The second phase of the beam energy scan at RHIC is currently ongoing with the goal to identify signals for the first order phase transition or the critical endpoint. One basic observable that needs to be understood before assessing more complex correlations and fluctuations is the rapidity distribution of net protons, that determines the net baryon density of the system.
The stopping dynamics of baryons is still lacking a quantitative understanding from the theory side. Here, we implement a string model for the description of hadronic interactions within a newly developed transport approach, SMASH (Simulating Many Accelerated Strongly-interacting Hadrons). The free parameters of the string model are determined by comparing to experimental measurements in elementary proton-proton collisions. With the adjusted parameters, we advance to heavy ion collisions, where the experimentally observed change of the shape of the net proton rapidity spectrum from a single peak structure to a double peak structure with increasing beam energy is reproduced.
By adjusting details of the formation process of string fragments, a good agreement with the measured rapidity spectra of protons and pions is achieved while insights on the fragmentation process are obtained.
Within the presented framework, dynamic 3-dimensional event-by-event initial conditions for hydrodynamic calculations can be provided.