Angular momentum conservation and pion production in intermediate-energy heavy-ion collisions

We have studied the effect of rigorous angular momentum conservation (AMC) in elastic, inelastic, and decay channels on pion production in intermediate-energy heavy-ion collisions based on the framework of an isospin-dependent Boltzmann-Uehling-Uhlenbeck (IBUU) transport model. We found that the constraint of AMC suppresses the absorption of both $\Delta$ resonances and pions, thus considerably enhances pion production and meanwhile reduces the charged pion yield ratio. The AMC effect on the charged pion yield ratio can not be simply compensated by a density-dependent in-medium $\Delta$ production cross section. Therefore, incorporating the constraint of AMC is important in obtaining the correct pion multiplicity and charged pion yield ratio by transport simulations, relevant for the extraction of the nuclear symmetry energy at high densities.