Researching something small often needs large amounts of effort. A well-known example of this is CERN with its particle accelerators measuring several kilometres in diameter in Geneva. But sometimes even this research apparatus is not big enough to characterise elementary particles. That’s why the OPERA project (1) has been running since this summer in which a beam of neutrinos, although generated at CERN (2), is then sent on a 732 kilometre journey through the Earth’s crust to an underground laboratory in the Gran Sasso mountains near Rome (3). The group of about 180 researchers from various countries taking part in the project, which also includes scientists belonging to ETH professor André Rubbia’s research group (4), want to use it to find out whether the neutrinos are transformed on the way. That is to say they want to know whether a proportion of the muon type neutrinos of which the beam initially consists are transformed into tau type neutrinos during the long journey.

But why this interest in the conversion of neutrinos? One reason is that elementary particle physicists have for a long time been wanting to determine the mass of these small electrically neutral particles, which has so far been unknown. Since a transformation is possible only if at least one of the neutrinos involved has a non-vanishing mass, detection of these extremely tiny masses is most likely to be possible by detecting their transformation, which is also called neutrino oscillation. By measuring the neutrino oscillation length, the OPERA experiment will provide more precise information about the difference of the mass squares of the neutrinos involved.

Because of the high density of neutrinos in space, the newly determined value might subsequently contribute significantly to an understanding of the relationships between the masses in the universe. In addition, the neutrino oscillation mechanism will solve the old riddle of why the number of electron neutrinos (the third type of neutrino in addition to the tau and muon neutrinos) reaching the earth from the sun is only about half as many as would be expected based on calculations.

A detector weighing 1,800 tons

However, it will be some time yet before the decisive experiments are carried out. In August this summer a low intensity beam was sent for the first time from Geneva to the detector weighing 1,800 tons and measuring 7 x 7 x 25 metres near Rome. The purpose of this first experiment was to optimise the beam settings and to check the functioning of the OPERA detector’s components. An important contribution by the ETH researchers also lies inside these components. What these researchers are doing is to supervise the production of a total of 206,000 sandwich bricks each consisting of about 12 million lead plates and photographic emulsions in which the neutrino reactions are measured very accurately. The emulsions also formed part of the OPERA project’s name, which stands for “Oscillation Project with Emulsion-tRacking Apparatus”. Other contributions by ETH scientists to the OPERA project, which began in 1998, consist of helping to construct the data acquisition system and to develop the analysis software.

A big instrument for tiny particles: This detector, a part of the OPERA project, is intended to detect transformations of muon neutrinos into tau neutrinos. (Photo: Public Affairs Office, Gran Sasso National Laboratory - INFN) large

The first beam captured

According to Andreas Badertscher of Rubbia’s research group, the August experiment led to more than 300 events in the OPERA detector that were exactly correlated in time with the neutrino beam pulses. In this process the neutrinos, being electrically neutral particles, are not detectable in the detector directly, but only based on the tracks of charged particles generated during a neutrino reaction. Although the experiment was very successful in this respect, further trial runs are needed before reaching the core experiment of oscillation detection. This is why neutrinos will again be sent from Geneva to Rome for further tests from 25 October to 7 November.

Overall the ETH researchers and their colleagues are confident that the direct proof of the transformation of muon neutrinos into tau neutrinos will be successful in the next few years. Up to now OPERA is the only so-called "tau appearance" experiment that can measure not only a neutrino deficit, which has already been done successfully, but also the appearance of tau neutrinos in a beam consisting originally of muon neutrinos. The assumption based on theoretical considerations that the missing muon neutrinos have been transformed mainly into tau neutrinos would then finally have a direct experimental foundation.

The experimental efforts will reach out even further. Work is already in progress on the “ICARUS” project, a supplement to OPERA in which a novel detector with liquid argon is being used. This is intended to capture even more messages from the neutrinos realm in the next few years.

The first neutrinos from Geneva are in Rome: the tracks of particles from a neutrino reaction are depicted at the left on a diagrammatic representation of the detector. (Photo: OPERA Collaboration)