Astroparticle physics: rare phenomena under the mountains
The Gran Sasso Laboratories
Some phenomena occur only at energies so high as to be beyond
the reach of accelerators, but also happen spontaneously, if
very rarely. Such events are easy to confuse with much more
commonplace phenomena due to cosmic rays and to naturally occurring
radioactivity. A laboratory such as that at Gran Sasso, where
a natural shield of 1400 meters of rock blocks the passage of
all but one particle for every million incident on the surface,
is therefore an ideal place for the study of such phenomena.
The Gran Sasso Laboratories are, both in size and in technology,
the most advanced underground laboratories on the worldwide
research scene. The Laboratories were completed in 1987, and
today host a dozen experiments conducted by about 600 researchers
from all over the world. The study of neutrino properties represents
one of the principal areas of research at the Gran Sasso Laboratories.
The GNO experiment is measuring neutrinos emitted from the Sun,
and will soon be joined by the BOREXINO and ICARUS experiments,
which are under construction. The LVD experiment is waiting
to detect the neutrinos emitted from the explosion of a supernova.
Delicate experiments are trying to measure the mass of the electron
neutrino. In the ambitious CNGS (CERN Neutrinos to Gran Sasso)
program, a beam of neutrinos will be produced at CERN, in Geneva,
and directed towards detectors installed at Gran Sasso. Other
important experiments (DAMA, CRESST, and HDMS) are searching
for the “dark matter,” which probably makes up the
majority of the universe.
The large experiment MACRO came to a conclusion recently after
having conducted a vast spectrum of studies, from the observation
of neutrinos produced in the atmosphere, to the search for new
particles.
Geologists and biologists as well are capitalizing on the unique
characteristics of the Laboratories in order to conduct cutting-edge
experiments.
The Sun shines with neutrinos
Neutrinos from the Sun carry real-time information about the
fusion reactions that take place in the solar interior, and
allow physicists to investigate the mechanisms involved.
The fact that neutrinos rarely interact with matter means that
the Gran Sasso massif is completely transparent to them, and
they can therefore reach the experiments in the underground
laboratories without being stopped.
GALLEX was the first experiment in the world to detect electron
neutrinos from the thermonuclear fusion reactions that produce
energy at the center of the Sun. Even if a billion trillion
neutrinos traverse the experiment each day, on average, only
one is absorbed by a nucleus of gallium. GALLEX demonstrated
a singular phenomenon which occurs during the neutrinos’
long voyage from the center of the Sun to the Earth: some of
the neutrinos—about half—disappear, or rather, presumably
transform into other types of neutrinos which GALLEX is not
capable of detecting. In place of GALLEX, another, more advanced
experiment, GNO, has been running since 1998.
BOREXINO will allow further progress, using specially developed,
cutting-edge techniques, and will study neutrinos in real time
and in a specific energy interval in which a behavior hypothesized
according to the new understanding of neutrino properties should
be most visible. The detector consists of a central part, which
is sensitive to neutrinos, surrounded by two shielding liquids,
which offer protection from outside disturbances. On rare occasions,
a neutrino will strike an electron in the central part of the
detector, giving rise to a small flash of light, which will
be captured by sensors. The extreme purity required of the detector
materials has already been attained, and the construction of
the experiment is nearing conclusion.
A 730-kilometer subterranean journey
The CNGS project envisions the construction of a source of
muon neutrinos at CERN, in Geneva, to be directed towards the
Gran Sasso Laboratories, 730 kilometers away. When construction
is complete, in 2005, the neutrinos will travel from Geneva
to L’Aquila along a straight-line path. Neutrinos can
traverse enormous distances in material of whatever sort, with
the same ease with which light travels through a windowpane.
 Neutrinos
are the most difficult to study of the elementary particles,
but they play a fundamental role in our understanding of nature
on both the microscopic and cosmic scales. Our current understanding
of elementary particles is based on the hypothesis that all
neutrinos are massless. During the last few years, however,
experiments in underground laboratories, including GALLEX and
MACRO at Gran Sasso, have shown that this might not be the case.
Signs of the so-called neutrino oscillation phenomenon, a quantum
effect in which neutrinos of one type can turn into another,
have been observed.
Experiments at Gran Sasso will search for tau neutrinos created
in transformations of this type during the voyage from CERN
to Gran Sasso.
 Two
planned experiments, ICARUS and OPERA, are currently under preparation.
Each uses a different methodology to produce precise event images.
One exploits the properties of liquid argon; the other exploits
those of photographic emulsions. OPERA can detect both the point
at which a tau particle is produced from a tau neutrino and
the point at which the tau particle subsequently decays, about
one millimeter away, thereby unambiguously identifying the phenomenon.
ICARUS, on the other hand, will yield precise information about
each event, allowing the presence of the tau to be reconstructed
using statistical methods.
|
