Neutrino oscillations make their first appearance in OPERA

1400 metres underground in the INFN Gran Sasso Laboratory, the Opera experiment has just observed its first candidate for neutrino oscillation – the phenomenon that confirms that neutrinos have mass. It is the first time that an experiment has observed the direct appearance of the new type of neutrinos produced in the oscillation. Opera uses a dedicated beam produced at CERN’s Super Proton Synchrotron (SPS).


Tracks of first candidate event observed by the OPERA experiment.

Neutrinos, abundant in cosmic rays, are involved in several of the nuclear reactions that take place in the Sun, and also in radioactive decays. Numerous as they are, they continue to hold many secrets for scientists. One is the fact that the three types of neutrinos—electron, muon and tau neutrinos—can change into each another. This physical phenomenon, known as neutrino ‘oscillations’, was originally described in an article by Bruno Pontecorvo and Vladimir Gribov in 1969 based on Pontecorvo’s pioneering work in the 1950s. Its occurrence in nature can be used to prove that neutrinos have a non-vanishing, although very small, mass.

The beam that CERN sends along the 732 km to the INFN Gran Sasso laboratory in Italy consists almost entirely of muon neutrinos, with a small residual contamination of antineutrinos, which does not affect the measurements performed by OPERA. “Each day of the run, CERN sends billions of muon neutrinos to our experiment”, says Antonio Ereditato, OPERA spokesperson. “However, only about 20 events are registered per day as neutrino interactions in our experimental target. We then have to carefully analyse these events to see whether there is the signature of a tau neutrino, not present in the original beam.” Such an appearance would provide unambiguous proof for neutrino oscillation, and hence for the non-zero mass of neutrinos.

The first evidence of neutrino oscillations came from SuperKamiokande in 1998. “Several other experiments in Europe, US and Japan have investigated and are presently studying this phenomenon”, explains Ereditato. “They study neutrino oscillations by measuring the number of neutrinos of a certain type that have ‘disappeared’ from a given beam. OPERA is the only experiment in the world dedicated to the ‘appearance’ of tau neutrinos arising from the oscillation of muon neutrinos, which should occur in flight as they make the 732 km-long trip from CERN”.

The same event displayed at a 200 micrometer scale.

OPERA exploits a dedicated beam produced at CERN by colliding high-energy protons from the SPS with a graphite target. Among the several types of particle created in the interaction are positive pions and kaons, which go on to decay and produce muon neutrinos. “CERN and the INFN Gran Sasso Laboratory have collaborated to define the energy and properties of the beam in order for it to best meet the experiments’ requirements”, says Lucia Votano, Director of Gran Sasso Laboratory. “OPERA has now received about one fifth of the total number of particles expected for the whole duration of its programme. It has recorded its first tau candidate event but we will have to continue the data taking and the subsequent data analysis in order to provide the scientific community with the final conclusive results”, adds Ereditato.

Although not a multipurpose detector, the OPERA apparatus is very complex, with a large ancillary infrastructure. Its core consists of 150 000 ‘bricks’ (see box) that register the tracks of the elusive particles. “We have analysed about 10 000 bricks so far. To do it, we use tens of automatic microscopes distributed in the participating institutes around the world. The read out is so accurate that we attain the accuracy of less than one micron in measuring the particles’ tracks”, says Ereditato.

“The results coming from the current neutrino experiments around the world will determine the future of neutrino physics”, concludes Lucia Votano. “I hope that CERN will continue to play an active role in neutrino physics and that CERN and Gran Sasso will continue to collaborate in this promising field.”


The Opera experiment

The OPERA apparatus has two main parts. The first is a series of 62 parallel walls made with 150 000 bricks of alternate lead/emulsion layers, which record the passage of particles arising from the neutrino interaction. Each brick has 56 layers of lead interleaved with 57 layers of photographic film. Altogether, the 150 000 bricks thus contain about 10 million films. The second part is a series of complementary electronic detectors (trackers, magnets etc.) that tag the neutrino interactions in real time, providing the precise spatial location of the bricks where the neutrino interaction occurred. The relevant bricks are then extracted from the walls so that the film can be developed and scanned with computer-controlled scanning microscopes.

by CERN Bulletin