The gg,qqbar --> t tbar b bbar process: matrix element implementation to PYTHIA6.1

The ttbb production at LHC is a dominant irreducible background for observability of the Standard Model (SM) and Minimal Supersymmetric Standard Model (MSSM) Higgs boson in the associated production, ttH, followed by the H-->bb decay. In this Note we describe implementation of the complete event generation using matrix element for the process gg, qqbar -->ttbb. This implementation is prepared as an external process in PYTHIA 6.1 Monte Carlo. The matrix element has been coded using MADGRAPH package, the phase space generation algorithm was adapted following importance sampling technique developed for NEXTCALIBUR package with multi-channel self-optimising approach. For the final optimisation also the VEGAS package has been used. The efficiency of the order of 10% was achieved for generating unweighted events. In the Note we discuss also the cross-section dependencies on the QCD energy scale defined for the process. In addition we show comparison between diferential distributions expected for the ttH signal and the ttbb background.