Rotation and Turbulence in Peripheral Heavy Ion Collisions

Abstract

In our computational fluid dynamics (CFD) calculations the Kelvin-Helmholtz instability (KHI) starts to develop in peripheral collisions. At high energies the shear viscosity of the Quark Gluon Plasma (QGP) flow is becoming small and the Reynolds number will exceed one, thus turbulent phenomena may start to occur, which enhance the rotation effect in the expanding system. Due to the large initial state angular momentum, a new directed flow structure may appear that the anti-flow peak observed at high energies rotates forward, and directed flow will start to peak at positive rapidities at sufficiently high beam energy, i.e. on the same side where the projectile spectator residues arrive after the collision. This is because the initial angular momentum leads to a faster rotating initial system, and this rotation moves the dominant directed flow peak forward before the expansion from the pressure which would slow down the rotation. The observation of this peak is not easy because of the beam directed fluctuations of the initial state, and the directed flow is very sensitive and subject to significant perturbations from these random fluctuations. Thus we can study other effects, which are induced by the rotation of the expanding system. One is the polarization method: by calculating the thermal vorticity it turns out that the Lambda polarization can reach several percent in peripheral heavy ion collisions. The other is the Differential Hanbury Brown Twiss method, which is sensitive to rotation and it has advantages compared with the flow harmonics analysis.