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We report on the calculation of the next-to-leading order QCD corrections to the production of tt̄ pairs in association with two hard jets at the Fermilab Tevatron and CERN Large Hadron Collider. Results for the integrated and differential cross sections are given. The corrections with respect to leading order are negative and moderate. A study of the scale dependence of our next-to-leading order predictions indicates that the residual theoretical uncertainty, due to higher order corrections, is 21% for the Tevatron and 15% for the LHC. In the case of the Tevatron, the forward-backward asymmetry of the top quark is calculated for the first time at next-to-leading order. With the inclusive selection of cuts, this asymmetry amounts to Due to image rights restrictions, multiple line equation(s) cannot be graphically displayed. at leading order and Due to image rights restrictions, multiple line equation(s) cannot be graphically displayed. at next-to-leading order. All results presented in this paper have been obtained with the help of the HELAC-NLO package. [ABSTRACT FROM AUTHOR]

Achieving a precise description of multi-parton final states is crucial for many analyses at LHC. In this contribution we review the main features of the HELAC-NLO system for NLO QCD calculations. As a case study, NLO QCD corrections for jet production at LHC are illustrated and discussed. [ABSTRACT FROM AUTHOR]