Neutrino shortage may signal new force

When the subatomic particles called neutrinos slam into atomic nuclei, the invisible missiles don’t behave quite as expected, new findings indicate. While the discrepancy is small, researchers say its significance could be great. It may point to a previously unrecognized fundamental force of nature.

So far, physicists have identified only four such forces: gravity, electromagnetism, and the weak and strong forces. An analysis unveiled late last month at Fermi National Accelerator Laboratory (Fermilab) in Batavia, Ill., suggests that another type of weak force may have influenced neutrinos in an experiment conducted there in 1996 and 1997.

During that time, experimenters fired an extraordinarily intense beam of the highest-energy neutrinos ever produced in a lab at steel plates in a 700-ton detector. The beam consisted solely of muon neutrinos, one of the three types of neutrinos. On the rare occasion when a muon neutrino struck an iron nucleus in the steel, the neutrino either would transform into a muon, which is a heavy cousin of an electron, or remain a neutrino.

After years of analysis of the neutrino-nucleus collisions, a surprise emerged: About 1 percent fewer collisions produced neutrinos than predicted by the standard model of particle physics.

If not just a fluke or evidence of some unexpected trait of neutrinos, the disparity may be the handiwork of a previously proposed extra force (SN: 1/15/00, p. 39), says Kevin S. McFarland of the University of Rochester (N.Y.), a member of the research team that conducted the experiments. A heavy particle known as Z9 would transmit the force, which would be about one-hundredth as strong as the weak force.

Recent upgrades to Fermilab’s particle collider are fueling hopes of directly producing the hypothetical Z9. In pursuit of that goal, “we’re first up at the plate,” McFarland says. No other lab will have the expected capability for years.