Wispy particles called neutrinos interact with matter slightly differently than expected, a team of physicists reports. The surprising observation could be evidence of a new force.
When a type of neutrino called a 鈥渕uon neutrino鈥� blasts through an atomic nucleus it can either remain a neutrino or change into a muon, a heavier cousin of the electron. Using the theory known as the standard model, particle physicists can predict precisely how often the neutrino will remain a neutrino.
However, the prediction is not quite right, say physicists working with the NuTeV experiment at Fermilab, outside Chicago. They studied more than three million collisions between high-energy muon neutrinos and iron nuclei. The neutrinos stayed neutrinos about one per cent less often than predicted, says Sam Zeller, a team member from Northwestern University in Evanston, Illinois.
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鈥淥ne percent may not sound like a lot, but this sort of measurement is so precise that one percent is a big deal,鈥� she says.
Extra weak force
The neutrinos interact with the quarks in a nucleus through the weak nuclear force, and the discrepancy might be evidence for a second weak force that interferes with the well-studied one.
The extra force would help explain other tantalizing oddities observed in the behavior of atoms, says Jens Erler, a theoretical physicist at the University of Pennsylvania in Philadelphia.
But the extra weak force might prove hard to reconcile with the results of other experiments at particle colliders, says Gordon Kane, a theorist at the University of Michigan in Ann Arbor. And it would not fit into the leading theories proposed to unify the four known forces, Kane says. 鈥淚鈥檓 very skeptical of this being a clue to new physics,鈥� he says.
The neutrino result is not part of any 鈥渄arling theory鈥� concedes NuTeV team member Kevin McFarland of the University of Rochester in New York. However, theorists do not always anticipate important results, he says: 鈥淪ometimes it happens the other way around, sometimes the experiments lead the theory.鈥�