  A search for dark matter is performed using events with large missing
transverse momentum and a Higgs boson decaying either to a pair of bottom
quarks or to a pair of photons. The data from proton-proton collisions at a
center-of-mass energy of 13 TeV, collected in 2015 with the CMS detector at the
LHC, correspond to an integrated luminosity of 2.3 inverse femtobarns. Results
are interpreted in the context of a Z'-two-Higgs-doublet model, where the gauge
symmetry of the standard model is extended to a U(1)[Z'] group, with a new
massive Z' gauge boson, and the Higgs sector is extended with four additional
Higgs bosons. In this model, a high-mass resonance Z' decays into a
pseudoscalar boson A and a light SM-like Higgs boson, and the A decays to a
pair of dark matter particles. No significant excesses are observed over the
background prediction. Combining results from the two decay channels yields
exclusion limits in the signal cross section in the m[Z']-m[A] phase space. For
example, the observed data exclude the Z' mass range from 600 to 1860 GeV, for
Z' coupling strength g[Z'] = 0.8, the coupling of A with dark matter particles
g[chi] = 1, the ratio of the vacuum expectation values tan[beta] = 1, and m[A]
= 300 GeV. The results of this analysis are valid for any dark matter particle
mass below 100 GeV.
