Blasting Neutrinos Under Wisconsin May Yield Big Payoff
Monday, August 17, 2009
Scientists are playing an exotic game of pitch and catch between Illinois and Minnesota. Their catcher's mitt is solid iron, weighs 5,500 tons, and is parked in northern Minnesota in an abandoned iron mine. With millions of dollars from the federal stimulus package, construction crews are now building a second mitt near the Canadian border. It's even heavier, some 15,000 tons, and is made of 385,000 liquid-filled cells of PVC plastic.
Five hundred miles to the south is the pitcher: Fermilab, a sprawling U.S. government laboratory west of Chicago where physicists do violent things with tiny particles.
The objects in flight are very strange particles called neutrinos. Fermilab scientists have figured out how to generate a beam of neutrinos and send it across Wisconsin to the big detectors in northern Minnesota.
Make that under Wisconsin. Because the Earth is round, anyone wishing to send an object in a straight line from one spot on the planet to another spot 500 miles away must aim through the planet itself.
Here's where the really weird physics kick in: Neutrinos blast right through the Earth with nary a spark. They interact so rarely and so weakly with normal matter that they can zip right through solid rock as though it were not even there -- much like light through a clear glass window. That's why, contrary to the hopes of some private contractors who heard about a big new experiment under construction, Fermilab does not need to dig a tunnel underneath Wisconsin.
A common adjective applied to neutrinos is "ghostly." They have no charge. Until recently, it was unclear if neutrinos had any mass at all (they do, but just a smidgen). Trillions of neutrinos from the sun pass through our bodies every minute, scientists say. You could be hit with a neutrino beam right between the eyes without getting so much as a blemish.
"These neutrinos are a type of matter that essentially form a shadow universe," said Marvin Marshak, a University of Minnesota physicist working on the new neutrino experiment, called Nova. "They share space with us, but they have very little interaction with us. So you have neutrinos going through your body all the time -- neutrinos from the sun, neutrinos from the cosmic rays coming down from space, neutrinos left over from the birth of the universe -- but they go right through you."
Nova had been shelved due to a funding shortfall, but it's back in business thanks to the American Recovery and Reinvestment Act of 2009 -- also known as the federal stimulus package. Forty million dollars are going to the Nova detector in Minnesota and another $103 million are heading to Fermilab for a variety of projects, including one to boost the power of the neutrino beam.
Fermilab is famous for its Tevatron, an atom-smasher housed in a tunnel beneath the Illinois prairie. The Tevatron is going full blast as it searches for the Higgs boson, a particle theorized to exist. In the Higgs search, Fermilab is in something of a race against the new accelerator near Geneva, the Large Hadron Collider, which is currently under repair after a problem-plagued start-up last year.
Fermilab is also busy generating neutrinos. The physicists accelerate protons to nearly the speed of light and smash them into a target. The collision creates a spray of new particles that, in turn, decay into neutrinos. A magnetic lens called a horn then focuses the particles into a beam. This neutrino beam is already being used for an experiment called Minos (for Main Injector Neutrino Oscillation Search). That uses the existing neutrino detector in the iron mine in Soudan, Minn. It's an octagonal cylinder made up of 485 layers of iron. It looks like a giant stop sign.
Of the countless neutrinos fired its way, Minos catches only one or two a day. That's in part because, by the time the neutrino beam reaches northern Minnesota, it has become more diffuse. The beam soon leaves the planet entirely and travels into outer space.
The new detector under construction is part of Nova, which will search for evidence of what is known as neutrino oscillation. There are three families of neutrinos: muon, electron and tau. One type can "oscillate" into another. Nova will look for evidence of muon neutrinos turning into electron neutrinos.