  First measurements of balance functions (BFs) of all combinations of
identified charged hadron $(\pi,\rm K,\rm p)$ pairs in Pb$-$Pb collisions at
$\sqrt{s_{\rm NN}} = 2.76$ TeV recorded by the ALICE detector are presented.
The BF measurements are carried out as two-dimensional differential correlators
versus the relative rapidity ($\Delta y$) and azimuthal angle ($\Delta\varphi$)
of hadron pairs, and studied as a function of collision centrality. The
$\Delta\varphi$ dependence of BFs is expected to be sensitive to the light
quark diffusivity in the quark$-$gluon plasma. While the BF azimuthal widths of
all pairs substantially decrease from peripheral to central collisions, the
longitudinal widths exhibit mixed behaviors: BFs of $\pi\pi$ and cross-species
pairs narrow significantly in more central collisions, whereas those of $\rm
KK$ and $\rm pp$ are found to be independent of collision centrality. This
dichotomy is qualitatively consistent with the presence of strong radial flow
effects and the existence of two stages of quark production in relativistic
heavy-ion collisions. Finally, the first measurements of the collision
centrality evolution of BF integrals are presented, with the observation that
charge balancing fractions are nearly independent of collision centrality in
Pb$-$Pb collisions. Overall, the results presented provide new and challenging
constraints for theoretical models of hadron production and transport in
relativistic heavy-ion collisions.
