The Oscillation Project with Emulsion-tRacking Apparatus (OPERA) was an instrument used in a scientific experiment for detecting tau neutrinos from

muon neutrino The muon neutrino is an elementary particle which has the symbol () and zero electric charge. Together with the muon it forms the second generation of leptons, hence the name muon neutrino. It was discovered in 1962 by Leon Lederman, Melvin Schwar ...

oscillations Oscillation is the repetitive or periodic variation, typically in time, of some measure about a central value (often a point of equilibrium) or between two or more different states. Familiar examples of oscillation include a swinging pendulum ...

. The experiment is a collaboration between

CERN The European Organization for Nuclear Research, known as CERN (; ; ), is an intergovernmental organization that operates the largest particle physics laboratory in the world. Established in 1954, it is based in a northwestern suburb of Gene ...

Geneva Geneva ( ; french: Genève ) frp, Genèva ; german: link=no, Genf ; it, Ginevra ; rm, Genevra is the List of cities in Switzerland, second-most populous city in Switzerland (after Zürich) and the most populous city of Romandy, the French-speaki ...

Switzerland ). Swiss law does not designate a ''capital'' as such, but the federal parliament and government are installed in Bern, while other federal institutions, such as the federal courts, are in other cities (Bellinzona, Lausanne, Luzern, Neuchâtel ...

, and the

Laboratori Nazionali del Gran Sasso Laboratori Nazionali del Gran Sasso (LNGS) is the largest underground research center in the world. Situated below Gran Sasso mountain in Italy, it is well known for particle physics research by the INFN. In addition to a surface portion of the ...

(LNGS) in

Gran Sasso Gran Sasso d'Italia (; ) is a massif in the Apennine Mountains of Italy. Its highest peak, Corno Grande (2,912 metres), is the highest mountain in the Apennines, and the second-highest mountain in Italy outside the Alps. The mountain lies wit ...

Italy Italy ( it, Italia ), officially the Italian Republic, ) or the Republic of Italy, is a country in Southern Europe. It is located in the middle of the Mediterranean Sea, and its territory largely coincides with the homonymous geographical re ...

and uses the

CERN Neutrinos to Gran Sasso The CERN Neutrinos to Gran Sasso (CNGS) project was a physics project of the European Organization for Nuclear Research (CERN). The aim of the project was to analyse the hypothesis of neutrino oscillation by directing a beam of neutrinos from CE ...

(CNGS) neutrino beam. The process started with protons from the

Super Proton Synchrotron The Super Proton Synchrotron (SPS) is a particle accelerator of the synchrotron type at CERN. It is housed in a circular tunnel, in circumference, straddling the border of France and Switzerland near Geneva, Switzerland. History The SPS was de ...

(SPS) at CERN being fired in pulses at a carbon target to produce

pion In particle physics, a pion (or a pi meson, denoted with the Greek letter pi: ) is any of three subatomic particles: , , and . Each pion consists of a quark and an antiquark and is therefore a meson. Pions are the lightest mesons and, more gene ...

s and

kaon KAON (Karlsruhe ontology) is an ontology infrastructure developed by the University of Karlsruhe and the Research Center for Information Technologies in Karlsruhe. Its first incarnation was developed in 2002 and supported an enhanced version of ...

s. These particles decay to produce

muon A muon ( ; from the Greek letter mu (μ) used to represent it) is an elementary particle similar to the electron, with an electric charge of −1 '' e'' and a spin of , but with a much greater mass. It is classified as a lepton. As wi ...

s and

neutrino A neutrino ( ; denoted by the Greek letter ) is a fermion (an elementary particle with spin of ) that interacts only via the weak interaction and gravity. The neutrino is so named because it is electrically neutral and because its rest mass ...

s. The beam from CERN was stopped on 3 December 2012, ending data taking, but the analysis of the collected data has continued.

Detector

OPERA, in Hall C of the Gran Sasso underground labs, was built in 2003–2008. The

taus Taus may refer to: * Domažlice (German: Taus), a town of the Czech Republic * Taus, Wisconsin, United States, an unincorporated community * Melek Taus, "The Peacock Angel", the Yazidis' name for the central figure of their faith * Taus (instrumen ...

resulting from the interaction of tau neutrinos are observed in "bricks" of photographic films (

nuclear emulsion A nuclear emulsion plate is a type of particle detector first used in nuclear and particle physics experiments in the early decades of the 20th century. https://cds.cern.ch/record/1728791/files/vol6-issue5-p083-e.pdf''The Study of Elementary Partic ...

) interleaved with lead sheets. Each brick weighs 8.3 kg; the two OPERA supermodules contain 150,000 bricks arranged into parallel walls interleaved with plastic

scintillator A scintillator is a material that exhibits scintillation, the property of luminescence, when excited by ionizing radiation. Luminescent materials, when struck by an incoming particle, absorb its energy and scintillate (i.e. re-emit the absorbed ...

counters. Each supermodule is followed by a magnetic

spectrometer A spectrometer () is a scientific instrument used to separate and measure spectral components of a physical phenomenon. Spectrometer is a broad term often used to describe instruments that measure a continuous variable of a phenomenon where the ...

for momentum and charge identification of penetrating particles. During data collection, a neutrino interaction and its corresponding brick are tagged in real time by the scintillators and spectrometers. These bricks are extracted from the walls asynchronously with respect to the beam for film development, scanning and for the topological and kinematic search of tau decays.

Tau neutrinos

In total, five tau neutrinos were detected. On 31 May 2010, OPERA researchers observed the first tau neutrino candidate event in a muon neutrino beam. On 6 June 2012, OPERA announced the observation of a second tau neutrino event. On 26 March 2013, the experiment caught for the third time a muon neutrino oscillating into a tau neutrino during travel from

LNGS Laboratori Nazionali del Gran Sasso (LNGS) is the largest underground research center in the world. Situated below Gran Sasso mountain in Italy, it is well known for particle physics research by the INFN. In addition to a surface portion of the ...

. The fourth one was found in 2014, and the fifth was seen in 2015.

Time-of-flight measurements

In September 2011, OPERA researchers observed

s apparently traveling faster than the

speed of light The speed of light in vacuum, commonly denoted , is a universal physical constant that is important in many areas of physics. The speed of light is exactly equal to ). According to the special theory of relativity, is the upper limit ...

. In February and March 2012, OPERA researchers blamed this result on a loose fibre optic cable connecting a GPS receiver to an electronic card in a computer. On 16 March 2012, a report announced that an independent experiment in the same laboratory, also using the CNGS neutrino beam, but this time the

ICARUS In Greek mythology, Icarus (; grc, Ἴκαρος, Íkaros, ) was the son of the master craftsman Daedalus, the architect of the labyrinth of Crete. After Theseus, king of Athens and enemy of Minos, escaped from the labyrinth, King Minos suspe ...

detector found no discernible difference between the speed of a neutrino and the speed of light. In May 2012, the Gran Sasso experiments BOREXINO, ICARUS, LVD and OPERA all measured neutrino velocity with a short-pulsed beam, and obtained agreement with the speed of light, showing that the original OPERA result was mistaken. Finally in July 2012, the OPERA collaboration updated their results. After the instrumental effects mentioned above were taken into account, it was shown that the speed of neutrinos is consistent with the speed of light. This was confirmed by a new, improved set of measurements in May 2013.

Detector

Tau neutrinos

Time-of-flight measurements

References

External links