In a paper published in the journal Physical Review Letters, the OPERA collaboration reports the observation of a total of 10 muon to tau-neutrino conversion events, demonstrating unambiguously that muon neutrinos oscillate into tau neutrinos on their way from CERN, where muon neutrinos were produced, to OPERA at the Gran Sasso Laboratory some 730 km away.

Researchers designed the OPERA experiment to conclusively prove that muon-neutrinos can convert to tau-neutrinos through a process called neutrino oscillation and were awarded the 2015 Nobel Physics Prize.

The new report represents the experiment’s final results. The OPERA collaboration has also made its data public through the CERN Open Data Portal, allowing researchers outside the OPERA Collaboration have the opportunity to conduct novel research with them. The datasets come with rich context information to help interpret the data. A visualizer enables users to see the different events and download them.

The OPERA collaboration observed the first tau-lepton event (evidence of muon-neutrino oscillation) in 2010, followed by four additional events reported between 2012 and 2015, when the discovery of tau neutrino appearance was first assessed. Thanks to a new analysis strategy applied to the full data sample collected between 2008 and 2012—the period of neutrino production—a total of 10 candidate events have now been identified, with an extremely high level of significance.

“We have analyzed everything with a completely new strategy, taking into account the peculiar features of the events,” says Giovanni De Lellis, spokesperson for the OPERA collaboration. “We also report the first direct observation of the tau neutrino lepton number, the parameter that discriminates neutrinos from their antimatter counterpart, antineutrinos. It is extremely gratifying to see today that our legacy results largely exceed the level of confidence we had envisaged in the experiment proposal.”

Beyond the contribution of the experiment to a better understanding of the way neutrinos behave, the development of new technologies is also part of the legacy of OPERA. The collaboration was the first to develop fully automated, highspeed readout technologies with sub-micrometric accuracy, which pioneered the large-scale use of the so-called nuclear emulsion films to record particle tracks. Nuclear emulsion technology finds applications in a wide range of other scientific areas from dark matter search to volcano and glacier investigation. It is also applied to optimize the hadron therapy for cancer treatment and was recently used to map out the interior of the Great Pyramid, one of the oldest and largest monuments on Earth, built during the dynasty of the pharaoh Khufu, also known as Cheops.