  A search for single production of vector-like quarks, T and B, decaying into
a Z boson and a top or a bottom quark, respectively, is presented. The search
is performed using data collected by the CMS experiment at the LHC in
proton-proton collisions at sqrt(s) = 13 TeV, corresponding to an integrated
luminosity of 2.3 inverse femtobarns. An exotic T quark production mode through
the decay of a heavy Z' resonance is also considered. The search is performed
in events with a Z boson decaying leptonically, accompanied by a bottom or a
top quark decaying hadronically. No excess of events is observed over the
standard model background expectation. Products of production cross section and
branching fraction for T and B quarks from 1.26 and 0.13 pb are excluded at 95%
confidence level for the range of resonance mass considered, which is between
0.7 and 1.7 TeV. Limits on the product of the Z' boson production cross section
and branching fraction, with the Z' boson decaying to the Tt final state, are
set between 0.31 and 0.13 pb, for Z' boson masses in the range from 1.5 to 2.5
TeV. This is the first search at 13 TeV for single production of vector-like
quarks in events with a Z boson decaying leptonically accompanied by boosted
jets.
