Probing freeze out sensitivity in nuclear collisions using exact solutions of fluid dynamics

One of the primary goals in the field of high-energy nuclear collisions is to create and study a novel phase of matter known as quark-gluon plasma (QGP), a substance which can be extremely well described by fluid dynamics. However, two different criteria are often used to identify the point of freeze out, which defines when the fluid dynamical simulation of a nuclear collision should be terminated: either when the QGP fluid has cooled below a given temperature, or when it has reached a certain Knudsen number (a quantity that estimates how well the system can be modeled by a fluid). Moreover, whereas the former is more convenient to use in simulations, the latter is more physically realistic. This study investigates how the choice of freeze-out surface, defined by either constant temperature or constant Knudsen number, may influence the predictions of one’s fluid dynamical model. Using an exact solution of the fluid dynamical equations to model the nuclear collision, we identify both freeze-out surfaces and employ them to make predictions for two sets of widely studied observables in nuclear collisions: the “spectra”, which count the number of particles produced by the collision as a function of particle momentum, and the “HBT radii”, which roughly reflect the size and shape of the system. We find that the choice of freeze-out criterion may have measurable consequences for both sets of observables and thus may also affect our understanding of the QGP and the collisions which create it.