OPERA (experiment)

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OPERA experiment during construction

The OPERA experiment ( acronym for O scillation P roject with E mulsion t R acking A pparatus , in German about: Project for the study of neutrino oscillations with a photosensitive apparatus ) is a physical experiment to neutrino oscillations . It examines the oscillation of muon - neutrinos to τ-neutrinos. It was the first appearance experiment , that is, it demonstrated the appearance of τ neutrinos in a purely muonic neutrino beam. Construction began in 2003 and the first neutrinos of the CNGS beam were detected in a test in August 2006, when most of the electronic detectors were ready for use. In 2008 the OPERA detector went into full operation. Operations ceased at the beginning of December 2012. The evaluation of the data will, however, be continued.

construction

Neutrino source

OPERA used the CNGS beam from CERN as a neutrino source . On PLC - particle accelerator at CERN were hadrons produced which produced muonic by its decay neutrinos. The beam was characterized by a low level of contamination with electron neutrinos and an extremely low level of contamination with τ neutrinos. Both contaminations were known in terms of their strength. Due to the low interaction with matter, the neutrino beam traversed the distance of about 730 km through the earth's crust to the detector in Italy without measurable losses.

detector

The core of the experiment was the neutrino detector. It was located in Hall C of the LNGS underground laboratory in the Gran Sasso massif near L'Aquila ( Abruzzo ) in Italy, with an area of ​​around 20 m in length, 10 m in height and 10 m in width. The total mass of the detector was almost 5000 t and had 200,000 individual sensors . Approx. 1300 t of lead in 1 mm thick lead plates served as the target . In each individual sensor there were 56 of these lead plates, which alternated with photo plates (photo emulsion ). The photo plates are known as Emulsion Cloud Chambers (ECC) and are used to detect charged particles that arise during a neutrino reaction. Furthermore, electronic trackers were built into the target, which made it possible to localize a reaction in real time. The affected parts of the target were then removed fully automatically, the photographic plates developed and evaluated with microscopes . A muon spectrometer was also located behind the target. This arrangement (the so-called super module ) was set up in identical form a second time behind the first super module. Since the OPERA detector had both passive elements (photoemulsions) and active elements (electronic detectors), it was called a hybrid detector .

Proof of τ neutrinos

The τ neutrinos were detected by the decay of the τ lepton , which was created when the neutrino reacted with the target. The disintegration into z. B. a muon (and two neutrinos invisible to the detector) produced a characteristic kink in the particle track. This kink could be recognized by the high-resolution particle trajectory reconstruction using the photo plates. Because of this strong signature of the τ lepton, this measurement had a very low background . A parameter of the neutrino oscillation could be calculated from the number of detected τ-neutrinos . According to the currently known value for from previous measurements of the Japanese Super-Kamiokande experiment and the American MINOS experiment, only 11 τ neutrinos were expected in the five-year term (at the intensity of the neutrino beam predicted by CERN ). Because of the small subsurface, this measurement was nevertheless highly significant .

On May 31, 2010, the OPERA researchers observed the first candidate for a τ neutrino event. A second τ neutrino event was observed on June 6, 2012. After two further τ-neutrino events were detected in 2013 and 2014, the 5th detected event has reached a statistical significance of 5 sigma, the probability of a statistical fluctuation without a signal is therefore less than one in a million. The discovery was announced on June 16, 2015. The evaluation of the data continues.

Investigation of the neutrino velocity

On September 23, 2011, OPERA published a paper in which the measurement of a superluminal speed of neutrinos was reported, in contradiction to previously recognized fundamental laws of physics. However, errors were found in the conduct of the experiment, and a new measurement by ICARUS showed agreement with the speed of light. In their final publication, the sources of error were taken into account and, as with ICARUS, there was a correspondence with the speed of light. Another measurement carried out in 2012 confirmed this result.

Web links

Commons : OPERA  - collection of images, videos and audio files

Individual evidence

  1. ^ H. Pessard: Status of the OPERA neutrino experiment . In: Proceedings of the European Physical Society. Europhysics Conference on High Energy Physics . 2009. arxiv : 0910.5701 .
  2. ↑ a b Robert Gast: The cable that shook physics. (No longer available online.) ASTROnews, January 9, 2013, archived from the original on January 30, 2013 ; accessed on January 30, 2013 . Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice. @1@ 2Template: Webachiv / IABot / www.sterne-und-weltraum.de
  3. ↑ N. Agafonova et al. (OPERA Collaboration): Observation of a first ν τ candidate event in the OPERA experiment in the CNGS beam . In: Physics Letters B . 691, No. 3, 2010, pp. 138-145. arxiv : 1006.1623 . bibcode : 2010PhLB..691..138A . doi : 10.1016 / j.physletb.2010.06.022 .
  4. ↑ OPERA Observes the Second tau neutrino . INFN press release. June 6, 2012. Retrieved June 8, 2012.
  5. ↑ OPERA detects its fifth tau neutrino on the CERN website
  6. ↑ OPERA collaboration: Measurement of the neutrino velocity with the OPERA detector in the CNGS beam . In: Journal of High Energy Physics . No. 10, 2012, p. 93. arxiv : 1109.4897v4 . doi : 10.1007 / JHEP10 (2012) 093 .
  7. ↑ OPERA collaboration: Measurement of the neutrino velocity with the OPERA detector in the CNGS beam using the 2012 dedicated data . In: Journal of High Energy Physics . No. 1, 2013, p. 153. arxiv : 1212.1276 . doi : 10.1007 / JHEP01 (2013) 153 .