Spurio - PARTICLES AND ASTROPHYSICS: a multi-messenger approach

Springer International Publishing Switzerland 2015

Maurizio Spurio 1

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The Standard Model (SM) of particle physics, which includes the theory of electroweak interaction and quantum chromodynamics for strong interaction, explains quite well all available experimental results in particle physics (Braibant et al. ). The SM predictions had precise confirmations from the measurements performed at the LEP and SLAC electron-positron colliders, with the discovery of the quark at the Tevatron collider. The theory was recently crowned by the discovery at the LHC of the last missing piece of the theory: the Higgs boson.

On the other hand, few physicists believe that the SM is the ultimate theory. Some considerations show that the SM is incomplete and represents a sort of low energy limit of a more fundamental theory, which should reveal itself at higher energies. These considerations are based upon the facts that:

The threshold for this higher energy limit could be so high that no accelerator on Earth, also in the far future, will be able to reach it. For instance, Grand Unified Theories (GUTs) of electroweak and strong interactions predict that new physics would appear at extremely high energies, GeV. It is in this context that astroparticle physics plays a fundamental role.

There are important connections between astrophysics, particle physics, and cosmology, in particular in the early Universe, which is commonly described as a gas of very energetic particles. As time proceeded, the Universe, Following the the Universe expanded, the energy per particle decreased, phase transitions took place, the nature of particles changed, and there was a symmetry breaking from unified to nonunified interactions, Fig.. In recent years, some indication from the study of the early Universe pointed out some features that are completely outside the SM, namely:

Grand Unification Theories foresee the nonconservation of the baryon and lepton numbers. Some models in the 1980s predicted proton lifetimes of the order of years. This lifetime is much longer than the age of the Universe, but experimentally measurable. This motivated the construction of large detectors aimed at a search for proton decay. The experiments contained more than 1 ton ( nucleons) of material, and were located underground to shield the experiment from the radiation due to cosmic rays. Only the penetrating component of secondary radiation, namely muons and neutrinos, is able to reach the detectors at such depths. Contrary to the optimistic expectation, no proton decays were observed (actually, the measured lifetime turned out to be longer than y) but the background events induced by the atmospheric neutrinos were particularly important. The events induced by atmospheric were roughly in agreement with the expectation. However, the number of events induced by was lower than expected. This was attributed to the phenomenon of neutrino oscillations , caused by a quantum-mechanical mixing between massive neutrinos. The definitive discovery of a nonzero neutrino mass in 1998 using atmospheric neutrinos was the result of long experimental investigation. This represented the first experimental hint for physics beyond the Standard Model.