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## The Neutrino, A Weird Particle With Magical Properties

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This stealth particle known as the neutrino has always been a mystery draped into complexity. And this goes back to its birth! At that time, in 1930, Wolfgang Pauli was fighting to spare the holy principle of energy conservation while explaining the continuous spectrum observed for beta decay. He came to the idea of a no-mass, no-charge and weak-interacting particle: the neutrino. Christened first "neutron" by Pauli, its name was soon switched to neutrino ("little neutral" in italian) by Enrico Fermi after 1932, when Chadwick discovered the neutron in the atom. Nonetheless, its first public appearance did not occur until 1956 when Frederick Reines and Clyde Cowan observed the very first neutrino in the reversed beta-decay process.

### From the Standard Model to the Majorana neutrino

Particle physics research during the second half of the 20th century helped establishing a simple and comprehensive description known as the Standard Model. 12 elementary particles, 6 quarks and 3 lepton pairs, grouped in 3 families, and 12 associated anti-particles:

 Elementay particles 1st Family 2nd Family 3rd Family Quarks u "up" d "down" c "charm" s "strange" t "top" b "bottom" Leptons electron electronic neutrino muon muonic neutrino tau tauic neutrino

All four known interactions (gravity, electromagnetism, weak and strong interactions) are described through an exchange of bosons as mediating particles between the quarks and leptons.

The neutrino of the Standard Model is massless and appears to have three flavors (e, μ et τ). Its only way to interact is the weak one: for instance, during a β decay, a u quark turns into a d quark by emitting a charged particle, the W- boson, and subsequently decays into an electron/anti-neutrino pair.

Fig. 1. Décroissance β

The Standard Model has long earned its reputation as a reliable tool for the understanding of particle physics and for numerous and precise predictions which turned out to be correct. Nonetheless, as a model, some limits to its capabilities are to be found.

The very fact that neutrinos oscillate (transition from one flavor to the other while the neutrino moves), which was observed in the Super-Kamiokande experiment in 1998 and later confirmed by many other experiments like SNO and KamLAND, proves that the neutrinos are not massless. This implies physical processes beyond the Standard Model.

Oscillation experiments do not give an absolute mass, but only mass differences. Now, it is crucial to measure the neutrino mass, which we already know is extremely small (at least a thousand times less than that of the electron), and this alone is already an enigma.

Not only the mass, but the nature of the neutrino has to be inquired. Ettore Majorana demonstrated that, as a neutral particle, the neutrino could be its own anti-particle. We call such a neutrino a Majorana particle, whereas it would be a Dirac particle in the opposite case. In order to explain the formation of the Universe with its mater/anti-matter asymmetry, most of the models rely on a Majorana neutrino-type! Talk about the huge impact in the Universe of such a tiny particle!

If the neutrino happens to be a Majorana one, then it opens the door for a process forbidden by the the Standard Model: the double-beta decay without emission of a neutrino. And then, apart from solving the enigma of the nature of the neutrino, what was out of reach for oscillation experiments, we have found the way to measure its absolute mass.

### The Double-Beta Decay

The double-beta decay with neutrino emission (2β2ν) is a seldom process with a half-life of some 1019 years. Seldom, but observed: for some nuclei, a simple β decay to the daughter nucleus is not energetically possible, whereas two simultaneous decays are allowed.

For a Majorana neutrino, one of the two emitted anti-neutrinos might well be seen as a neutrino by the second neutron and consequently absorbed during the second process instead of being emitted. In this particular case, there is no neutrino emission. Such a sequence of events should occur far less often then the 2β2ν decay, with a half-life around 1025 years!

Fig. 2. Décroissance double beta

How to distinguish the two scenarii? By measuring precisely the energy of the two emitted electrons, in the case of emitted neutrinos, the value will be only a portion of the total Qββ energy involved in the decay. Conversely, il there is no neutrino emitted, the total energy carried by the two electrons will be exactly Qββ and we will see a discrete line in the spectrum at a well-defined energy.

Such a signal is the observable signature of the neutrinoless double-beta decay that experiments like NEMO look for.

Spectre double beta