We analyze published data from the ALICE Collaboration in order to obtain the first extraction of the recently proposed rapidity-even directed flow observable $v_1$. An accounting of the correlation due to the conservation of transverse momentum restores the factorization seen by ALICE in all other Fourier harmonics and thus indicates that the remaining correlation gives a reliable measurement of directed flow. We then carry out the first viscous hydrodynamic calculation of directed flow, and show that it is less sensitive to viscosity than higher harmonics. This allows for a direct extraction of the dipole asymmetry of the initial state, providing a strict constraint on the nonequilibrium dynamics of the early-time system. A prediction is then made for $v_1$ in Au-Au collisions at RHIC.

• Received 6 March 2012

DOI:https://doi.org/10.1103/PhysRevLett.108.252302