Questions relating to neutrinos
Matter in its most elementary form is divided into two families of particles,
the leptons and the quarks.
electron - electron neutrino
Each family consists of six particles grouped in pairs. In particular
the leptons are comprised of the:
muon - muon neutrino
tau - tau neutrino
When a particle is discovered, one of our jobs is to measure its fundamental
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 :
Is the neutrino stable ?
Are there other neutrino species ?
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.
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.
Is the neutrino its own
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
For our Universe, the critical density has been estimated to be several
proton masses per m3.
- 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.
Due to the large number of neutrinos present in the Universe, its mass
can play a principle role :
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 :
What kind of experiments can be performed
How and where are neutrinos produced ?
How do neutrinos interact with material and
what effects can be used to detect them?
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.