the world's highest-energy particle accelerator, experimenters created a beam of neutrinos that were directed at a giant particle detector, a 700-tonne 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 another particle called 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.

The Tevatron: The world's highest-energy particle accelerator

If they saw nothing leaving, they knew a neutrino had come and gone. The scientists measured the ratio of muons to neutrinos and compared it with the predicted values according to the so-called Standard Model, which other experiments have verified to an accuracy of 0.1%.

But that is not what they observed.

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

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 that may solve this puzzle - or find new ones.

A paper describing the result has been submitted to Physical Review Letters.