Physicists shooting neutrinos underground 185 miles across Japan have for the first time detected muon neutrinos transforming into electron neutrinos, an observation that is a first step toward understanding why normal matter predominated over antimatter in the Big Bang that created the universe.
Researchers have previously observed other kinds of neutrino oscillations — muon to tau and tau to electron — but it was not clear if the muon-to-electron transformation occurred frequently enough to allow researchers to observe and measure it. The fact that the transformation can be observed “has the whole neutrino community very excited,” said MIT physicist Janet Conrad, who was not involved in the research.
Once the observations have been confirmed, scientists will then be able to conduct a direct test of the symmetry between matter and antimatter by performing the same experiment with anti-muon neutrinos.
Physicists suspect that neutrinos and antineutrinos might behave slightly differently in the presence of gravity and electromagnetic fields, a so-called charge-parity or CP violation. Such violations might account for the now-overwhelming abundance of normal matter in the universe.
CP violations have previously been observed for the fundamental particles called quarks — a feat that won two Nobel prizes — but “the amount of the CP violation is not enough to explain the asymmetry” between matter and antimatter in the universe, said physicist Chang Kee Jung of Stony Brook University, a spokesman for the research collaboration that includes scientists from 12 countries.
Physicists now hope to trace the rest of that CP violation to neutrinos, which would be “a major step forward in our understanding of the nature of neutrinos and the universe,” he said.
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 passing through a window pane. They have no electrical charge and there are three types: muon, tau and electron, each named for the particle produced when they collide with an atom.
Neutrinos were originally thought to have no mass, but results obtained with the Super-Kamiokande detector in 1998 proved for the first time that they do. Super-Kamiokande is buried in an old zinc mine 3,250 feet under Mt. Ikena near Kamioka in the Japanese Alps. The cylindrical detector contains 12.5 million gallons of ultra-pure water and is lined with an acre of photomultiplier tubes that detect light emitted when neutrinos are destroyed in collisions with water molecules.
In the Tokai to Kamioka, or T2K, collaboration, muon neutrinos created at the Japan Proton Accelerator Research Complex in Tokai are aimed at Super-Kamiokande, nearly 200 miles away. Detectors at Tokai determine how many muon neutrinos leave the facility and detectors at Kamioka measure how many arrive there. It also observes electron neutrinos directly.
The most precise measurements of CP violation in neutrinos can be made only by monitoring the muon-to-electron neutrino transformation, Conrad said. At least three different major collaborations are now trying to observe this transformation, and it appears that the Japanese project is now in the lead.
The team in Japan observed what it believed were six muon-electron transformations between the time the project began in 2010 and March of this year, when the experiment was cut short by the magnitude-9 Tohoku earthquake. Theory predicts that background radiation would have produced only 1.5 such transformations.
The six events represent about 2% of the total that scientists had expected to observe during the course of the experiment. But the team announced its preliminary results at a news conference Wednesday because it will be unable to make any more measurements until at least the end of the year, when the earthquake damage to the proton accelerator should be repaired.