Search for the Nonresonant Production of a Pair of Additional Higgs Bosons in the Type-X Two-Higgs-Doublet Model in Proton-Proton Collisions at sqrt[s]=13  TeV

A search is presented for the production of two additional Higgs bosons from an off-shell Z boson, where both additional particles decay to τ lepton pairs. The search is performed with a data sample collected with the CMS detector from proton-proton collisions at the LHC at sqrt[s]=13  TeV, corresponding to an integrated luminosity of 138  fb^{-1}. No deviation from the standard model background is observed. Exclusion limits are set on the type-X two-Higgs-doublet model alignment scenario. These results exclude previously unexplored regions of parameter space and provide constraints complementary to those from earlier searches.