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CERN Set to Study Sterile Neutrinos

A new experimental facility to detect a hypothetical particle that many physicists think probably doesn't exist could be up and running at the CERN laboratory near Geneva, Switzerland, within 3 years, assuming that the lab's member states approve spending roughly $110 million to build it.

The sterile neutrino, if it exists, would be an obscure variety of an already otherworldly subatomic particle. Ordinary neutrinos, which have no charge and almost no mass, come in three varieties: electron, muon, and tau. They are very hard to detect because their interaction with ordinary matter is extremely feeble, but over the years physicists have detected enough of them to observe that as they travel through space they can "oscillate" from one flavour to another.

This oscillation phenomenon, which means that neutrinos cannot be entirely massless, has been confirmed by many different experiments. But one such experiment produced results at odds with the rest. That was the Liquid Scintillator Neutrino Detector (LSND) at the Los Alamos National Laboratory in New Mexico, which in data acquired between 1993 and 1998 showed muon antineutrinos to be oscillating into electron antineutrinos far more readily than expected.

The Los Alamos researchers suggested that sterile neutrinos might explain the anomaly by providing an invisible missing link. The idea is that the muon antineutrinos oscillate into sterile antineutrinos—which cannot be detected directly—and that these in turn oscillate into electron antineutrinos. Because the ease with which one neutrino oscillates into another is related to the difference in those particles' masses, a suitably heavy sterile neutrino could explain the greater than expected number of electron antineutrinos. Plus, if they were discovered, sterile neutrinos might account for a significant fraction of dark matter in the universe.

The idea has proved controversial, however, because while some other experiments have also obtained tentative evidence for sterile neutrinos many others have seen no sign of them at all. The new project aims to try and resolve the issue once and for all. It would involve tapping protons from CERN's existing Super Proton Synchrotron (SPS) accelerator, building a tunnel to direct those protons onto a solid target. The resulting beam of muon neutrinos would then pass through two pairs of detectors, with one pair at 400 meters from the target and the other at 1600 meters. One detector in each pair would be built from iron and would largely measure muon neutrinos while the other would contain liquid argon and mainly detect electron neutrinos, which the muon variety would oscillate into.

According to the Italian-led experimental collaborations behind the proposal—ICARUS and NESSiE—the simultaneous measurement of muon and electron neutrinos at both near and far detectors would provide much stronger evidence for or against the existence of sterile neutrinos than is possible with just a single detector like the one used for LSND. They argue that two pairs of detectors would be needed to pin down two key unknown parameters: the mass of a hypothetical sterile neutrino and the characteristic distance over which the particle would oscillate.

The proposed neutrino facility, which would involve testing new detector technologies as well as searching for sterile neutrinos, is considered a "high-priority" in a strategy document on European particle physics that is currently in the draft stage. The project was given the thumbs-up by CERN's SPS and Proton Synchrotron Experiments Committee at a meeting in mid-January, and it must next be approved by the lab's Research Board when it meets at the end of February. Final approval to spend CERN funds on the experiment would then need to come from CERN's Council of Member States, possibly at a meeting in June when an updated 5-year plan for the lab is due to be presented.

CERN Director-General Rolf-Dieter Heuer refuses to be pinned down on the precise cost of the facility but says that it would be below $110 million. Assuming that funding can be made available, he says, "it makes a lot of sense" to set up a new neutrino beam at CERN "given that every laboratory should have a diverse program." Regarding the sterile neutrino experiment in particular, he adds that "as far as I can judge, it would be close to giving us a definitive answer" on whether or not the hypothetical particle really exists.

The ICARUS-NESSiE collaborations propose recycling ICARUS, a 600-tonne argon detector currently operating at the Gran Sasso National Laboratory in central Italy, to provide one of the sterile neutrino experiment's four detectors as well as reusing parts from OPERA, also at Gran Sasso, which was made famous for its claimed, and then refuted, detection of faster-than-light neutrinos. The researchers say that the experiment should be operating by the end of 2015 to ensure it is competitive with any other similar facilities that get built -- there being worldwide at least 20 sterile neutrino experiments currently on the drawing board.