Initial baryon stopping and angular momentum in heavy-ion collisions

Noncentral heavy-ion collisions create fireballs with large initial orbital angular momentum that is expected to induce strong vorticity in the hot bulk fluid and generate global spin polarization of the produced particles. As the collision beam energy $\sqrt{{s}_{\mathrm{NN}}}$ decreases to approach the two-nucleon-mass threshold, this initial angular momentum approaches zero. One may thus expect that the observed global spin polarization should reach a maximum and then drop to zero as increased stopping competes with decreased initial momentum. Recent experimental measurements, however, appear to show a continual rise of hyperon polarization even down to $\sqrt{{s}_{\mathrm{NN}}}=$ 2.42 GeV, suggesting a peak very near threshold, which is difficult to interpret and calls for a better understanding of angular-momentum initial conditions, especially at low energy. Here, we develop a new Glauber-based initial-state model (``$\mathrm{Glauber}+$'') to investigate the initial distribution of angular momentum with respect to rapidity as well as the dependence of this distribution on initial baryon stopping across a wide range of collisional beam energy. We estimate that the angular momentum per produced final charged particle at midrapidity peaks around 5 GeV, which may present a potential challenge to an interpretation of the spin-polarization measurements near threshold as being a consequence of the initial angular momentum of the colliding system.