An Overview of Astroparticle Physics

Abstract

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 [1Br11,1Br12]. The SM predictions had precise confirmations from the measurements performed at the LEP and SLAC electron-positron colliders, with the discovery of the $top$ quark at the Tevatron $p\overline{p}$ 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 SM has many free parameters which need an experimental input (the masses of leptons, quarks, and gauge bosons; the mass of the Higgs boson; the coupling constants;...); the three-family structure of lepton and quarks remains unexplained; the SM does not contain gravity; there are several unresolved “fine-tuning” problems; there are several unresolved “aesthetic” problems, such as the fact that the electric charge of the fundamental fermions and bosons is quantized in multiples of $\frac{1}{3}e$, without a deeper justification. 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 the electroweak and strong interactions predict that new physics would appear at extremely high energies, $>$ $10^{14}$ GeV. It is in this context that astroparticle physics plays a fundamental role.