The top quark is the heaviest known elementary particle and it is expected to play a special role in electroweak symmetry breaking. Top-quark pair production is a crucial process at the CERN LHC and it enters in many backgrounds to new-physics searches. Theoretical particle physicists (group Massimiliano Grazzini) at the University of Zurich have completed a new computation of the top-pair production cross section that includes perturbative corrections at the second order in Quantum Chromo Dynamics (QCD). The calculation is obtained with techniques which combine scattering amplitudes generated with automated tools developed in Zurich, with sophisticated analytic methods and advanced numerical integration techniques. Such result, which is the first independent confirmation of a landmark result obtained in 2013, paves the way to new and more efficient Monte Carlo simulations for this process.