  A measurement of the top quark pair production (t-tbar) cross section in
proton-proton collisions at the centre-of-mass energy of 8 TeV is presented
using data collected with the CMS detector at the LHC, corresponding to an
integrated luminosity of 19.6 inverse femtobarns. This analysis is performed in
the t-tbar decay channels with one isolated, high transverse momentum electron
or muon and at least four jets, at least one of which is required to be
identified as originating from hadronization of a b quark. The calibration of
the jet energy scale and the efficiency of b jet identification are determined
from data. The measured t-tbar cross section is 228.5 +/- 3.8 (stat) +/- 13.7
(syst) +/- 6.0 (lumi) pb. This measurement is compared with an analysis of 7
TeV data, corresponding to an integrated luminosity of 5.0 inverse femtobarns,
to determine the ratio of 8 TeV to 7 TeV cross sections, which is found to be
1.43 +/- 0.04 (stat) +/- 0.07 (syst) +/- 0.05 (lumi). The measurements are in
agreement with QCD predictions up to next-to-next-to-leading order.
