Physicists looking to shed more light on dark matter

British and Japanese scientists at the multinational T2K particle-physics project in Japan said Friday that they have observed the experiment’s first neutrino to travel 185 miles underground across the Asian country, indicating that the project is now ready to begin doing physics.

Although not on the massive scale of Europe’s new Large Hadron Collider, which is also just beginning to conduct experiments, the T2K project is expected to shed light on the oscillations of the mysterious elementary particles known as neutrinos and, in the process, perhaps explain why there is more matter than antimatter in the universe.

Neutrinos are ghost-like particles that travel at the speed of light and interact with matter so weakly that they can travel through the entire Earth with the ease of a light beam traveling through a windowpane. They have no electrical charge -- hence the name, meaning “little neutral one.”

Physicists rarely see neutrinos. Instead, they observe the debris left behind on the rare occasions when a neutrino strikes an atom head on. They now know that there are three types of neutrino: electron, muon and tau neutrinos, each named for the particle that is produced in the collision. Each of those also has an antimatter counterpart.

The key player in studying the neutrino is the massive Super-Kamiokande detector, buried in an old zinc mine 3,250 feet under Mt. Ikena near Kamioka in the Japanese Alps. The massive cylindrical detector contains 12.5 million gallons of ultra-pure water and is lined with an acre of photomultiplier tubes, which detect light from neutrino collisions and convert it into an electrical signal.


Neutrinos were originally thought to have no mass. But results obtained with Super-Kamiokande in 1998 startled researchers by showing that they do.

The detector monitored neutrinos produced by cosmic rays striking the atmosphere directly above Japan and those produced in the same fashion on the opposite side of the Earth, about 8,000 miles away, detecting an average of about 5.5 neutrinos daily. Physicists found about half as many neutrinos coming from the opposite side of the Earth as from directly above. Because the detector cannot observe tau neutrinos, their finding meant that the neutrinos were oscillating, changing from muon neutrinos to tau neutrinos as they passed through the extra distance to the detector.

And if they are oscillating, that means they have to have mass. Not much, only about one ten-millionth the mass of an electron. But for each electron in the universe, there are an estimated 50 billion neutrinos, so neutrinos could account for a large part of the bulk of the universe -- the so-called dark matter.

The T2K, or Tokai to Kamioka, project was set up to explore those oscillations. British and Japanese researchers constructed a beamline to produce neutrinos at the J-PARC accelerator in Tokai, about an hour north of Tokyo by train, and pointed it at the Super-Kamiokande. An accelerator at J-PARC produces a beam of protons that are focused and directed at a carbon rod target, where they produce particles known as pions. The pions are focused into a helium-filled container, where they decay into neutrinos that can then proceed to Kamioka.

Two neutrino detectors at Tokai measure the neutrinos’ properties as they leave the laboratory, while the third at Super-Kamiokande measures them after they have had a chance to oscillate. Among other things, the observatory hopes to detect differences between the behavior of neutrinos and that of anti-neutrinos.

Detecting the first neutrino from Tokai at Super-Kamiokande “is a big step forward,” Takashi Kobayashi, a spokesman for the project, said in a statement. “We have been working for more than 10 years to make this happen.”

Added physicist Dave Wark of Imperial College London, “The observation of this first neutrino means that the hunt has just begun.”