Broadly defined, particle physics aims to answer the fundamental questions of the nature of mass, energy, and matter, and their relations to the cosmological history of the Universe.
As the recent discoveries of the Higgs Boson, neutrino oscillations, as well as direct evidence of cosmic inflation have shown, there is great excitement and anticipation about the next round of compelling questions about the origin of particle masses, the nature of dark matter, and the role leptons, and in particular neutrinos, may play in the matter-antimatter asymmetry of the Universe.
The energy scales relevant for these questions range from the TeV to perhaps the Planck scale. Experimental exploration of these questions requires advances in accelerator and detector technologies to unprecedented energy reach as well as sensitivity and precision. New facilities coming online in the next decade promise to open new horizons and revolutionize our view of the particle world.
Particle theory addresses a host of fundamental questions about particles, symmetries and spacetime. As experiments at the Large Hadron Collider (LHC) directly probe the TeV energy scale, questions about the origin of the weak scale and of particle masses become paramount. Is this physics related to new strong forces of nature, to new underlying symmetries that relate particles of different spin, or to additional spatial dimensions that have so far remained hidden? Will this physics include the particles that constitute the dark matter of the universe, and will measurements at the LHC allow a prediction of the observed cosmological abundance? String theory remains the leading candidate for a quantum theory of gravity, but a crucial debate has emerged as to whether its predictions are unique, or whether our universe is part of a multiverse. All of these fundamental questions about particles and spacetime lead to corresponding questions about the early history of the universe at ever higher temperatures. The most compelling links between cosmological observations and fundamental theory involve dark matter, inflation, the cosmological baryon excess and dark energy.