  A search is performed for the production of heavy resonances decaying into
top-antitop quark pairs in proton-proton collisions at sqrt(s) = 8 TeV. Data
used for the analyses were collected with the CMS detector and correspond to an
integrated luminosity of 19.7 inverse femtobarns. The search is performed using
events with three different final states, defined by the number of leptons
(electrons and muons) from the t t-bar to W b W b decay. The analyses are
optimized for reconstruction of top quarks with high Lorentz boosts, where jet
substructure techniques are used to enhance the sensitivity. Results are
presented for all channels and a combination is performed. No significant
excess of events relative to the expected yield from standard model processes
is observed. Upper limits on the production cross section of heavy resonances
decaying to t t-bar are calculated. A narrow leptophobic topcolor Z' resonance
with a mass below 2.4 TeV is excluded at 95% confidence level. Limits are also
derived for a broad Z' resonance with a 10% width relative to the resonance
mass, and a Kaluza-Klein excitation of the gluon in the Randall-Sundrum model.
These are the most stringent limits to date on heavy resonances decaying into
top-antitop quark pairs.
