Initial state of ultrarelativistic heavy ion collisions

A model for energy, pressure, and flow velocity distributions at the beginning of ultrarelativistic heavy ion collisions is presented, which can be used as an initial condition for hydrodynamic calculations. Our model takes into account baryon recoil for both target and projectile, arising from the acceleration of partons in an effective field $F^{\mu \nu }$ produced in the collision. The typical field strength (string tension) for RHIC energies is about 5–12 GeV/fm, which allows us to talk about “string ropes.” The results show that a quark-gluon plasma forms a tilted disk, such that the direction of the largest pressure gradient stays in the reaction plane, but deviates from both the beam and the usual transverse flow directions. Such initial conditions may lead to the creation of “antiflow” or “third flow component” [L. P. Csernai and D. Röhrich, Phys. Rev. Lett. B 458, 454 (1999)].