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Experiment refutes previous finding about the nature of neutrinos

Neutrino

New results shed light on the behaviour of fundamental particles<em> - News</em>

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By Danielle Reeves
Tuesday 17 April 2007

The MiniBooNE magnetic focusing horn is used to create the neutrino beam used in the experimentResults of an international particle physics experiment announced last week have solved a long-standing question about the nature of neutrinos, one of the fundamental particles that make up the universe. The MiniBooNE experiment has taken significant steps towards refuting some unexpected results that occurred during a neutrino experiment in the 1990s, which suggested a new type of neutrino exists. The new MiniBooNE results also help to clarify the overall picture of how neutrinos behave.

Neutrinos are elementary particles which travel close to the speed of light, and which do not have an electrical charge. They are created as a result of certain types of radioactive decay or nuclear reactions. Currently three types, or 'flavours' of neutrinos are known to exist: electron neutrinos, muon neutrinos and tau neutrinos, and each neutrino has an associated antimatter partner called an antineutrino. In the last 10 years, several experiments have shown that neutrinos can oscillate from one flavour to another and back.

A key experiment in the 1990s, called LSND, suggested that a fourth, or 'sterile', type of neutrino exists, with different properties from the three standard neutrinos. The existence of sterile neutrinos would throw serious doubt on the current structure of particle physics, known as the Standard Model of Particles and Forces.

These sterile neutrinos were theorised due to an excess of electron antineutrinos in LSND's data. If this excess were due to simple flavour oscillations then MiniBooNE should have seen an excess of electron neutrinos with a particular energy distribution. However, MiniBooNE found no excess matching the predictions from the LSND signal. Combining the results of LSND and MiniBooNE together gives a probability of just 2 per cent that the LSND excess stems from neutrino oscillations. Without an oscillation interpretation of the LSND event excess there is no evidence for the existence of sterile neutrinos.

Dr Morgan Wascko   from Imperial College London's Department of Physics, who contributed to the new MiniBooNE experiment and is the co-spokesperson of the upcoming SciBooNE experiment at Fermilab, explains the significance of the team's results, saying: "Sterile neutrinos are the easiest physics explanation for the LSND signal, and the MiniBooNE result has ruled out the simple explanation. The next obvious steps are to check the result with MiniBooNE antineutrino data, and to apply SciBooNE data to confirm MiniBooNE's background event estimates."

The scientific team behind the MiniBooNE experiment say that although it has answered some questions about the behaviour of fundamental neutrino particles, there are many other aspects of their behaviour that need clarification – for which additional analyses of the MiniBooNE data will be necessary.

Fermilab physicist Steve Brice, analysis coordinator for MiniBooNE said: "We have been studying the bulk of our data for several years. There are remaining analyses that we are eager to do next. They will include detailed investigation of data we observe at low energy that do not match what we expected to see, along with more exotic neutrino oscillation models and other exciting physics."

The MiniBooNE experiment, which is based at Fermilab in the States, relies on a detector made of a 250,000-gallon tank filled with ultrapure mineral oil, which is clearer than water from a tap. A layer of 1280 light-sensitive photomultiplier tubes, mounted inside the tank, detects collisions between neutrinos made by an accelerator, and the carbon nuclei of oil molecules inside the detector.

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