The strong interactions, which describe the forces between quarks and their confinement into nucleons and other hadrons, appear to be invariant under inversion of the spatial coordinates (P-symmetry) and its combination with charge-conjugation (CP-symmetry). In fact, the violation of P and CP is described by the dimensionless parameter θ, which is constrained by the experimental limit on the neutron electric dipole moment: θ < 10^-10. Hence θ is the smallest parameter of the SM. Its small value is particularly puzzling because we know that P and CP are not symmetries of the SM, as they are experimentally observed to be largely violated in the weak interactions. The dimensionless parameter describing CP violation in the weak interactions is order unity. Therefore, the strong CP problem is the lack of theoretical understanding of the smallness of θ. My work aims to advance our comprehension of the landscape of theories and mechanisms that could address this puzzle and to pinpoint the most promising experimental directions to test them.

Modern cosmological models necessarily include an electromagnetically neutral, non- baryonic and non-relativistic matter species, generically referred to as “cold dark matter” (DM). For the benchmark ΛCDM cosmology the DM accounts for ∼ 84.4% of the total matter density in the Universe and its fundamental makeup is, as of yet, unknown. To investigate the nature of the DM, my work proceed on two fronts:

• I study the phenomenological connections with other theoretical problems and the embedding of their solutions in a common model. A prime example of this work includes studies related in connection to the strong CP problem and the flavor puzzle.

• I study the signatures of various DM scenarios and how to probe them directly or indirectly. Especially important for this is the complementarity between laboratory experiments and astrophysical observation to probe motivated DM scenarios featuring novel particles and interactions. In fact, astrophysical environments, such as supernovae and neutron stars, allow to access scales of temperature and density not reproducible in a laboratory and processes otherwise not observable.

Many open questions remain about the fundamental properties of the matter fields in the Standard Model (SM). For instance, the SM does not explain why there are three generations/flavors of fermions, the origin of the large hierarchy in the fermion spectrum and their mixing, or the origin of neutrino masses. Collectively, these unresolved issues are referred to as the 'flavor puzzle'. The unique experimental testability and the significant ongoing experimental effort make this sector a prime playground for building and testing theories beyond the SM.

My work aims to explore solutions to these puzzles, their embedding within complete extensions of the SM, their consistency with available experimental data and their testability. The outcome of this study is the development of theoretically motivated models, whose experimental investigation could potentially lead to the direct discovery of new physics. In this context, I wish to emphasize the importance of the complementarity between low- and high-energy physics in my research.