  A search for narrow vector resonances decaying into quark-antiquark pairs is
presented. The analysis is based on data collected in proton-proton collisions
at sqrt(s) = 13 TeV with the CMS detector at the LHC, corresponding to an
integrated luminosity of 35.9 inverse femtobarns. The hypothetical resonance is
produced with sufficiently high transverse momentum that its decay products are
merged into a single jet with two-prong substructure. A signal would be
identified as a peak over a smoothly falling background in the distribution of
the invariant mass of the jet, using novel jet substructure techniques. No
evidence for such a resonance is observed within the mass range of 50-300 GeV.
Upper limits at 95% confidence level are set on the production cross section,
and presented in a mass-coupling parameter space. The limits further constrain
simplified models of dark matter production involving a mediator interacting
between quarks and dark matter particles through a vector or axial-vector
current. In the framework of these models, the results are the most sensitive
to date, extending for the first time the search region to masses below 100
GeV.
