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Neutrinos and other probes for new physics

Final Report Summary - NEUPROBES (Neutrinos and other probes for new physics)

The particle physics Standard Model is an extremely successful and predictive theory of the constituents of matter. However, there is solid evidence for new unknown physics beyond it to be discovered. Venturing beyond this frontier in our present understanding and unveiling the new physics behind represents the main goal of research in particle physics.
While many aspects of the Standard Model indicate the existence of a more fundamental theory, neutrino physics, together with dark matter, represents our only experimental evidence so far and thus a most promising window to probe the underlying theory beyond. In 2015, the physics Nobel prize was awarded to Takaaki Kajita and Arthur B. McDonald "for the discovery of neutrino oscillations, which shows that neutrinos have mass". This breakthrough indeed requires the SM to be extended so as to accommodate the neutrino masses and mixings observed in oscillation experiments.
In particular, the extreme lightness of neutrino masses, as compared to all other fundamental particles, hints at a qualitatively different origin for them. Indeed, neutrinos are special among the other SM fermions in that could be their own antiparticles. This breaking of the separation between particles and antiparticles in the neutrino sector could in turn help us understand the origin of the particle excess in the Universe to which we owe our existence and that remains an open mystery in the Standard Model. The main goal of the Neuprobes project was the study of the open questions in the neutrino sector so as to shed light into these fundamental issues.

The present experimental situation hints that neutrinos and their antiparticles (antineutrinos) could behave differently, breaking the particle-antiparticle symmetry, otherwise almost exact in the rest of the Standard Model. The breaking of this symmetry is a necessary ingredient to understand the origin of the matter (over antimatter) excess that explains the existence of galaxies, stars, planets and life. The Neuprobes project has actively participated in the studies and proposals of three new neutrino oscillations facilities: T2HK (in Japan), DUNE (in the USA) and the ESSnuSB (in Europe) planned to explore this important open question. The sensitivities of all three facilities to the unknown parameter that controls the violation of the particle-antiparticle symmetry have been extensively simulated and optimizations to their design have been proposed so as to maximize this sensitivity. We can conclude that any of these three projects, if approved and built, would either confirm or rule out with high significance the present tantalizing hint for the violation of this symmetry.

Another line of research explored in Neuprobes has been the analysis of present precision data to explore the existence of extra heavy, very weakly interacting neutrinos. The existence of these extra neutrinos at some mass scale is a general prediction of most beyond the Standard Model theories explaining the non-vanishing but extremely small neutrino masses discovered. Neuprobes has found an interesting hint for their existence, but its significance is still too low to be conclusive. Future measurements to further analize this hint have also been proposed. At higher statistical significance, a large part of previously allowed parameter space has been excluded where no extra heavy neutrinos have been found. These studies have had significant impact in the community, constituting the most exhaustive and updated set of constraints on this source of new physics that could explain our present evidence for neutrino masses.

Finally, Neuprobes has also explored alternative explanations to the particle nature of the mysterious dark matter component of the Universe. In this context, alternative theories with dark matter candidates have been proposed and the best search strategies to probe for them have been described. Since other more well-studied theories have not been confirmed experimentally so far, it is important to widen in this way our theoretical framework, to avoid narrowing too much our experimental probes and be able pin down the particle properties (such as its mass and interactions), that could determine potential future applications, of dark matter.

The original scientific research carried within the Neuprobes project can be found the following webpage: http://projects.ift.uam-csic.es/neuprobes/#disseminations.