Seminar in Hadronic Physics

Bayesian constraints on the initial stage and the neutron skin of Pb-208 using Trajectum

Govert Nijs

Center for Theoretical Physics, MIT

In the first part of the talk, we discuss the results from recent Bayesian analyses which focus on the initial stage of heavy ion collisions. Specifically, we examine the effect of the nucleon width on the total hadronic PbPb cross-section and find that to reach agreement with recent ALICE measurements one needs a much smaller nucleon width than what was found in previous Bayesian analysis. In addition, we modify the TRENTo formula to allow for more general scaling behavior of the energy deposition into the initial state, and interpolate between two different pre-hydrodynamic stage models, in both cases using Bayesian analysis to find which behavior is preferred by experimental data.

For the second part of the talk, we use similar techniques to investigate the shape of the Pb nuclei themselves. Whereas the proton distribution within a Pb nucleus is well-known from electron scattering experiments, the distribution of neutrons is much harder to measure this way given that both the charge and electric dipole moment of the neutron vanish. For this reason, the difference between the radii of the neutron and proton distributions, the so-called neutron skin, is not known very precisely. Heavy ion observables are sensitive to the geometry of the collision, and indeed in our most recent Bayesian analysis we are able to extract the neutron skin. Our extracted value is compatible with nuclear theory predictions as well as existing experimental measurements, and we achieve a precision competitive with the PREX experiment.