  We perform a detailed study of NLO parton shower matching uncertainties in
Higgs boson pair production through gluon fusion at the LHC based on a generic
and process independent implementation of NLO subtraction and parton shower
matching schemes for loop-induced processes in the Sherpa event generator. We
take into account the full top-quark mass dependence in the two-loop virtual
corrections and compare the results to an effective theory approximation. In
the full calculation, our findings suggest large parton shower matching
uncertainties that are absent in the effective theory approximation. We observe
large uncertainties even in regions of phase space where fixed-order
calculations are theoretically well motivated and parton shower effects
expected to be small. We compare our results to NLO matched parton shower
simulations and analytic resummation results that are available in the
literature.
