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We present a next-to-leading order (NLO) calculation of $t\bar{t}$ productionin hadronic collisions interfaced to shower generators according to the POWHEGmethod. We start from an NLO result from previous work, obtained in the zerowidth limit, where radiative corrections to both production and decays areincluded. The POWHEG interface required an extension of the POWHEG BOXframework, in order to deal with radiation from the decay of resonances. Thisextension is fully general (i.e. it can be applied in principle to any processconsidered in the zero width limit), and is here applied for the first time. Inorder to perform a realistic simulation, we introduce finite width effectsusing different approximations, that we validated by comparing with publishedexact NLO results. We have interfaced our POWHEG code to the PYTHIA8 showerMonte Carlo generator. At this stage, we dealt with novel issues related to thetreatment of resonances, especially with regard to the initial scale for theshower that needs to be set appropriately. This procedure affects, for example,the fragmentation function of the b quark, that we have studied with particularattention. We believe that the tool presented here improves over previousgenerators for all aspects that have to do with top decays, and especially forthe study of issues related to top mass measurements that involve B hadrons orb jets. The work presented here also constitutes a first step towards a fullyconsistent matching of NLO calculations involving intermediate resonancesdecaying into coloured particles, with parton showers.

Top-pair production and decay at NLO matched with parton showers