The non-linear flow modes, $v_{ 4,22}$, $v_{ 5,32}$ and $v_{ 6,33}$ as a function of transverse momentum ($p_{ \mathrm{ T}}$) have been measured for the first time in Pb--Pb collisions at $\sqrt{ s_{ \rm{ NN}}} = 5.02$ TeV for identified particles, i.e. charged pions, kaons and (anti-)protons. These observables probe the contribution of the second and third initial anisotropy coefficients to the higher order flow harmonics. Interestingly, all the characteristic features observed in previous $p_{ \mathrm{ T}}$-differential as well as $p_{ \mathrm{ T}}$-integrated measurements (e.g. $v_{ 2}$ and $v_{ 3}$) for various particle species are present in these measurements, i.e. increase of magnitude with increasing centrality percentile, a mass ordering in the low $p_{ \mathrm{ T}}$ region and a particle type grouping in the intermediate $p_{ \mathrm{ T}}$ region. The results cover 0--1\% (ultra-central collisions), 10--20\% (mid-central collisions) and 40--50\% (mid-peripheral collisions) centrality intervals and allow to test models that attempt to describe initial conditions and the mode-coupling of different harmonics.
Categories: nucl-ex