The rapidity-dependent directed flow of particles produced in a relativistic heavy-ion collision can be generated in the hydrodynamic expansion of a tilted source. The asymmetry of the pressure leads to a buildup of a directed flow of matter with respect to the collision axis. The experimentally observed ordering of the directed flow of baryons, pions, and antibaryons can be described as resulting from the expansion of a baryon inhomogeneous fireball. An uneven distribution of baryons in the transverse plane leads to a difference in the collective push for protons and antiprotons. Precise measurements of the collective flow of identified particles as a function of rapidity could serve as a strong constraint on mechanism of baryon stopping in the early phase of the collision.

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• Collective flow
• Hydrodynamic models
• Relativistic heavy-ion collisions