New frontiers in particle physics

Question: Now that the Higgs boson has been found, what are the next challenges for particle physicists?

(1) One challenge is to complete our knowledge of the properties of the particles that exist in the "Standard Model of particle physics". In particular, there are some properties of neutrinos that we still do not know. We only know five of the seven numbers that describe the masses of the three types of neutrinos and how they mix with each other.

(2) Another challenge is to find out what the "dark matter" particles are, which compose about one-quarter of the mass in the universe.

(3) A third challenge is to solve the mystery called the "gauge hierarchy problem". There are known quantum mechanical effects that contribute a very large amount to the mass of the Higgs particle, and something is causing the actual mass of the Higgs particle to be many orders of magnitude less than that. It was hoped that the Large Hadron Collider (LHC) would turn up clues that would help solve this mystery, but so far it has not.

The "next" significant discovery will probably be in one of those three areas.

(4) Finally, there is a list of very important unanswered questions that will be difficult or even impossible for us to answer: Are any of the "grand unified theories" that unify the non-gravitational forces correct? Why do the particles of the Standard Model have the masses they do? What is the right way to combine Einstein's theory of gravity with quantum mechanics? In particular, is "superstring theory" (which does this) correct?

-Stephen Barr