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From neutrino mass phenomenology to the particle physics theory beyond the Standard Model and related signatures in cosmology and colliders

Final Activity Report Summary - BEYOND NEUTRINO MASS (From neutrino mass phenomenology to the particle physics theory beyond the Standard Model and related signatures in cosmology and colliders)

The Standard Model describes precisely most of the observed phenomena in particle physics, however it cannot explain by itself the small but non-zero mass of the neutrinos, the matter-antimatter asymmetry of the Universe, and the existence of dark matter. Moreover, the Standard Model depends on about 20 parameters whose values are not explained, including the strength of gauge interactions as well as the lepton and quark masses and mixing angles.

In this project we studied the theories for particle physics beyond the Standard Model, that can address the above issues, and we built new models that provide correlations between the various observables and testable predictions for future experiments.

One part of the project's efforts was devoted to models of flavour, that is, the attempt to explain the masses and mixing of the three flavours of quarks and leptons. Recent neutrino oscillation experiments showed that a lepton mixing angle and a ratio among neutrino masses are both small; we identified the minimal flavour symmetry that can relate the two parameters and thus predict the value of one as a function of the other. The other two lepton mixing angles are observed to take large values (close to 30 and 45 degrees); in theories where quarks and leptons are unified, this is unexpected because quark mixing angles are known to be small; we built the minimal models that account at the same time for the specific values of lepton mixing angles and for the small mixing and hierarchical masses of the three quark flavours.

The second part of the project dealt with two very appealing ideas to extend the Standard Model. On the one hand, supersymmetry is a property of particles that may be discovered at the next generation of collider experiments. It associates to each standard particle a partner with a different spin (still to be discovered) and it provides a very elegant explanation of the smallness of the Standard Model energy scale (about 100 GeV) with respect to the scale of gravity (about 10 to the power 19 GeV). On the other hand, grand unified theories merge the different values of the 3 gauge couplings into a unique one and also explain the quantization of the electric charge.

We built a fully realistic and predictive class of supersymmetric grand unified theories, based on the gauge symmetry group SO(10). In addition to the above motivations, these new models provide a common origin for the masses of neutrinos and the matter-antimatter asymmetry of the Universe. The two phenomena are related to the same set of couplings between the leptons and a very heavy scalar particle, that realize the so-called seesaw mechanism of type II. The seesaw particles are extremely heavy and not accessible directly in experiments, but they leave a number of footprints in the low energy observables.

In fact, besides explaining the value of neutrino parameters and of the matter density of the Universe, the seesaw particles also modify the masses and mixing of the supersymmetric partners of the Standard Model fermions. These so-called sfermions induce rare particle processes that change the flavour of fermions and may also break the charge-parity symmetry. We examined the predictions of the new class of SO(10) models for these processes as well as for the sfermion mass spectrum that can be observed at colliders.