They are neutral (i.e. they carry no electrical charge) and the probability that they will interact with matter is extremely low. This is clearly illustrated by the fact that everyone on earth is traversed by the enormous quantity of 400,000 billion neutrinos from the sun every second. Neutrinos pass through our planet with a very low probability of interaction with matter, which shows both their relative harmlessness and how hard they are to capture in physics experiments.

The difficulty of detecting neutrinos partly explains the mystery still surrounding them in particle physics. It has now been established that neutrinos have either only a very small mass or no mass at all. Choosing between these two possibilities is of capital importance both for the standard model and for our understanding of the Universe. We know that the Universe contains some 100 millions of "relic" neutrinos per cubic metre (originating from the first minutes after the "Big Bang" but only 0.5 protons in the same volume of space. If neutrinos have mass, then they make up part of galactic "dark matter". In that case the Universe could have sufficient gravity for it one day to stop expanding and to start contracting.

Neutrinos are divided into three families, called n_e, n_m and n_t, indicating that they are associated with the three types of charged particles: the electron (e), muon (m) and tau (t) (c.f. Figure 1 on page 5). If neutrinos have a small mass, it is possible that the three neutrino types are in some way or other "mixed" (c.f. Figure 2 on page 7); for example, that a neutrino of type n_m could turn into a neutrino of type n_t. This phenomenon is what we call "neutrino oscillation".

Neutrinos of type n_e come from the sun and other natural sources. The second family of neutrinos, the n_m, are produced for example in the decay of particles resulting from collisions between two protons:

These collisions between protons can occur in the earth’s atmosphere (protons are among the cosmic rays) as well as in particle accelerators. The pions and kaons produced are particles from the so-called "meson" group. The pion consists of a quark u and an antiquark , the kaon also contains a quark u, but is accompanied by a heavier partner, an antiquark .

The first neutrino beam at CERN was built in the early nineteen sixties. In 1973, the "Gargamelle" bubble chamber in the PS¹ neutrino beam gave CERN one of its most important discoveries, called the «neutral current interaction»: neutrinos can interact with other particles while remaining neutrinos.

¹ PS, "Proton Synchrotron", accelerator built at CERN in 1959, with a proton energy of 28 GeV, was the most powerful accelerator of its day. It still serves today for experiments and as pre-accelerator of the SPS ("Super Proton Synchrotron").

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