Gran Sasso catches the first tau neutrino
The OPERA experiment at INFN's Gran Sasso National Laboratory (LNGS) on 31st May 2010 has announced the first direct observation of the transformation of the neutrino of one type into another. When confirmed by a few more such events, this observation will complete the evidence for "neutrino oscillations", a phenomenon that is possible only if neutrinos have mass, and that is evidence of physics beyond the Standard Model. ------ Neutrinos are neutral leptons, assumed to have no mass in the Standard Model. They exist in three types, each being the brother of a charged lepton: electron, mu and tau. Neutrinos are assumed to be stable in the model. Here already the Italian contribution was remarkable with the discovery of the leptonic nature of the mu in 1947 by Marcello Conversi, Ettore Pancini and Oreste Piccioni, and in 1967 with the hypothesis of and subsequent search for the third lepton at ADONE by Antonino Zichichi (a lepton later discovered in 1975 at SPEAR by Martin Perl and called tau). In the Standard Model neutrinos are assumed to be stable. However, over the past forty years experiments have shown that neutrinos of a type can spontaneously disappear, when travelling over long distances. This happens, it is assumed, because they change from a type to another one. Up to now, there was no direct evidence for that. OPERA is the first experiment to measure the appearance of a tau-neutrino in a beam of mu-neutrinos, thus confirming that neutrinos oscillate between different types. ------ The contributions of Italian physicists also concerning neutrino oscillations have been outstanding since the very beginning. The hypothesis of oscillations between different neutrino types was first advanced by Bruno Pontecorvo (then in Russia) in 1967, to try to explain the disappearance of electron neutrinos from the Sun, observed in an underground experiment then starting in the Homestake Mine in the USA by Raymond Davis. For that, Davis employed the electron neutrino detection method suggested, once more, by Pontecorvo (then in Canada) back in 1946. ------ Thirty years ago, Antonino Zichichi, then President of the INFN, designed a large, high technology, underground laboratory under the Gran Sasso Mountain and obtained the necessary approvals and funding. He oriented its halls in the CERN direction in order to efficiently host, in a future, experiments detecting neutrinos produced at the accelerators of the European Centre. In the following years LNGS provided the infrastructures necessary to experiments on neutrinos from the Sun, GALLEX, GNO and BOREXINO, and on neutrinos produced by cosmic rays in the atmosphere, MACRO, which substantially contributed in providing the above quoted evidence for neutrino disappearance. ------ OPERA, designed in 1997, was built and is ran by a collaboration of scientists from 12 Countries (Belgium, Bulgaria, Croatia, France, Germany, Israel, Italy, Japan, Korea, Russia, Switzerland, Tunisia and Turkey), including nine Italian Institutions, under the leadership, along the years, of the spokesmen Paolo Strolin, Yves Déclais and, presently, Antonio Ereditato. In 1999 the CERN Council approved the CNGS (CERN Neutrinos to Gran Sasso) proposal of the Director General, Luciano Maiani, to build a mu-neutrino beam optimised for tau-neutrino appearance experiments. The OPERA and ICARUS experiments became strong part of the INFN programme and commitment, when Enzo Iarocci was its President and, in particular, of LNGS, when Alessandro Bettini was its Director. ------ If tau neutrinos are present in the beam when it reaches Gran Sasso, it may happen, but very rarely, that one of them interacts with a nucleus (Pb) of the detectors. Then, a tau particle is produced, amongst the others. However, its lifetime is very short and consequently the length of its track is only one millimetre or so. Only the photographic emulsion technique can provide the necessary spatial resolution. On the other hand, the thin emulsion sheets must be assembled in a kiloton size detector. OPERA employed the "emulsion chamber" technique originally developed in Japan over the years by the Nagoya group, led by Kiyoshi Niu in the 1970s and later by Kimio Niwa. The very large amount of emulsions to be assembled and processed required a strong commitment of the European and Japanese groups of OPERA to bring the technique to unprecedented levels. ------ Since 2006 scientists of the OPERA experiment are collecting data on neutrino interactions of the CNGS beam searching for the track of a tau particle. That track has been found now, indicating that one of the billions upon billions of mu neutrinos transformed into a tau neutrino and interacted with OPERA. ------ Further details can be found at the OPERA web-site (http://operaweb.lngs.infn.it/).