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

Abstract

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 $Δ$ 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 $Δ$ 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.

Figures (6)

• Figure 1: Cartoons for the initial-state and final-state momenta in the C.M. frame of $N+N\rightarrow N+N$ scatterings without (a) and with AMC (b), $N+N\rightarrow N+\Delta$ scatterings with AMC (c), and $N+\Delta\rightarrow N+N$ scatterings with AMC (d).
• Figure 2: Comparison of the collision rates as a function of time (left) and C.M. energy (right) for $N+N\rightarrow N+N$ scatterings (first row), $N+N\rightarrow N+\Delta$ scatterings (second row), $N+\Delta\rightarrow N+N$ scatterings (third row), and net $\Delta$ production (fourth row) with and without the constraint of AMC in $^{132}$Sn+$^{124}$Sn collisions at the beam energy of 270 AMeV and an impact parameter $\text{b}=4$ fm.
• Figure 3: Comparison of the collision rates as a function of time (left) and C.M. energy (right) for $n+n\rightarrow p+\Delta^{++}$ and $p+p\rightarrow n+\Delta^{-}$ scatterings (first row), inverse processes (second row), and net $\Delta$ production (third row) with and without the constraint of AMC in $^{132}$Sn+$^{124}$Sn collisions at the beam energy of 270 AMeV and an impact parameter $\text{b}=4$ fm.
• Figure 4: Comparison of the reaction rates as a function of time (left) and C.M. energy (right) for $\Delta \rightarrow N+\pi$ processes (first row) and $N+\pi \rightarrow \Delta$ processes (second row) with and without the constraint of AMC in $^{132}$Sn+$^{124}$Sn collisions at the beam energy of 270 AMeV and an impact parameter $\text{b}=4$ fm.
• Figure 5: Production of pion-like particles with time evolution with and without the constraint of AMC in $^{132}$Sn+$^{124}$Sn collisions at the beam energy of 270 AMeV and an impact parameter $\text{b}=4$ fm, with the upper panel for the case without Pauli blocking and lower panel for the case with Pauli blocking.