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Standard model extensions with massive neutrinos :Phenomenology, model building and cosmological implications

Final Activity Report Summary - NEUTR-COSMO-ACCEL (Standard model extensions with massive neutrinos: Phenomenology, model building and cosmological implications)

The project is in the field of fundamental research in High Energy Physics, and focuses on interdisciplinary studies that bring together Theory with Experiment, and Elementary Particle Physics with Astrophysics and Cosmology. It also combines analytical calculations with computations based on modern software tools. The project addresses the following topics:
1. Phenomenological and theoretical study of Standard Model extensions with massive neutrinos and lepton number violation, in association with the generic fermions mass problem. These models are motivated by the neutrino data of the recent years.
2. Detailed analysis of the implications of such theories for searches for new particles and interactions in present and future experiments, with particular focus at the LHC at CERN. In this respect, concrete collider search channels have been put forward and analysed.
3. Study of related cosmological implications and predictions for Leptogenesis-Baryogenesis, Dark Matter, Dark Energy and Inflation. The impact of symmetry breaking on density fluctuations and structure formation in the universe has also been analysed.

Here, we summarise some of the key points:
Neutrino textures that match the experimental data have been identified and compared to the ones expected in unified theories. Their predictions for exotic rare decays of charged leptons and kaons have been analysed, and observables that could be tested in future experiments have been proposed. In doing so, the most recent experimental and observational data was constantly monitored and used to derive constrains on the structure of the fundamental theory of nature.

Detailed studies have been performed on CP-violation in the leptonic sector (an open question that attracts a lot of attention, particularly in view of neutrino factories), and its relevance for leptogenesis-baryogenesis. In all calculations higher order quantum corrections have been included, and concrete correlations to test different theoretical constructions have been put forward. An elaborate set of codes has been composed, in order to enable the combined analysis of many different (but inter-dependent) observables, leading to additional predictions for the neutrino mass and mixing parameters that are currently less constrained by the data.

In the collider searches part, we looked at different manifestations of lepton number violation (either through mixing effects and quantum corrections, or via new interactions). Emphasis has been given on the analysis of novel cascade decays in experiments such as the LHC but also the Linear Collider. In doing so, a detailed analysis of backgrounds had to be performed, and combined with studies of the supersymmetric parameter space. In this case as well, significant effort has been invested on computational aspects, and the development of programs that enable a precise analysis of events with multi-particle final states and novel multi-lepton and multi-jet signatures.

A interesting topic in this respect is to identify the origin of dark matter in models where additional interactions make the lightest supersymmetric particle unstable, thus changing its behaviour not only in colliders, but also in the early universe. Along these lines, we studied for the first time radiative gravitino decays to a photon and a neutrino, generated via couplings that violate lepton or baryon number. We found that here is a wide region of parameters for which gravitinos have a lifetime that is longer than the age of the Universe. This scheme is testable by (i) concrete signals in colliders, (ii) neutrino mass terms, and (iii) photonic signatures, which have also been analysed and compared to recent data.

In view of the extensive input from WMAP (Wilkinson Microwave Anisotropy Probe), we enhanced the initial proposal with Dark Matter studies, and respective constraints on the parameter space of our theoretical models. Additional work, on Cold Dark Matter in non-standard scenarios that also account for Dark Energy in the Universe, has been performed; this indicated that for a wide range of parameters it is possible to evade some of the dangerous constraints that often appear in conventional schemes.

Leptogenesis in the early universe has been studied in different particle physics models and viable schemes have been identified. New ways to realise lepton, and subsequently the observed baryon asymmetry of the Universe through flavour and resonant effects have been proposed, and concrete classifications of theoretical constructions have been put forward.

Finally, we studied cosmological phase transitions due to symmetry breaking, particularly with domain walls acting as seeds of structure formation. Unlike what is expected in most theories, we found that biased phase transitions may generate sufficient density fluctuations at both large and smaller scales, while being compatible with the measurements on Cosmic Microwave Background Anisotropies. After identifying qualitatively viable theoretical models, we proceeded to a detailed numerical study of the evolution of domain wall networks in the early universe.