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Paris, 31 May 2010

OPERA experiment : detection of a first candidate in the search for direct proof of neutrino oscillation

The OPERA(1) experiment at the Gran Sasso near Rome, in which CNRS/IN2P3(2) takes part, has probably detected the first tau-type neutrino. This neutrino could stem from the transformation, during its 730 km journey, of one of the many muon-type neutrinos sent in the CERN's CNGS (CERN Neutrinos to Gran Sasso) beam. To make this observation possible, CERN accelerators produced billions of billions of muon-type neutrinos, which was necessary due to the very weak capacity of neutrinos to interact with matter. This result is important because the observation of several events of this kind could represent the long-awaited direct proof of neutrino oscillation (change of flavor), thus opening the way to new and fascinating physics, beyond the Standard Model of particles and their interactions.

Although the disappearance of the initial flavor of the neutrino has already been observed in several experiments over the last fifteen years, the “direct observation” of the change of flavor, or “oscillation”, of neutrinos constitutes the missing piece of the jigsaw. In this respect, the OPERA experiment is unique in the world. 

In 2006, the detection by OPERA, at the Gran Sasso Underground Laboratory of the Italian National Institute for Nuclear Physics (INFN), of the first “muon”-type neutrinos sent from CERN signalled the start of the operational phase of the experiment. Sophisticated research of very high spatial precision (at the micron scale) obtained on a 1300-ton target was then launched to observe the very particular signal that can be induced by “tau”-type neutrinos.

The OPERA detector comprises around 150,000 units known as bricks, each of them equivalent to a sophisticated camera. Using such bricks, made of alternating sheets of lead and special photographic film, OPERA researchers can detect all the details of neutrino events by a precise measurement of the elementary particles produced by the interaction of a neutrino with the brick.

After three years of experiments, during which thousands of neutrino interactions have been recorded and analyzed, the researchers may have succeeded in highlighting a first candidate for the interaction of a “tau”-type neutrino, observed by the OPERA detection device. The image shows the detail of the region surrounding the interaction point of the neutrino (coming from the left of the image), producing several particles whose trajectories leave a trace in the brick. The presence of a trajectory with a kink (red then turquoise) is the probable sign of the interaction of a tau neutrino, with a probability of around 98%. The image represents a volume of only several cubic millimeters, but is packed with valuable information for reconstructing the physics of the interaction.

This result, which is crucial for neutrino physics, is the fruit of a complex scientific undertaking, achieved through the expertise of a large number of researchers, technicians, scientists and students, with strong commitment from various partners, such as the host laboratories at Gran Sasso and CERN. This initiative also benefitted from major financial support from Italy and Japan, as well as significant contributions (in terms of human resources and funding) from France, Germany, Belgium and Switzerland. Several researchers from Israel, Korea, Russia, Tunisia and Turkey are also involved in the project.

Four IN2P3/CNRS  laboratories involved in the OPERA experiment

The four laboratories(3) contributed from the very outset in devising and setting up the project. They designed and made all of the detector's key elements, the scintillator trajectographs used to locate the brick in which the neutrino interactions occur, the readout electronics and the innovative acquisition system “with distributed intelligence” of the set of electronic detectors. They also developed the automated device used to handle the 150,000 bricks. Over the last four years, French laboratories have made major contributions to data analysis, including the computing environment and the events database, one of the largest interactive databases in the world. This activity has been strongly supported by the Centre de Calcul of IN2P3 (CNRS), which offered its computing resources to the team.

Image of the first candidate for a tau neutrino event

Image of the first candidate for a tau neutrino event, which shows a detail of the region surrounding the interaction point of the neutrino (coming from the left), producing several particles identified by their traces in the brick. The detection of the trace with a kink is the probable sign of the interaction of a tau neutrino (with a probability of around 98%). The image represents a volume of only several cubic millimeters, but is packed with useful information for OPERA physicists.


Notes:

(1) Oscillation Project with Emulsion-tRacking Apparatus
(2) Institut National de Physique Nucléaire et de Physique des Particules of CNRS
(3) The Institut de Physique Nucléaire de Lyon (CNRS/Université Claude Bernard-Lyon 1), the Institut Pluridisciplinaire Hubert Curien (CNRS/Université de Strasbourg), the Laboratoire de l'Accélérateur Linéaire (CNRS/Université Paris-Sud 11) which participated up to 2005, and the Laboratoire d'Annecy le Vieux de Physique des Particules (CNRS/Université de Savoie)

Contacts:

Researchers
Henri Pessard l 04 50 09 55 74 l henri.pessard@lapp.in2p3.fr
Dario Autiero l 04 72 44 84 51 l
d.autiero@ipnl.in2p3.fr

CNRS press officer
Claire Le Poulennec l T 01 44 96 51 51 l
claire.le-poulennec@cnrs-dir.fr


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