An NLO+PS generator for t t̄ t t ¯ and W t W t production and decay including non-resonant and interference effects

We present a Monte Carlo generator that implements significant theoretical improvements in the simulation of top-quark pair production and decay at the LHC. Spin correlations and off-shell effects in top-decay chains are described in terms of exact matrix elements for $$pp→ ^+ν _{{ }{ }{ }{ } }\, l^-ν ̄_{{ }{ }{ }{ }l}\,b \,b̄ $$ at order $$α ^4 α _{{ }{ }{ }{ } S}^2$$ plus full NLO QCD corrections, where the leptons $$ $$ and l belong to different families, and b quarks are massive. Thus, the contributions from $$tt̄$$ and Wt single-top production, plus contributions without top resonances and all relevant quantum interferences between different channels are fully included. Matrix elements are matched to the $$ Pythia8$$ parton shower using a recently proposed method that allows for a consistent treatment of resonances in the $$ POWHEG$$ framework. These theoretical improvements are especially important for the interpretation of precision measurements of the top-quark mass, for single-top analyses in the Wt channel, and for $$tt̄$$ and Wt backgrounds in the presence of jet vetoes or cuts that enhance off-shell effects. The new generator is based on a process-independent interface of the $$ OpenLoops$$ amplitude generator with the POWHEG-BOX framework.