Hadroproduction of $W^{+}{W}^{-}b\bar{b}$ at NLO accuracy matched with shower Monte Carlo programs

We present the computation of the differential cross section for the process $pp(\bar{p}) \to (W^+,W^-,b,\bar{b} \to);e^+,\nu_e,\mu^-,\bar{\nu}_\mu,b, \bar{b}+X$ at NLO QCD accuracy matched to Shower Monte Carlo (SMC) simulations using PowHel, on the basis of the interface between HELAC-NLO and POWHEG-BOX. We include all resonant and non-resonant contributions. This is achieved by fully taking into account the effect of off-shell t-quarks and off-shell W-bosons in the complex mass scheme. We also present a program called DECAYER that can be used to let the t-quarks present in the event files for $pp(\bar{p})\to t\bar{t}X$ processes decay including both the finite width of the t-quarks and spin correlations. We present predictions for both the Tevatron and the LHC, with emphasis on differences emerging from three different $W^{+}{W}^{-}b\bar{b}$ hadroproduction computations: (i) full implementation of the $p,p(\bar{p}) \to W^+,W^-,b,\bar{b}$ process, (ii) generating on-shell t-quarks pushed off-shell with a Breit-Wigner finite width and decayed by DECAYER, and (iii) on-shell t-quark production followed by decay in the narrow width approximation, as described by the SMC.