  We present the first measurement of the two-particle transverse momentum
differential correlation function, $P_2\equiv\langle \Delta p_{\rm T} \Delta
p_{\rm T} \rangle /\langle p_{\rm T} \rangle^2$, in Pb-Pb collisions at
$\sqrt{s_{_{\rm NN}}} =$ 2.76 TeV. Results for $P_2$ are reported as a function
of relative pseudorapidity ($\Delta \eta$) and azimuthal angle ($\Delta
\varphi$) between two particles for different collision centralities. The
$\Delta \phi$ dependence is found to be largely independent of $\Delta \eta$
for $|\Delta \eta| \geq$ 0.9. In 5% most central Pb-Pb collisions, the
two-particle transverse momentum correlation function exhibits a clear
double-hump structure around $\Delta \varphi = \pi$ (i.e., on the away side),
which is not observed in number correlations in the same centrality range, and
thus provides an indication of the dominance of triangular flow in this
collision centrality. Fourier decompositions of $P_2$, studied as a function of
collision centrality, show that correlations at $|\Delta \eta| \geq$ 0.9 can be
well reproduced by a flow ansatz based on the notion that measured momentum
correlations are strictly determined by the collective motion of the system.
