The Standard Model and Beyond

Abstract

The outstanding problems of the Standard Model of particle physics can conveniently be classified into 3 main categories. The most immediate is that of Mass: why are the quark, lepton and electroweak gauge boson masses non-zero? And why are they so small: mw/mp ∼ 10-10? The answer to the first question is presumably some variant of the Higgs mechanism, entailing the existence of at least one physical Higgs boson. The answer to the second question probably involves some additional symmetry, such as technicolour [1] or supersymmetry [2], entailing the existence of many more particles. The answer to both these questions should be found at energies ≤ TeV, putting the solution to the problem of Mass within reach of prospective experiments. Then there is the problem of Unification: is there a unifying group comprising all the particle interactions, some Grand Unified Theory (GUT) group [3] containing SU(3) × SU(2) × U(l)? This would entail interesting new phenomena such as proton decay and neutrino masses, but the characteristic energy scale would be at least 1015 GeV. Nevertheless, some indirect signatures may be found in detailed measurements at present energies, as we shall see later. Finally, there is the problem of Flavour: why are there so many types of matter particles, and what fixes the ratios of their masses and the charged current mixing angles? It is often suggested that quarks and leptons are composite, but the scale of such compositeness is most unclear, and there are no very attractive models, so I will not discuss this possibility in these lectures. All the above problems should find their resolution in a Theory of Everything (TOE), for which the only viable candidate is some variant of string theory. In the second lecture I will discuss ideas for the structure of this TOE, based in particular on the flipped SU(5) GUT [4].