ar X iv : h ep - p h / 99 12 27 9 v 1 9 D ec 1 99 9 Recent Developments in Physics Beyond the Standard Model 1

In this talk I discuss what I believe are the most interesting recent developments in physics beyond the Standard Model. After some initial comments on neutrino masses, I discuss the status of low-energy supersymmetry and finally turn to describing some recent work in theories with extra spatial dimensions. 1 Neutrinos The most concrete indication for the existence of physics beyond the Standard Model has recently emerged from the Superkamiokande data [1], which convincingly confirm the presence of an atmospheric neutrino anomaly [2]. The most reasonable explanation for these experimental observations relies on the assumption that neutrinos are massive and that the different flavour eigenstates can oscillate among each other. If this interpretation is correct, it implies evidence for new physics beyond the Standard Model. During this Conference, we have heard much discussion of the experimental status of neutrino masses and oscillations. Let me here make some comments on what I believe are the main lessons for theory we have learnt from these results which, if confirmed, represent one of the most important discoveries in physics in recent years. • We are finding evidence for a new mass scale much larger than the typical weak scale, but different from the Planck mass G −1/2 N. Indeed, including only Standard Model degrees of freedom, neutrino masses are described by dimension-5 operators of the form 1 Λ ℓ T L Cℓ L HH, (1) where ℓ L represents the lepton weak doublet and C is the charge-conjugation matrix. After electroweak symmetry breaking, the Higgs field H gets a non-vanishing vacuum expectation value, and the operator in eq. (1) leads to a Majorana neutrino mass m ν = H 2 /Λ. According to the most natural interpretation of the atmospheric neutrino anomaly, there exists a neutrino with mass of about 6 × 10 −2 eV. This