Faster than Light Experiment : OPERA

CERN Neutrinos to Gran Sasso Underground Structures

Credit : CERN

Published : 3rd October, 2011

The Oscillation Project with Emulsion-tRacking Apparatus (OPERA) is an experiment to test the phenomenon of neutrino oscillations. It exploits CERN Neutrinos to Gran Sasso (CNGS), a high-intensity and high-energy beam of muon neutrinos produced at the CERN Super Proton Synchrotron in Geneva and pointing to the Laboratori Nazionali del Gran Sasso (LNGS) underground laboratory, 733 km (455 mi) away at Gran Sasso in central Italy (Abruzzo region). OPERA is located in Hall C of LNGS and is aimed at detecting for the first time the appearance of tau neutrinos from the oscillation of muon neutrinos during their 3 millisecond travel from Geneva to Gran Sasso. Tau particles resulting from the interaction of tau neutrinos will be observed in "bricks" of photographic emulsion films interleaved with lead plates. The apparatus contains about 150,000 bricks, for a total mass of 1300 tons, and is complemented by electronic detectors (trackers and spectrometers) and ancillary infrastructure. Its construction was completed in spring 2008 and the experiment is currently collecting data.

On 31 May 2010, OPERA researchers announced the observation of a first tau neutrino candidate event in a muon neutrino beam. In September 2011, CERN and OPERA announced that time of flight measurements made by their collaboration had indicated muon neutrinos traveling at faster than lightspeed. While acknowledging that such a measurement would be a major discovery if correct, many physicists,and the OPERA team itself, have expressed skepticism that OPERA's measurements are sufficiently free of error. Experimental groups such as the MINOS Experiment at Fermilab and the T2K experiment are planning to attempt to replicate the result, while others in the physics community search for any experimental errors which might account for it.

OPERA needs an intense and energetic beam of muon neutrinos traveling a distance of hundreds of kilometers to detect the appearance of oscillated tau neutrinos. A beam of this type is generated by collisions of accelerated protons with a graphite target after focusing the particles produced (pions and kaons in particular) in the desired direction. The products of their decays, muons and neutrinos, continue to travel in generally the same direction as the parent particle. Muon neutrinos produced in this way at CERN pass through the Earth's crust reaching OPERA after a 730 km journey.

OPERA is located in Hall C of the Gran Sasso underground labs. Construction started in 2003, and the apparatus was completed in summer 2008. The taus resulting from the interaction of tau neutrinos will be observed in "bricks" of photographic films (nuclear emulsion) interleaved with lead sheets. Each brick has an approximate weight of 8.3 kg and the two OPERA supermodules contain about 150,000 bricks arranged into parallel walls and interleaved with plastic scintillator counters. Each supermodule is followed by a magnetic spectrometer for momentum and charge identification of penetrating particles. During the data collection, a neutrino interaction is tagged in real time by the scintillators and the spectrometers, which also provide the location of the bricks where the neutrino interaction occurred. These bricks are extracted from the walls asynchronously with respect to the beam to allow for film development, scanning and for the topological and kinematic search of tau decays.