mardi 16 juin 2015

OPERA detects its fifth tau neutrino

CERN - European Organization for Nuclear Research logo.

June 16, 2015

The OPERA (Oscillation Project with Emulsion-tRacking Apparatus (link is external)) experiment at the Italian National Institute for Nuclear Physics (link is external) (INFN) at Gran Sasso in Italy has detected the fifth occurrence of a tau neutrino in the muon-neutrino beam from CERN. A particle that left CERN as a muon neutrino underwent "oscillation" during its 730-kilometre journey to Gran Sasso, arriving as a tau neutrino.

Researchers at Gran Sasso announced the result yesterday, naming it "5 sigma" on the scale that particle physicists use to describe the certainty of results. One sigma could be a random statistical fluctuation in the data, 3 sigma counts as evidence, but only a result of 5-sigma or more is ranked as a clear observation. By definition, the probability that a 5-sigma result is wrong is less than one in a million.

Image above: Super Proton Synchrotron (SPS) is the second-largest machine in CERN’s accelerator complex. Measuring nearly 7 kilometers in circumference, it takes particles from the Proton Synchrotron and accelerates them to provide beams for the OPERA experiment. Image Credit: CERN.

“The detection of a fifth tau neutrino is extremely important: We…can definitely report the discovery of the appearance of tau neutrinos in a muon neutrino beam,” said spokesperson Giovanni De Lellis of INFN in Naples, Italy, on behalf of the international research team.

Three types or "flavours" of neutrino exist in nature: the electron neutrino, the muon neutrino and the tau neutrino. But it seems that neutrinos are the chameleons of the particle world: they can change from one flavour into another. This phenomenon, called “oscillation”, occurs as neutrinos travel long distances through matter. The process is directly related to the neutrinos' tiny mass.

OPERA was designed to search for tau neutrinos in the muon-neutrino beam from CERN, which ran during 2006-2012; detecting tau neutrinos in this beam is proof that oscillation occurred during their particles' 730 km long flight.

Image above: The Oscillation Project with Emulsion-tRacking Apparatus (OPERA) experiment is designed to test the phenomenon of neutrino oscillations. Image Credit: Worldpress.

CERN produced the neutrino beams for Gran Sasso by firing pulses of protons from the Super Proton Synchrotron (SPS) at a graphite target. The collisions created particles called pions and kaons, which were fed into a system of two magnetic lenses to focus the particles into a parallel beam in the direction of Gran Sasso. The pions and kaons then decayed into muons and muon neutrinos in a 1-kilometre long pipe underground. At the end of the pipe, a block of graphite and metal 18 metres thick absorbed protons as well as pions and kaons that did not decay. Muons were stopped by the rock beyond, but the muon neutrinos remained to streak through the rock on their journey to Italy. As they hardly interact at all with matter, the neutrinos could travel the 730 kilometres from CERN to Gran Sasso directly through the rocks of the Earth's crust. When they arrived at Gran Sasso, a small fraction of the incoming neutrinos interacted with the OPERA detector, which can determine which type of neutrino passed through.

After detecting the first few muon neutrinos produced at CERN in 2006, the experiment collected data from 2008 to the end of 2012. The first tau neutrino was observed in 2010. The second and third events were reported in 2012 and 2013 respectively, while the fourth one was published in 2014.

Image above: The OPERA detector is housed in a tunnel in the Gran Sasso laboratory. OPERA's core. Image Credit: INFN.

“The achievement reported yesterday was made possible thanks to the continuous effort of all the researchers involved in the project, to the excellent performance of the CERN neutrino beam and to the support of all the funding agencies”, said De Lellis.

Scientists will continue to analyse the data collected, searching for other tau neutrinos produced from the oscillation of muon neutrinos. Technologies developed for the OPERA experiment will be used in forthcoming experiments in neutrino physics and other fields.


CERN, the European Organization for Nuclear Research, is one of the world’s largest and most respected centres for scientific research. Its business is fundamental physics, finding out what the Universe is made of and how it works. At CERN, the world’s largest and most complex scientific instruments are used to study the basic constituents of matter — the fundamental particles. By studying what happens when these particles collide, physicists learn about the laws of Nature.

The instruments used at CERN are particle accelerators and detectors. Accelerators boost beams of particles to high energies before they are made to collide with each other or with stationary targets. Detectors observe and record the results of these collisions.

Founded in 1954, the CERN Laboratory sits astride the Franco–Swiss border near Geneva. It was one of Europe’s first joint ventures and now has 22 Member States.

Related links:

Oscillation Project with Emulsion-tRacking Apparatus (OPERA) experiment:

Italian National Institute for Nuclear Physics (INFN):

Super Proton Synchrotron (SPS):

For more information about the European Organization for Nuclear Research (CERN), visit:

Images (mentioned), Text, Credits: CERN/Cian O'Luanaigh.


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