TRENTo-3D: Modeling the Rapidity Evolution of Ultrarelativistic Heavy-Ion Collision Geometry*

We present TRENTo-3D, a new extension of the TRENTo initial-conditions ansatz for heavy-ion collisions that computes a fully three-dimensional energy density profile based on the nuclear thicknesses generated by TRENTo.  The TRENTo-3D model is designed to ensure energy conservation among a central fireball at midrapidity and two fragmentation regions at forward and backward rapidities, while apportioning the energy deposited by the collision according to the limiting fragmentation hypothesis and appropriate longitudinal fluctuation.  The design exploits accepted scaling relations as well as indications from past TRENTo-based analyses in order to achieve considerable flexibility with a modest number of parameters.  We demonstrate that TRENTo-3D produces initial conditions that enable good agreement with rapidity-dependent experimental data across an array of collision systems and energies, and through closure testing we explore the potential for experimental data to constrain the model's parameters.