"CESR, PEP, PETRA, ISABELLE, p-bar p colliders, LEP, the tevatron, and ep machines are at various levels of design or construction. They will study the properties of b-matter, see weak intermediaries, and perhaps find the t-quark and the Higgs boson. Never before was there such a bestiary waiting to be discovered; and what surprises will be found!" - S. L. Glashow ("The Future of Elementary Particle Physics," Quarks and Leptons, NATO Advanced Study Institutes Series Volume 61, 1980, pp 687-713).

The situation in 1980 was clearly different from the present situation in 2013, in which we face the very real possibilty that no new degrees of freedom will ever again be within reach of a collider. In an intriguing twist of fate, this very fact results in a sharp paradox for fundamental physics: the Higgs mass should be MP/m h ∼ 1017 times larger than it actually is, and the vacuum energy density of the universe should be (M P/A)4 ∼ (1031)4 times larger than it actually is, and apparently nature refuses to give us any more clues as to why. These together are what I would call the main problem of 21st century physics: despite all of the predictive success of particle physics so far, we must find a way to suitably modify the rules of quantum field theory, lest we accept the unproductive defeatist attitude that our universe is simply fine-tuned.

In the meantime, there is much interesting work to be done in more "traditional" particle physics: we have learned that neutrinos actually have tiny but nonzero masses, which is clear and unambiguous evidence for physics beyond the Standard Model. I will allocate the first third of this document to phenomena related to neutrino oscillations. In particular, I would like to argue that some of the apparent differences between neutrino mixing and quark mixing are to an extent illusory, and actually many aspects of the two sectors can be understood in a coherent framework for extending the Standard Model.

The remaining two-thirds of this document are devoted in one way or another to phenomena associated with the Higgs sector of the Standard Model. Now that the Large Hadron Collider has finally observed what is presumably the Higgs boson with mass mh ≈ 125 GeV, every parameter in the Standard Model is known. However, there remains an enormous freedom to extend the scalar sector of the Standard Model, and I will study a particular multi-Higgs framework, which is called the "Private Higgs" model. Within this framework it becomes clear that the ratios of Yukawa couplings may be completely uncorrelated with the ratios of the corresponding quark masses and that much of the Higgs sector may remain shrouded in mystery for years to come.