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CERN switches on neutrino beam to Gran Sasso
Geneva, Switzerland, September 11: CERN has switched on a new neutrino
beam, aimed through the earth to the INFN 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."
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