What is the ${W b \bar b}$, ${Z b \bar b}$ or ${t \bar t b \bar b}$ irreducible background to the light Higgs-boson searches at LHC?

Abstract. The ${W b \bar b}$, ${Z b \bar b}$ and ${t \bar t b \bar b}$ production at LHC are irreducible backgrounds for the possible observability of the standard model and minimal supersymmetric standard model light Higgs boson in processes involving associated WH, ZH and $t \bar t H$ production followed by the $H \to b \bar b$ decay. The comparison presented in this paper uses the background estimates obtained with (a) the complete massive matrix element implemented in the AcerMC Monte Carlo generator and (b) the PYTHIA implementation of the inclusive W, Z, and $t \bar t$ production, followed by the parton showering mechanism. Both approaches lead to the production of the final state of interest but differ in the approximations used. The focal point of this study is the comparison of the two approaches when estimating the background contributions to the light Higgs boson searches at the LHC.