Thierry Stolarczyk

Cosmic particle hunter

The neutrino, a ghost particle

The "chlorine" solar neutrino detector, in the Homestake underground laboratories, the first to have detected solar neutrinos in the early 70's.

Other pages on this site:

  • Particles
  • Gallex, detecting solar neutrinos
  • Nomad, looking at neutrino osicllations
  • Antares, an undersea telescope for cosmic neutrinos.
  • CTA, detecting cosmic very high energy photons.

    • The neutrino is a particle from the infinitely small, like the electron for instance. It can cross huge amount of matter without being stopped. Neutrino hunting is always delicate although contemporary discoveries allow a better knowledge of its properties. Because of its extremely weak interaction with matter, the neutrino makes the Universe "transparent".

    Neutrinos are emitted during various processes : in the heart of the Sun during the fusion of hydrogen into helium (solar neutrinos), in natural radioactivity (beta emission), in the interaction of cosmis rays in the upper layer of the atmosphere (atmospheric neutrinos), during fusion reactions in nuclear plants. They could also be emitted in cosmic objects like active galactic nuclei or supernova remnants when accelerated protons from these objects interact with the surrounding medium.

    Neutrino history began in December 1930 when Wolfgang Pauli postulated its existence to explain that in a β-decay process, a kind of natural radioactivity, the electron energy spectrum is continuous instead of being limited to a unique peak, as it was observed from the beginning of the century.
    In the β decay process, inside a nucleus, a proton decays into a neutron, and emits an electron and an anti-neutrino (or a neutron decays into a proton, and an anti-electron and a neutrino are emitted):

    p → n + e- + νe
    or
    n → p + e + + νe

    In the abscence of the neutrino, the electron would have a fixed energy corresponding to the mass difference between the initial and final nuclei (considered to be at rest). Experiments show that this is not true: the electron can have all energies from zero up to the maximum avalaible energy, the nuclei mass difference. This requires another particle that share the available energy with the electron.

    The "tiny neutral" received its Christian name from Enrico Fermi, at the time he wrote the "β-decay theory" nowadays called the Fermi theory. In 1956, twenty six years after having its existence postulated, the neutrino was discovered by C.L. Cowan and F. Reines when they observed the first interactions of anti-neutrinos emitted from a nuclear fission plant.

    Neutrinos, as they have no charge, only interact with matter through the "weak interaction" (see the particle page for more). A one GeV neutrino (1 GeV = 1 billion of electron-Volts), an energy typical of most of the cosmic-rays arriving at the top of the atmosphere, has only a chance over 10 millions to interact in crossing the whole Earth.

    The elementary particle standard model describes the matter as 6 quarks and 6 leptons organised in 3 families (see the particle section for more). Each lepton has a flavour, electron, muon or tau, associated to the "electron-neutrino", "muon-neutrino" and "tau neutrino" (respectivelyνe, νμ et ντ). Results obtained this last decennia in the experimental domain, in particular at CERN, validate the elementary particle standard model. However this later does not say anything on the neutrino mass. Results accumulated up to 2001 on neutrino oscillations experiments show without any ambiguity that the neutrino has a non-zero mass.

    Similarly to the cosmic microwave background at 3K, the Universe is filled by 2K neutrinos from the Big-Bang. This corresponds to an energy of 0.3 milli-electron-volt. Their density is about 100 per cm3 per flavour, whereas the proton density is only 10-7 per cm3. We know today that neutrino masses are too small to give a solution to the dark matter problem. With such an energy the interaction probability of the cosmological neutrinos is very small: they can travel  through our universe without interaction on distances as large as... 1032... light-years !