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Particle physics: Neutrinos' changing face

  • 2010/08/26
The standard model of particle physics may need revising after the first evidence of neutrino oscillations

Fig. 1: Neutrinos generated at a CERN facility in Switzerland are beamed through the Earth toward an underground laboratory at Gran Sasso in Italy. The OPERA experiment aims to detect a change in the type of a neutrino on its 2.4 millisecond journey.

An electron is an electron. It doesn't decay radioactively and doesn't change spontaneously on its own into, for example, a proton. Yet the electron has a close relative, the neutrino, which has been postulated to change its type periodically. Any observation of these 'oscillations' between types would force revisions to our fundamental understanding of the Universe, because in the conventional standard model of particle physics, the existence of such oscillations is forbidden.

The first direct evidence of a neutrino oscillation has now been obtained by researchers involved in the international OPERA experiment using the facilities of the European Organization for Nuclear Research (CERN) in Switzerland and Italy1. Twenty-four Nagoya University researchers participated in the study, including Mitsuhiro Nakamura from the Quest for Fundamental Principles in the Universe GCOE program. "Although we must accumulate more data for a firm confirmation, this is the first direct observation of neutrino oscillations," says Nakamura.

Neutrinos come in three types known as 'flavors': electron, muon and tau. In 1962, theoretical physicists Ziro Maki, Masami Nakagawa and Shoichi Sakata from Nagoya University first proposed that neutrinos can change their type; for example, a muon neutrino could transform spontaneously into a tau neutrino.

Previous experiments have inferred indirectly the existence of such oscillations, but direct verification has remained elusive. The experiments are difficult because neutrinos are notoriously elusive particles that interact very little with matter. In fact, more than 50 trillion neutrinos travelling at close to the speed of light pass through our bodies every second.

The OPERA experiment is based on an automated nuclear emulsion read-out system that studies neutrino interactions in 1,300 tons of emulsion 'bricks', consisting of nuclear emulsion films and lead plates, housed in an underground facility at Gran Sasso National Laboratory in Italy. The neutrinos are produced at the CERN particle physics laboratory in Geneva, Switzerland, some 730 km away -- just 2.4 milliseconds of flight time for a neutrino (Fig. 1).

The latest research is the culmination of two years of experiments, including about 3,000 detected neutrino interactions. What the scientists were looking for was evidence of the arrival of a tau neutrino from the stream of muon neutrinos sent from CERN. What they found after analyzing the two-years of data was the first strong candidate for a tau neutrino based on its characteristic energy and decay pattern.

More experiments are needed for an unambiguous verification of neutrino oscillations, and the OPERA experiment will collect data until 2012. The researchers are optimistic that the observations will strengthen their findings. Beyond experimental confirmation, physicists are also eager to learn more about these unique particles, says Nakamura. "Precise measurement of the neutrino oscillation probabilities will be the next step."

Affiliated Researchers

The Nagoya University affiliated researchers mentioned in this highlight are from the Quest for Fundamental Principles in the Universe (QUEST) GCOE program.

Reference
  1. Agafonovaa, N., Aleksandrovb, A., Altinokc, O., Ambrosiod, M., Anokhinae, A., Aokif, S., Arigag, A., Arigag, T., Autieroh, D., Badertscheri, A. et al. Observation of a first ντ candidate in the OPERA experiment in the CNGS beam. Phys. Lett. B 691, 138-145 (2010). | article

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