Neutrino Ettore Majorana Observatory

	Neutrinos are the second most abundant particles in the Universe. There are on average about 3 millions of them in a cubic meter. They are among very few truly fundamental particles on which we build our Standard Model of particle physics. Yet, neutrinos are the least understood objects in particle physics.
	In the Standard Model neutrinos are assumed to have exactly zero mass. However, now we have a pretty solid evidence that this is actually not true. This evidence comes from the experiments studying a phenomenon known as neutrino oscillations using atmospheric, solar and reactor neutrinos.
	Experiments like SuperKamiokande, KamLAND, SNO etc. observed a dissapearance of the neutrino of a particular flavour (electron, muon and tau) as it travels through the space-time which is consistent with the idea of oscillations between the three known to us neutrino flavours.
	This can only take place if mass differences between different neutrino flavours is not zero which, in turn, means that at least one of the neutrino states has a non-zero mass. Despite the fact that it proves the Standard Model incomplete, the observation of neutrino oscillations is actually a very good and exciting news.
	For a while physicists have been suspecting that the Standard Model is part of something bigger &mdash some kind of an ultimate theory which would unify all existing in nature interactions known as Grand Unification theories or GUT. It turns out that a small but non-zero neutirno mass is a requirement of most of the GUT theories.

There are many experiments, currently running, being constructed or planned that will use artificial neutrino beams created at particle accelerators to explore the neutrino oscillation phenomenon in great details. The UCL HEP group is actively participating in one of these projects &mdash the MINOS experiment.

However there are two questions which are absolutetly fundamental for our understanding of neutrinos and which neutrino oscillation can not answer:

1. What is the absolute value of neutrino mass?(recall that neutrino oscillations measure mass differences, or to be precise mass-squared diffrences, between different neutrino states, not the absolute mass).
2. Given that neutrino mass is not zero, why is it so much smaller compared to the masses of other fermions in the Standard Model?

It is clear that we have to provide an answer on the first question. But the second question might be even more fundamental. The answer on this question may be found if we can understood whether neutrino is a Majorana or a Dirac particle. All the fermions in the Standard Model are Dirac particles by nature, where the antiparticle is the charge conjugate of the particle and has equal but opposite quantum numbers. Neutrinos, however, have an extra possibility due to their neutral charge. It is not forbidden for neutrinos to have a Majorana mass which would imply that the neutrino and antineutrino are the same particle. This possibility seems to be favoured by the theoretical community, indeed it is a requirement of many GUTs. The Majorana nature of the neutrino leads to the non-conservation of the full lepton number and can address many fundamental questions. In particular it can explain a tiny assymetry between matter and antimatter shortly after the Big Bang which was necessary to make the surrounding us world evolve the way it did.

Experiments which can answer both questions are searching for neutrinoless double beta decay &mdash 0&nu&beta&beta. In fact, 0&nu&beta&beta is the only practicaly way to understand whether neutrino is a Majorana or a Dirac particle.

Double beta decay is a process in which, unlike the ordinary beta decay, two neutrons are simulataneously transformed into two protons.