Geneva, 11 September 2006. CERN1 has switched on a new neutrino beam, aimed through the earth to the INFN2 Gran Sasso Laboratories some 730km away near Rome. This is the latest addition to a global endeavour to understand this most elusive of particles and unlock the secrets it carries about the origins and evolution of our Universe. The start of the project was marked today by a ceremony at the Gran Sasso Laboratories attended by Italian Minister for Universities and Research, Fabio Mussi, and CERN Director General Robert Aymar.
“CERN has a tradition of neutrino physics stretching back to the early 1960s,” said Dr Aymar, “this new project builds on that tradition, and is set to open a new and exciting phase in our understanding of these elusive particles.”
The CNGS beam and the experimental devices constructed in the Gran Sasso Laboratories to study neutrino interactions are part of a project aimed at shedding light on the mysterious phenomenon of the oscillation of these particles.
Neutrinos are continuously produced in nuclear reactions within the stars, and they are the most abundant particles in the Universe after photons. Our planet is constantly traversed by their flux: each second, 60 billion neutrinos go through a space the size of a fingertip. They interact so weakly with other particles that they can go through any form of matter without leaving a trace. This peculiarity makes neutrinos so elusive that a great sensitivity is required in the design of experiments to study them. Neutrinos are divided into three families: electron, muon and tau. Experimental evidence obtained through both cosmic and man-made neutrinos shows that they can oscillate from one type into another. This important phenomenon implies that each type of neutrino has a mass, and that the masses of the three types are different.
“The existence of a mass for these particles sheds light on some of the most important problems of modern physics,” explains INFN president Roberto Petronzio. “For example, the existence of neutrino mass could help to explain the so-called asymmetry between matter and antimatter, that is to say the prevalence of matter in the Universe, in spite of the nearly perfect similarity of their fundamental interactions.”
By virtue of the oscillation phenomenon, a beam of neutrinos that is initially homogeneous, detected after some time, would contain within it another kind of neutrino. Experiments at the Gran Sasso Laboratories, which use the neutrino beam from CERN, will be able to demonstrate in particular the transformation of muon neutrinos into tau neutrinos, a phenomenon so far never observed. Only muon neutrinos will be produced at CERN, but after 2,5 milliseconds, when the beam arrives at Gran Sasso after having covered about 730km at almost the speed of light, a very small number of tau neutrinos are expected to be detected by the researchers. According to some theoretical calculations, among many billions of billions of muon neutrinos arriving at Gran Sasso, only about 15 tau neutrinos will be identified.
At CERN, neutrinos are generated from collisions of an accelerated beam of protons with a target. When protons hit the target, particles called pions and kaons are produced. They quickly decay, giving rise to neutrinos. Unlike charged particles, neutrinos are not sensitive to the electromagnetic fields usually used by physicists to change the trajectories of particle beams. Neutrinos can pass through matter without interacting with it; they keep the same direction of motion they have from their birth. Hence, as soon as they are produced, they maintain a straight path, passing through the earth's crust. For this reason, it is extremely important that from the very beginning the beam points exactly towards the laboratories at Gran Sasso.
At Gran Sasso two experiments will be waiting for the neutrinos from CERN: Opera and Icarus, the latter still under construction. Opera is an enormous detector weighing 1800 tons, made up of photographic plates interleaved with lead layers. The very few tau neutrinos produced from neutrino oscillation, interacting with the lead layers, will generate very short-lived charged particles (called tau leptons) whose decay products will leave marks in the photographic emulsions. The reconstruction of these traces will allow experimenters to identify the tau lepton and so detect the presence of tau neutrinos in the beam. The Icarus apparatus will use a detector of 600 tons of liquid argon. The products of the interaction among neutrinos and argon atoms will be registered by a series of sophisticated sensors plunged into the liquid itself. The experiments are located at the Gran Sasso Laboratories where they are sheltered by 1440 metres of rock, a very powerful screen against the cosmic rays produced in the atmosphere by primary cosmic radiation. Cosmic rays produce a storm of charged particles that constantly hit our planet. Without the protection of rock, the noise from cosmic rays would drown out the very weak signal of the few interactions of neutrinos in the detectors.
Neutrino experiments are an integral part of the strategy for particle physics approved by the CERN Council on 14 July in Lisbon. The development of a common strategy for nuclear and particle physics in Europe is necessary because of the scale of research in this field for the near future. Coordination between CERN, research centres and national laboratories is therefore more necessary than ever. A joint experiment between CERN and the Laboratories of Gran Sasso represents an ideal inauguration of the new direction approved in Lisbon.
The CNGS project complements similar projects in the US and Japan, both of which look for disappearance of neutrinos of a particular type from the initial beam. In the US, a beam is sent from Fermilab near Chicago to a deep underground mine in Minnesota. “I offer warmest congratulations from Fermilab on the magnificent achievement of the CERN to Gran Sasso neutrino beam,” said Fermilab director Piermaria Oddone, “Of all the known particles, neutrinos are the most mysterious. In the years ahead, neutrino experiments at Gran Sasso and around the world will discover the fascinating secrets of neutrinos and how they shaped the Universe we live in.”
In Japan, the K2K project sent a neutrino beam from the KEK laboratory to the distant Kamioka mine from 1999 to 2004. “The neutrino is now becoming one of the central issues in elementary physics,” said Atsuto Suzuki, Director General of KEK and former spokesperson of KamLAND, another type of neutrino detector that found neutrinos generated at the centre of the Earth. “There are many exciting challenges in this area. One of the most important milestones for the development of neutrino physics is to verify experimentally that the oscillation of muon-neutrinos to tau-neutrinos is the one that has been discovered in atmospheric neutrino observations. I am very pleased that the CERN and Gran Sasso experiments will soon answer this important question.”
James Gillies, CERN Communication group
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Eugenio Coccia, director of National Laboratories of Gran Sasso
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Barbara Gallavotti, head of the Office for Communication of INFN
Telephone: + 39 06 68 68 162; + 39 335 6606075
Video material available from Silvano de Genaro, CERN Communication group
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