Theoretical overview on top pair production and single top production

Abstract

Abstract. In this talk I will give an overview on theoretical aspects of top quark physics. The focus lies on top pair production and single top production. Basic facts about the top quarkThe top quark is the heaviest elementary particle known up to today. It has been discovered at the Tevatron and it is currently studied at the LHC. The top quark can be characterised by three essential numbers. These are the top quark mass, the top quark width and the branching ratio for the decay into a bottom quark. The current experimentally measured values are [1]The first number tells us that the top quark is heavier than all other known elementary particles, from the second one we deduce that the lifetime of the top quark is shorter than the characteristic hadronisation time scale. The third number indicates that the top quark decays predominately into a bottom quark and a W-boson. These numbers have several implications: The top quark mass is close to the electroweak symmetry breaking scale v = 246 GeV. If there is new physics associated with electro-weak symmetry breaking, top quark physics is a place to look for. Secondly, the large top mass sets a hard scale. The short lifetime of the top quark implies that the top quark decays before it can form bound states. Therefore, top quark physics is described by perturbative QCD. The absence of hadronisation effects allows also that spin information of the top quark is transferred to its decay products. Finally, the large top mass implies also that the top quark contributions are relevant to precision physics. For example, the top gives a significant contribution to the Higgs self-energy and a precise knowledge of the top mass is required for an indirect determination of the Higgs mass from electro-weak precision fits. This raises immediately the question how precise the top quark mass can be extracted from experiments. There are some theoretical subtleties which should be taken into account. The starting point for a theoretical description is the Lagrange density, the relevant part reads It should be stressed that Z m and hence m renorm depend on the renormalisation scheme. Popular choices for a renormalisation scheme are the on-shell scheme, where the the mass m pole is defined as the pole of the propagator (and m pole is therefore called the pole mass), or the MS-scheme, in which the MS-mass m MS (µ) is scale-dependent. Within perturbation theory one can convert between the different schemes. Naively, the pole mass seems to be the natural choice. However, the exact definition of the pole mass assumes the concept of stable colour-less particle. The top quark is neither stable nor colour-less, and this re-introduces non-perturbative effects. As a result, the pole mass is ambiguous by an amount O Λ QCD [2][3][4][5]. This ambiguity limits the precision by which the pole mass can be extracted from experiment. As an alternative one can use a mass definition, which is only sensitive to short distances. The MS-mass m MS (µ) is an example of a short-distance mass. The direct extrac...