Bayesian analysis of heavy ion collisions with the heavy ion computational framework Trajectum

We introduce a model for heavy ion collisions named trajectum, which includes an expanded initial stage with a variable free streaming velocity vfs and a hydrodynamic stage with three varying second order transport coefficients. We describe how to obtain a Gaussian emulator for this 20-parameter model and show results for key observables. This emulator can be used to obtain Bayesian posterior estimates on the parameters, which we test by an elaborate closure test as well as a convergence study. Lastly, we employ the optimal values of the parameters found in Nijs et al. [G. Nijs, W. van der Schee, U. Gürsoy, and R. Snellings, Phys. Rev. Lett. 126, 202301 (2021)10.1103/PhysRevLett.126.202301] to perform a detailed comparison to experimental data from PbPb and pPb collisions. This includes both observables that have been used to obtain these values as well as wider transverse momentum ranges and new observables such as correlations of event-plane angles.