Lepton Universality

The topic of lepton universality is directly related to the structure of the fundamental fermion elds and the dynamics generating it. It can be investigated independently for the charged and the neutral weak interactions. The precise measurements of the neutral couplings achieved at LEP and SLC are the key ingredients for the investigation of electroweak radiative corrections and the indirect investigation of the Higgs sector. Since it is heavy enough to decay into hadrons, the lepton has turned into a nice source of QCD studies, including the most precise determination of the strong coupling constant. c 1997 by Michel Davier. Introduction During recent years, a great deal has been learned about leptons through the experimental program at LEP and SLC. It is well-known that ee annihilation is a democratic supplier of leptons and quarks. In addition, the large cross section provided by the existence of the Z resonance enabled the experiments to register important statistics of leptons, hence opening a eld of precision measurements unheard of before in high-energy physics. These lectures emphasize the properties of the leptonic couplings to the gauge bosons. In the Standard Model, a universal structure is assumed for the couplings of leptons and quarks to the , W , and Z bosons. The new experimental data can test this structure with great accuracy. Nobody questions today the fascinating universality of the electric charge of leptons which measures their coupling to the photon. This property is imbedded in the universal QED theory which, however, tells us nothing about the existence of quark and lepton families. The situation might be di erent with the electroweak interaction, whose structure could have some relation to the particle spectrum. The driving force for investigating so aggressively the problem of universality rests on the possibility that violations might be discovered, thereby unveiling some clue about the puzzling family structure of matter. The lectures are organized as follows: (i) Leptonic W couplings: Universality of the couplings in the weak charged current is investigated in order to con rm the structure assumed in the Standard Model or nd departures. (ii) Leptonic Z couplings: The same approach is applied to the electroweak neutral current. In addition to testing universality, precise determination of the leptonic couplings provides a fascinating tool to explore the electroweak vacuum, with information on very massive fermions like the top quark and the Higgs sector of the theory. (iii) The , a peculiar standard lepton: The lepton has standard electroweak properties, as discussed in the previous lectures. But, owing to its relatively large mass, it can decay into hadrons as well as into leptons, opening a eld of study of the strong interactions (QCD) in a very clean and favorable environment. The recent results from these studies will be presented and discussed. 1 Leptonic W Couplings 1.1 Structure of Leptonic Charged-Current Interaction The general four-fermion interaction is well-described theoretically under the assumptions of locality, a derivative-free Lagrangian, and lepton number conservation. The most general amplitude for the transition shown in Fig. 1(a) contains a priori 12 complex coupling constants: