Fine-Tunings at Particle Scales

Particle physics, like most fundamental sciences, has a number of open questions that are generally perceived as being of utmost importance. Accordingly, trying to address those questions motivates most of the experimental and theoretical activity in the field. In particle physics, these questions can be classified into two main categories. First, there are questions that arise from experimental observations – for instance, what are the constituents of dark matter, or what is the origin of the matter-antimatter asymmetry seen in our Universe? These questions have a well-defined, unique answer which must be understood before a complete understanding of the fundamental theory that governs all interactions can be claimed. Additionally, there are questions that are more related to aesthetic arguments and to our belief that the fundamental theory should obey certain criteria of symmetry and simplicity that we often refer to as the beauty or elegance of a theory. The most common queries in the second category include, for example, why are there three generations of quarks and leptons in the Standard Model (SM) of particle physics, why is the mass spectrum of fermions hierarchical, what is the origin of neutrino masses, what resolves the strong charge-parity (CP) problem, and why are the electroweak (EW) scale and the Planck scale so widely separated? These questions, some which will be discussed in detail in the following, are sometimes considered even more fundamental and interesting than those of the first type. For instance, discovering that dark matter is an axion particle would be a great achievement but would not fundamentally advance the understanding of our ultimate theory. On the other hand, it is not clear whether all questions of the second type have a well-defined answer.