  First measurements of dihadron correlations for charged particles are
presented for central PbPb collisions at a nucleon-nucleon center-of-mass
energy of 2.76 TeV over a broad range in relative pseudorapidity, Delta(eta),
and the full range of relative azimuthal angle, Delta(phi). The data were
collected with the CMS detector, at the LHC. A broadening of the away-side
(Delta(phi) approximately pi) azimuthal correlation is observed at all
Delta(eta), as compared to the measurements in pp collisions. Furthermore,
long-range dihadron correlations in Delta(eta) are observed for particles with
similar phi values. This phenomenon, also known as the "ridge", persists up to
at least |Delta(eta)| = 4. For particles with transverse momenta (pt) of 2-4
GeV/c, the ridge is found to be most prominent when these particles are
correlated with particles of pt = 2-6 GeV/c, and to be much reduced when paired
with particles of pt = 10-12 GeV/c.
