Questions relating to neutrinos


Matter in its most elementary form is divided into two families of particles, the leptons and the quarks.
Each family consists of six particles grouped in pairs.  In particular the leptons are comprised of the:

electron - electron neutrino

muon - muon neutrino

tau - tau neutrino


When a particle is discovered, one of our jobs is to measure its fundamental parameters.

 In the case of the neutrino moreover, where 25 years separate its proposed existence and its eventual discovery, the mystery looms larger. A list of questions can be asked about them :

Does the neutrino have a mass ?

If yes, what are the consequences?  As an aside, it is interesting to note that all the elementary building blocks of matter have mass.
Is the neutrino stable ?
Are there other neutrino species ?
Is the neutrino its own antiparticle?
Among these important questions we have chosen to emphasize the first, which has undoubtedly sparked the most interest among physicists.  Measurements of the  mass of the Universe are given in terms of its density relative to a "critical" density above which the Universe would expand forever according to Einstein's theory of General Relativity.

This quantity is analogous to the escape velocity of an object propelled towards the sky, the velocity above which the object can escape the Earth's gravitational attraction.

For our Universe, the critical density has been estimated to be several proton masses per m3.
Due to the large number of neutrinos present in the Universe, its mass can play a principle role :

- The neutrino mass only has to be a millionth ( 10-6 ) of the electron mass for the Universe to be dominated by neutrinos.
- If the neutrino has ten thousandths ( 10-4 ) of the electron mass, the Universe will stop its expansion and recontract.

From this, we can clearly see the importance that is assumed by the existence of a non zero neutrino mass.
Effectively, neutrinos are directly connected to cosmology since:
- the future of our Universe depends on them,
- they can partially resolve the equally burning question of the "dark matter" of the universe.

So its clear why nuclear, particle, and astrophysicists are interested in the neutrino problem.
In order to try to better understand this particle, we are forced to ask ourselves additional questions :
 

How and where are neutrinos produced ?
How do neutrinos interact with material and what effects can be used to detect them?
What kind of experiments can be performed ?

On that last point, there exists the remarkable consequence that if neutrinos have mass, then each "species" of neutrino could continuously transform itself from one to another in an effect called neutrino oscillations.  Inversely if neutrinos oscillate, then they must have a non zero mass.   This is why several experiments are based on the phenomenon of oscillation. The basic idea is:
For a beam of neutrinos of a known species, to detect on its trajectory neutrinos of another species. This is called an appearance experiment.
Or alternatively to compare the number of neutrinos of a known source (a reactor) with the number still present in detector further along the trajectory. This is called a disappearance experiment.