New Physics in Top Events at the LHC

Abstract

It is widely believed now that new physics beyond the Standard Model (SM) will appear at the electroweak scale. In almost all models of electroweak symmetry breaking, top either couples strongly to new particles or its properties are modified in some way, and can be therefore considered as a powerful tool for new discoveries findings. The top quark, discovered about 10 years ago [1], is currently being studied in detail by the CDF and D0 experiments at the Tevatron. Its production cross section has been measured in a variety of channels; its mass has been determined to better than 2%, and can be used to constrain the mass of the Higgs. Top quark decays have been tested and nonstandard production mechanisms searched for. So far, all of the top quark’s properties are consistent with the SM, but these studies have to deal with a quite low statistics. LHC instead will be a top factory, with a NLO σ($t\bar t$ ) equal to about 830 pb for a luminosity of 2 × 10³³ cm⁻² s⁻¹: this implies the production of two $t\bar t$ events per second. This enormous sample available will allow to increase the precision of the previously mentioned measurements and will also open up new possibilities such as observation of spin correlations, FCNC in top production and decay, and perhaps even CP violation in the top sector. Many different studies are being actively pursued by tha ATLAS and CMS experiments.