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Thursday, 20 July, 2000, 13:21 GMT 14:21 UK
Neutrinos fired through Earth's crust
Super-Kamokande Consortium
A typical flash of light from a detected neutrino
By BBC News Online science editor Dr David Whitehouse

Japanese scientists have sent a beam of neutrinos through the Earth's crust in an experiment to probe the particles' fundamental properties.

Physicists have long suspected that neutrinos, which travel at almost the speed of light, can change between three types.

But the observational evidence gathered so far has yet to convince all researchers.

Now, the K2K (KEK-to-Kamioka) Long Baseline Neutrino Oscillation Experiment appears to confirm the effect.

Stellar evolution

Neutrinos, sub-atomic particles, are among the fundamental building blocks of matter.

They are released when the Sun fuses hydrogen to make helium - the basic nuclear process that provides the energy to drive life on Earth.

KEK
The neutrinos set off from the KEK
Detecting and understanding all the different types of neutrino would provide a very direct test of what we believe goes on inside stars and how they evolve over millions of years.

But picking them up is not an easy task because neutrinos interact so rarely with normal matter, and the few facilities that can manage it are built on a grand scale.

In the Japanese experiment, the KEK particle generator fired the so-called muon-type neutrinos through the Earth's crust at the Super-Kamokande detector 250 km (155 miles) away.

In firing the beam of neutrinos, the scientists hoped the particles would have time to change by the time they arrived at the detector. And this is precisely what appears to have happened - fewer neutrinos were picked up because some had changed during the trip to an undetectable type.

Giant water tank

The neutrinos were detected in the Super-Kamokande facility's 250,000-tonne tank of water. Neutrinos will interact with water molecules to produce a brief but detectable flash. Most neutrinos, however, will pass through the tank unhindered.

Super-Kamokande Consortium
The particles were picked up in the Super-Kamokande detector tank
The experiment saw its first detection event in June 1999. By March 2000, 17 neutrinos had been detected.

A neutrino detector back at the KEK particle generator gave physicists some idea of how many neutrinos were being generated and this information allowed them to calculate how should be picked up in the Super-Kamokande tank.

If neutrinos did not change from one type to another, it was estimated that about 29 neutrino events should be expected at the Super-Kamokande detector.

That 17 were found, the researchers say, is statistically significant and shows that neutrinos do indeed oscillate between three types called the electron-type, the tau-type and the muon-type.

Observed and expected

To confirm that the neutrinos came from the generator, the time of each detection at the Super-Kamokande detector had to coincide with the time at which the neutrino beam was shot from the KEK.

Account also had to be taken of the particles' time of flight between the two locations. The agreement between the observed and expected times was found to be better than a millionth of second.

Last year, Super-Kamokande scientists reported strong evidence for neutrino change when they examined particles travelling through the Earth's atmosphere. It seems that muon-type neutrinos change to tau-type neutrinos as they pass through our planet's gaseous envelope.

Physicists announced the results of the K2K experiment at the Neutrino 2000 Conference held in Sudbury, Canada.

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See also:

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