Among elementary particles, the neutrino has a mysterious charm. It
is very difficult to observe, its dimensions are difficult to detect and
its mass is apparently null (it is probably extremely small). Yet the neutrino
has the same spin as ordinary electrons: a puzzling massless spinning top
, whose spin effects can only be contemplated by quantum mechanics. In
1896 Becquerel observed a phenomenon that is now explained assuming that
neutrinos exist: the "nuclear 
decay". Pauli was the first to assume the existence of neutrinos in
1930. In 1932 Fermi put forward the first theory describing its interactions,
which could only be observed in 1956. Despite its long history, the neutrino
still leaves unsolved enigmas, whose implications to astrophysics and cosmology
make this particle all the more interesting.
Becquerel observed that uranium minerals emit radiations that remain impressed in photographic emulsions. During this "magic" process, elementary particles are created apparently from nothing and the chemical element emitting them is spontaneously transformed into a different element. Should the transmuting elements be iron and gold respectively, the ancient dream of alchemists would become true!
An atom is made of a nucleus, that is a very dense cluster of protons and neutrons. very tightly linked together, surrounded by as many electrons as there are protons. The charge of protons is equal and opposite to that of electrons, thus the global electric charge is zero. An atom' s chemical properties are determined by the way the more external electrons (in contact with other atoms) are laid out.
The extraordinary phenomenon discovered by Becquerel is due to the fondamental "weak" (meaning infrequent) interaction that changes an electrically charged proton into a neutron or vice versa. The total charge of the nucleus thus changes; but since the atom must be electrically neutral, the number of its electrons has to change and this is how its chemical properties change! Let us make an example. An argon nucleus has 18 protons and 22 neutrons. An argon nucleus produced artificially or spontaneously in nature with an insufficient number of neutrons is unstable. Thus, it decays and by the weak interaction that changes a proton into a neutron the balance is restored. A nucleus with 18-1=17 protons is a chlorine nucleus. Chlorine is chemically very reactive and totally different from argon, that is defined "noble" because it has difficulty in making chemical bonds.
This is how chemical elements are transmuted. Let us now explain the
simultaneous radiation that made Becquerel's discovery possible by remaining
stamped on photographic emulsions,. When a neutron is transmuted into a
proton, the conservation of the total electric charge (a law that has never
been violated) requires that a particle with the same charge as an electron
(equal and opposite to the proton's) is emitted. Indeed an electron is
emitted or, in inverse transmutation, a positron - its antimatter particle,
with opposite charge. These particles are emitted directly by the nucleus
at the relatively high energies that come into play in nuclear reactions.
Following Becquerel's discovery, these emissions were called
rays, hence the name given to the decay. Only later it became clear that
they were electrons or positrons.
In decays, the
available energy E comes from the mass difference 
between the initial and final nuclei, according to equation 
. The energy of the visible particle that is emitted (an electron or a
positron) does vary in each event. Where does the "missing" energy
go? As an "extreme remedy" to explain this enigma, Pauli boldly
assumed the existence of a neutrino particle (indicated as 
) in 1930: this particle, invisible to us, would take away the apparently
missing energy (Figure 1).
"Neutrino" is the obvious name for such a particle. It is
neutral: had it got any electric charge, it would be visible to
our instruments, as are electrons and positrons. Its mass is extremely
small or null, otherwise a significant part of the available energy would
be spent in the neutrino mass (again 
), and electrons and positrons could never have as much energy as the one
avallable, as is sometimes the case.
Neutrinos are very difficult to detect because of their very low probability of interacting with matter, or more technically speaking, because of their "weak interaction". Out of one hundred thousand solar neutrinos reaching the earth, all but one pass through the earth unnoticed! Hence, the probability that a neutrino interacts with an instrument is very low, thus its detection probability is minimal. To make up for such a low probability, very massive and at the same time technically refined instruments are needed. The figure 2 shows a neutrino interaction from the experiment CHARM II, recently carried out at CERN in Geneva. Its mass of about 700 tons is equipped with a multitude of electronic channels to read the signals produced by the neutrino interaction.
More than 70 years after Pauli's hypothesis, these experimental difficulties
have left the basic properties of neutrinos a mystery. We still do not
know its mass, we can only say it cannot be higher than a certain value!
Yet neutrinos are not at all rare particles artificially produced for the
use of scientists. In nature they are abundantly created in the core of
stars and in particular our Sun, during the nuclear reactions originating
the electromagnetic radiation (including light) that carries to the earth
the energy needed for animal and vegetal life. In a second, the tip of
our finger (about 1 
) is crossed by a flux of about 100 billion solar neutrinos! They do not
cause any biological hazard because their probability of interaction is
practically null.
Because of their impressive power of penetration through layers of matters without interaction, the neutrinos produced inside the Sun reach its surface and the Earth and reveal significant astrophysics informations on the processes taking place in the core of the Sun itself. This is not the case with light or electromagnetic radiation, carrying information only on the outer layers. Neutrinos play a crucial role for understanding the mechanisms that keep the Sun on, similarly to x rays' role in radiography. Neutrinos also carry with them the weak and still ill-detected signals of far astrophysics phenomena such as star collapses, because they are not absorbed in outer space. Like classic astronomy uses light as a carrier of information, maybe one day there will be a "neutrino astronomy" using the big detectors of cosmic neutrinos instead of telescopes.
Moreover, in the first instants of the Universe along with the well
known "background cosmic radiation", a huge number of neutrinos
must have been produced, but we have not yet observed them due to their
very low energy. If one thinks that their present concentration is estimated
to be a hundred per 
, how many there could be in the whole cosmos! We know that gravitational
forces not only determine the motion of celestial bodies, but also the
entire evolution of the universe: gravitation tends to brake its expansion,
as is shown by Hubble's law (Figure 3). Multiplying
even a very small number for each neutrino mass by the number of neutrinos
in the cosmos, can make their total mass so considerable as to influence
the future of the universe ?