By Dr David Whitehouse
BBC News Online science editor
A study involving the world's largest underground tank of water has concluded neutrino particles are a mystery.
The photomultipliers of Japan's Super-Kamiokande detector
Neutrinos do not interact very much with matter, but they can be detected as flashes of light in the 50,000-tonne Super-Kamiokande detector in Japan.
Many of them come from the Sun while others are formed in the Earth's atmosphere by cosmic ray impacts.
The new data confirms that they change as they travel through space which is contrary to current theories of matter.
For five years a team of more than 100 physicists from around the world have studied the flashes of light detected by the photomultiplier tubes that line the Super-K tank, a facility sited in the Kamioka Mozumi mine.
This so-called Cerenkov radiation is given off by charged particles created in the tank by the passage of neutrinos.
Neutrinos are an enigma. They interact only very weakly with normal matter; billions of them come streaming out of the Sun's core unhindered by the mass of the star.
Scientists look for flashes in the tank
And some neutrinos are created by high-energy cosmic rays from deep space striking air molecules in the Earth's upper atmosphere. These neutrinos have proven very profitable to study.
In 1998, Super-K researchers established that neutrinos could change their type, or oscillate, as they travel through space.
Now, further work, to be published in the journal Physical Review Letters on behalf of the Super-Kamiokande Collaboration, essentially confirms and refines the earlier observations.
This presents some problems for physicists. The data indicates that the so-called Standard Model, proposed in the 1970s to describe the fundamental forces and particles that make up all matter, is incomplete.
"These findings show that the Standard Model needs to be modified to better explain the fundamental forces that make up all matter," says Professor James Stone, of Boston University, US, and a co-principal investigator on the Super-K experiment.
The latest research confirms the cosmic-ray produced neutrinos come in two "flavours", called electron-type and muon-type.
According to current ideas, neutrinos should only be able to change their flavours if they have mass - but the Standard Model does not allow for this.
After the first indications that neutrinos could oscillate, there remained a few unusual theories that roughly explained the phenomenon. But these ideas have now been ruled out by the latest results, says the Super-K Collaboration.