Zapped neutrinos zip through the Earth

• 12:57 07 March 2005
• NewScientist.com news service
• Kelly Young

Scientists are to zap neutrinos through the Earth to better understand the mysterious particles' shifting nature with a new experiment at the Fermi National Accelerator Laboratory in Illinois, US.

Neutrinos are subatomic particles with no charge. They exist in three forms - or "flavours" - called electron, muon and tau neutrinos. The $180 million experiment, called the Main Injector Neutrino Oscillation Search, was launched on Friday to study how neutrinos oscillate between flavours. Previous experiments to detect this shifting state have been in uncontrolled environments - using neutrinos from space. But this new project is the first controlled experiment of its kind.

"Whenever you do an experiment, you much prefer to do it in such a way that you can control the experimental situation," explains Stan Wojcicki, a physics professor at Stanford University, California, US.

Scientists at Fermilab will zap neutrinos underground from Illinois, across the state of Wisconsin, through Lake Superior to a particle detector in the Soudan Underground Mine in Minnesota State Park, US. A trip of 457 miles (735 kilometres).

Building blocks

Neutrinos are all around the universe - cooked up in the nuclear reactions of stars. They zip through all matter with little effect. They have no charge and almost no mass, which makes them extremely difficult to detect. Yet researchers suspect they could account for a bulk of the mass of the universe, including elusive dark matter. Neutrinos may also play a role in the creation of protons, neutrons and electrons - the building blocks of atoms.

For this new experiment the Neutrinos at the Main Injector (NuMI) instrument - buried underground at Fermilab - will generate a beam of protons. The beam travels through a 1.2-kilometre-long tunnel on a slight downward angle and to the north.

In the tunnel, the beam will pass through a piece of carbon. The protons interact with the carbon and produce short-lived particles called pions, which mainly decay into muon neutrinos. A detector near Fermilab will measure the neutrinos as they start their journey. The neutrinos will travel underground from Illinois to the far away detector in Minnesota.

Altered states

When the project was being designed, putting the detector in a mine was an easy way to shield it from cosmic rays, which would interfere with the measurements.

The neutrinos travel close to the speed of light. The 457-mile trip to the detector takes just 2.5 milliseconds. But in that time, as the muon neutrinos pass through the earth, some of them may change states to become tau and electron neutrinos.

By comparing the results from the near detector with those of the target detector, researchers hope to see exactly how the composition of the particle beam has changed.

Trillions of neutrinos will pass through the MINOS detector annually. But the detector will only pick up about one in a million of them because neutrinos rarely interact with other materials.