CERN launches groundbreaking neutrino experiment

September 13, 2006

Brussels, 12 Sep 2006

Modern physics took one gigantic step forward on 11 September when scientists at CERN, the European Organisations for Nuclear Research, started to send a beam of neutrinos through the Earth to the Gran Sasso laboratory near Rome, Italy, 760km away. The experiment is part of a global effort to understand these particles, which, despite never having been observed directly, are believed to carry the secrets of the origins and evolution of our Universe.

Neutrinos are invisible elementary particles which are produced in nuclear reactions within stars. They are the second most abundant type of particle in the universe after photons.

Because they interact so weakly with other particles, neutrinos can pass through matter, leaving little or no trace which makes them very hard to detect. Trillions of neutrinos pass through the Earth every second of the day and night.

Scientists believe that thanks to this weak interaction with other particles, neutrinos carry untainted information about supernovae. Understanding this information is key element to understanding our universe.

Neutrinos are thought to oscillate between three different 'types': electron, muon and tau. The researchers hope that the Gran Sasso laboratory will be able to detect the transformation of muon neutrinos into tau neutrinos, a phenomenon so far never observed.

Starting out at CERN, muon neutrinos will be generated from the collision between an accelerated beam of protons and a special target. The muon neutrinos generated in this collision will travel the 730km to Gran Sasso in 2.5 milliseconds at close to the speed of light. At Gran Sasso, the researchers hope to detect a small number of tau neutrinos, which will have changed from muon neutrinos during the journey. Calculations predict that among the many billions of muon neutrinos arriving at Gran Sasso, about 15 tau neutrinos will be detected.

The detection of these tau neutrinos by the Gran Sasso facility is what distinguishes it from other neutrino experiments in the US and Japan, which have typically measured the number of muon neutrinos that disappear, rather than the number of muon neutrinos that appear.

Gran Sasso has two neutrino detectors, named Opera and Icarus. Only the 1800-ton Opera is currently operational - it 'sees' neutrinos by using photographic plates to detect interactions between lead and neutrinos. Icarus will use 600 tons of liquid argon to detect the neutrinos.

'The neutrino is now becoming one of the central issues in elementary physics,' said Atsuto Suzuki, Director General of High Energy Accelerator Research Organization (KEK) and former spokesperson for KamLAND, another neutrino detector that found neutrinos generated at the centre of the Earth.. 'There are many exciting challenges in this area. One of the most important milestones for the development of neutrino physics is to verify experimentally that the oscillation of muon-neutrinos to tau-neutrinos is the one that has been discovered in atmospheric neutrino observations. I am very pleased that the CERN and Gran Sasso experiments will soon answer this important question.'

Scientists will use these experiments to decide whether neutrinos have mass, and if so, whether this mass differs depending on the type of neutrino. Current theory predicts that neutrinos are without mass, since they hardly interact with other matter.

'The existence of a mass for these particles sheds light on some of the most important problems of modern physics,' explains Roberto Petronzio, president of Italy's National Institute of Nuclear Physics (INFN) where the Gran Sasso laboratory is located. 'For example, the existence of neutrino mass could help to explain the so-called asymmetry between matter and antimatter, that is to say the prevalence of matter in the Universe, in spite of the nearly perfect similarity of their fundamental interactions.'

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