  A systematic study of the factorization of long-range azimuthal two-particle
correlations into a product of single-particle anisotropies is presented as a
function of pt and eta of both particles, and as a function of the particle
multiplicity in PbPb and pPb collisions. The data were taken with the CMS
detector for PbPb collisions at sqrt(s[NN]) = 2.76 TeV and pPb collisions at
sqrt(s[NN]) = 5.02 TeV, covering a very wide range of multiplicity.
Factorization is observed to be broken as a function of both particle pt and
eta. When measured with particles of different pt, the magnitude of the
factorization breakdown for the second Fourier harmonic reaches 20% for very
central PbPb collisions but decreases rapidly as the multiplicity decreases.
The data are consistent with viscous hydrodynamic predictions, which suggest
that the effect of factorization breaking is mainly sensitive to the
initial-state conditions rather than to the transport properties (e.g., shear
viscosity) of the medium. The factorization breakdown is also computed with
particles of different eta. The effect is found to be weakest for mid-central
PbPb events but becomes larger for more central or peripheral PbPb collisions,
and also for very high-multiplicity pPb collisions. The eta-dependent
factorization data provide new insights to the longitudinal evolution of the
medium formed in heavy ion collisions.
