Monday, July 11, 2011

Neutrinos and antineutrinos might "disappear" differently, going against the standard model

Scientists working on the MINOS experiment at Fermilab say they may have detected for the first time a difference in the way muon neutrinos and antineutrinos disappear. If they're right, it could change particle physics as we know it.

[The MINOS experiment Far Detector, located in northern Minnesota. Photo credit: Jon 'ShakataGaNai' Davis.]

A fundamental of the standard model of particle physics called "CPT symmetry" requires that the way a particle and its antiparticle disappears be the same. The MINOS experiment has shown that this may not be the case.

When a neutrino or antineutrino disappears, it doesn't actually go away. Instead, it changes flavors. This change is known as an oscillation. MINOS studied the oscillations of both neutrinos and antineutrinos and found that they were different.

The difference may be a fluke, however, for a couple of reasons, says physicist William C. Louis who wrote a Physics Viewpoint about the research. It could just be a statistical fluctuation - a phantom in the data. Or, it could be due to the way the neutrinos' and antineutrinos' energies are measured.

MINOS will take more data in the coming years and study it for the same fluke. Several other experiments could also be tasked to look for the difference. Fermilab's SciBooNE and MiniBooNE experiments are currently doing joint analysis of their data on neutrino and antineutrino disappearances to see what they find. The IceCube neutrino detector, recently completed at the South Pole, also measures high-energy atmospheric neutrinos and antineutrinos.

If more experimental data shows there is a difference, it would mean that scientists might have to consider the possibility of physics outside the standard model. That, says Louis, would have a huge impact in the nuclear and particle physics world.

The MINOS (Main Injector Neutrino Oscillation Search) experiment at Fermilab detects neutrinos produced by the NuMI (Neutrinos at the Main Injector) beam. The experiment consists of two neutrino detectors, one, the Near Detector, a kilometer away from the NuMI beam near Chicago, Ill., and the other, the Far Detector, 735 km away in northern Minnesota.

The research was published in Physical Review Letters on July 5.

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