Exotic Particle Changes Flavor as Scientists Watch

The MINOS neutrino experiment is located in a cavern half a mile deep in the Soudan Underground Laboratory, Minnesota. A mural of famous scientists is painted onto the rock wall. CREDIT: Fermilab View full size image

Scientists have observed the rare phenomenon of one type of exotic particle transforming into another, which could reveal secrets about the evolution of the universe.

The particles are two types of chargeless, nearly massless species called neutrinos, which come in three flavors: muon, electron and tau. In past experiments, physicists have measured the change of muon neutrinos to tau neutrinos and electron neutrinos to muon or tau neutrinos, but no one has definitively seen muon neutrinos turn into electron neutrinos.

Now, two separate experiments — one in Japan and one in Minnesota — have both found evidence for this transformation as well.

Detecting neutrinos

Scientists of the Main Injector Neutrino Oscillation Search (MINOS) experiment at the Department of Energy's Fermi National Accelerator Laboratory announced their findings today (June 24). The results are consistent with, and significantly constrain, a measurement reported 10 days ago by the Japanese Tokai-to-Kamioka (T2K) experiment, which announced an indication of this type of transformation. [Strange Quarks and Muons, Oh My! Nature's Tiniest Particles]

The MINOS study sent a beam of muon neutrinos 450 miles (735 kilometers) through the Earth, from the Main Injector accelerator at Fermilab in Batavia, Ill., to a 5,000-ton neutrino detector, located half a mile underground in the Soudan Underground Laboratory in northern Minnesota.

The neutrinos' trip from Fermilab to Soudan takes about four hundredths of a second, giving the neutrinos enough time to change their identities.

MINOS recorded a total of 62 electron neutrino-like events, which is a likely indication that there were 62 electron neutrinos present at Soudan. If muon neutrinos didn't transform into electron neutrinos, MINOS should have seen only 49 events. The T2K experiment showed 71 such electron-neutrino events, though the two experiments use different methods and analysis techniques to look for this rare transformation.

The balance of matter

The new finding could have major implications for our understanding of the history of the universe. If muon neutrinos can transform into electron neutrinos, neutrinos could be the reason that the Big Bang produced more matter than antimatter, leading to the universe as it exists today. To solve this mystery, scientists want to calculate how often different flavors of neutrinos change into each other, and compare that with the rate of change among neutrinos' antimatter partners, antineutrinos.

If it turns out that the rules of transformation are different between neutrinos and antineutrinos, that asymmetry could help explain why matter vastly outnumbers antimatter in the universe.

MINOS will continue to collect data until February 2012. The T2K experiment was interrupted in March when the severe earthquake in Japan damaged its muon neutrino source. Scientists expect to resume operations of the experiment at the end of the year.

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