  The cross section for pair production of top quarks (t-tbar) with high
transverse momenta is measured in pp collisions, collected with the CMS
detector at the LHC with sqrt(s) = 8 TeV in data corresponding to an integrated
luminosity of 19.7 inverse femtobarns. The measurement is performed using
lepton+jets events, where one top quark decays semileptonically, while the
second top quark decays to a hadronic final state. The hadronic decay is
reconstructed as a single, large-radius jet, and identified as a top quark
candidate using jet substructure techniques. The integrated cross section and
the differential cross sections as a function of top quark pt and rapidity are
measured at particle level within a fiducial region related to the
detector-level requirements and at parton level. The particle-level integrated
cross section is found to be sigma[t-tbar] = 0.499 +/- 0.035 (stat+syst) +/-
0.095 (theory) +/- 0.013 (lumi) pb for top quark pt > 400 GeV. The parton-level
measurement is sigma[t-tbar] = 1.44 +/- 0.10 (stat+syst) +/- 0.29 (theory) +/-
0.04 (lumi) pb. The integrated and differential cross section results are
compared to predictions from several event generators.
