  Normalized double-differential cross sections for top quark pair (t t-bar)
production are measured in pp collisions at a centre-of-mass energy of 8 TeV
with the CMS experiment at the LHC. The analyzed data correspond to an
integrated luminosity of 19.7 inverse femtobarns. The measurement is performed
in the dilepton e+/- mu-/+ final state. The t t-bar cross section is determined
as a function of various pairs of observables characterizing the kinematics of
the top quark and t t-bar system. The data are compared to calculations using
perturbative quantum chromodynamics at next-to-leading and approximate
next-to-next-to-leading orders. They are also compared to predictions of Monte
Carlo event generators that complement fixed-order computations with parton
showers, hadronization, and multiple-parton interactions. Overall agreement is
observed with the predictions, which is improved when the latest global sets of
proton parton distribution functions are used. The inclusion of the measured t
t-bar cross sections in a fit of parametrized parton distribution functions is
shown to have significant impact on the gluon distribution.
