THE NEUTRINO:
WHAT IS IT ?

It's nothing, almost nothing. As would say F.Reines, it is "... the most tiny quantity of reality ever imagined by a human being". Despite that (or because of that!), this particle never ceased to question physicists and to give headaches to the one who wants to detect it.

Within the standard model

After many works, mistakes or successes, the particle physicists have finally obtained a model quite satisfying for now, allowing to describe the particles of matter and the way they interact. This model, called "Standard Model", has reduced considerably the number of "elementary elements" the matter is composed of; those elements seems as beautiful as what, near 400 BC, the greek philosopher Democrite was calling "atoms".

According to the Standard Model, 12 particles are the base of matter: 6 quarks (a silly name taken from a James Joyce novel) and 6 leptons (a word coming from the greek word leptos which means light, tiny).

Apart from the nu_tau

neutrino, all the particles, leptons and quarks, have been put into evidence experimentally, thanks to particles accelerators and detectors or bubble chambers or both. Their existence has a very stable experimental fundement. To each charged lepton (electron, muon, tau) is associated a neutral lepton or neutrino ( nu_e

for the electron, nu_mu

for the muon, nu_tau

for the tau). Same rule for the quarks which are grouped by pairs.
Within the standard model, the neutrino has a zero mass, a zero charge and a spin 1/2. It can be of 3 different types or families presented upper. All this is like a directory perfectly ordered. In fact, many important questions concerning the neutrino are still not yet experimentally resolved.

The uncatchable neutrino

The neutrinos, under the three types nu_e

, have kept since 1956 (year of their first experimental evidence) many mysteries. We know 4 fundamental interactions between the particles: the strong interaction. the electromagnetic interaction, the weak interaction and the gravitational interaction. The neutrinos are only concerned with the weak interaction and this allows to them to pass through the earth without any deviation or so. They interact at the best only one time over one billion in the huge apparatus built to detect them.

Today, in 1996, the three questions of 1956 are still opened:

Is its mass zero ?
Has it a magnetic spin ?
Is it identical to its anti-particle ?

Following is a very short summary of experimental knowledge in 1998 about the neutrino.

Spin	Mass	Magnetic spin	Cross section on nucleon at 1 GeV
1/2	< 2.8 eV	< 5.8 10^-20 MeV/T	about 10^-38 cm²
1/2	< 170 keV	< 4.3 10^-20 MeV/T	about 10^-38 cm²
1/2	< 18.2 MeV	< 3.1 10^-17 MeV/T	about 10^-38 cm²

The incredible neutrino

Moreover, the neutrino seems to have a remarkable property: its spin is always oriented in the direction opposite to its velocity (it is said to be of left helicity). The anti-neutrino is always of right helicity (spin in the same direction as the velocity).

This hypothesis was imagined in 1956 by T.D. Lee et C.N. Yang and was confirmed in the following year by Ambler, Hayward, Hoppes, Hudson and Wu, observing an asymmetry of the electrons coming from beta decay of Cobalt 60 nuclei. (Phys. Rev. 105, 1413 (1957)).

If Majorana is right (neutrino identical to the anti-neutrino) or if the neutrino is massive, then the neutrino will not always respect the following observed rule: a neutrino is always left and an anti-neutrino is always right.

With astonishment, we found also this asymmetry in the macroscopic world of life, where almost all amino-acids are of left chirality, while ADN uses only right chirality sugars.

The biological phenomena and the the weak interactions are, at first sight, the only physical processes which differ from their mirror image.

The oscillating neutrino:

The quarks composing matter are not independent from each other, a "quantum mixing" exists between them. In the same way, the neutrinos nu_e , nu_mu and nu_tau , if they were massive, could be mixed by quantum mechanics: a neutrino traveling in space would then be a mixture of nu_e , nu_mu and nu_tau .

This oscillation between neutrino families could help to explain the deficit observed in the solar neutrino flux and could be a good experimental tagging of the fact that neutrinos are massive. Many experiments near nuclear plants or at particles accelerators have tried to explore this way since more than 20 years, in vain up to now. But since 1996, more and more indices in favour of neutrinos oscillation appear.

To access to the neutrino mass by detecting its eventual oscillations is not an easy task. This indirect measurement depends on two main parameters: the quantum mixing angle between the concerned types of neutrinos and the difference of their squared masses.

and

The result of an experiment looking for the neutrinos oscillation is often presented on a plot where is shown

as a function of

A simple relation gives the probability of oscillation between two types of neutrinos
(for instance nu_e and nu_mu )

where L is the length (in meters) needed for one oscillation cycle and E is the energy (in MeV) of the neutrinos. You can remark that the lighter is the neutrino, the longer is the oscillation length. Thus, a neutrino nu_e

of mass 0.001 eV/c², coming from the sun (it has an energy of about 1 MeV) and oscillating with a neutrino nu_mu

of mass 0.1 eV/c² will become a nu_mu

after a travel of 400 meters and will be again a nu_e

after 800 meters.

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Last update: 26/06/1999 : http://wwwlapp.in2p3.fr/neutrinos/ankes.html