What Is the Standard Model of Elementary Particles and Why We Have to Modify It

We introduce the standard model of elementary particles and discuss the reasons why we have to modify it. Emphasis is put on the indications from the neutrinos and on the role of the Higgs particle; some promising theoretical ideas, like quark-lepton symmetry, existence of super-heavy “right-handed” neutrinos, grand unification and supersymmetry at the weak scale, are introduced and shortly discussed. 1 The standard model of elementary particles In this section we define the standard model of elementary particles (SM) by introducing the particle content and the parameters of this model. We aim at giving a short introduction, at illustrating how this model is used, and at discussing (some of) its merits and limitations (to go deeper in the subjects touched, one could make reference to the seminal papers on the SM [1], to books and reviews [2, 3], and to the Particle Data Group biannual report [4]). Presented at the Vulcano 2000 Workshop “Frontier objects in astroparticle and particle physics”, May 22-27, Vulcano, Italy. In the spirit of the Workshop, I tryed to provide an essential vocabulary and perhaps entry points for colleagues working in different fields of research. I renounced to present a lists of references regarding experiments, and I mostly limited the discussion to few ideas and theoretical leitmotifs; I apologize for the arbitrariness of the selection.