The v^1/3_3/v^1/2_2 ratio in PbAu collisions at √(s_NN) = 17.3 GeV – a hint of a hydrodynamic behavior

The Fourier harmonics, v_2 and v_3 of negative pions are measured at center-of-mass energy per nucleon pair of √(s_NN)= 17.3 GeV around midrapidity by the CERES/NA45 experiment at the CERN SPS in 0–30% central PbAu collisions with a mean centrality of 5.5%. The analysis is performed in two centrality bins as a function of the transverse momentum p_T from 0.05 GeV/c to more than 2 GeV/c. This is the first measurement of the v^1/3_3/v^1/2_2 ratio as a function of transverse momentum at SPS energies, that reveals, independently of the hydrodynamic models, hydrodynamic behavior of the formed system. For p_T above 0.5 GeV/c, the ratio is nearly flat in accordance with the hydrodynamic prediction and as previously observed by the ATLAS and ALICE experiments at the much higher LHC energies. The results are also compared with the SMASH-vHLLE hybrid model predictions.