Abstract
Initial fluctuation is one of the ingredients that washes fingerprints of the nuclear symmetry energy on observables in heavy-ion collisions. By artificially using the same initial nuclei in all collision events, the effect of the initial fluctuation on isospin-sensitive observables, e.g., the yield ratio of free neutrons with respect to protons Nn/Np, 3H/3He yield ratio, the yield ratio between charged pions π−/π+, and the elliptic flow ratio or difference between free neutrons and protons v2n/v2p (v2n-v2p), are studied within the ultrarelativistic quantum molecular dynamics (UrQMD) model. In practice, Au + Au collisions with impact parameter b = 5 fm and beam energy Elab = 400 MeV/nucleon are calculated. It is found that the effect of the initialization on the yields of free protons and neutrons is small, while for the yield of pions, the directed and elliptic flows are found to be apparently influenced by the choice of initialization because of the strong memory effects. Regarding the isospin-sensitive observables, the effect of the initialization on Nn/Np and 3H/3He is negligible, while π−/π+ and v2n/v2p (v2n-v2p) display a distinct difference among different initializations. The fingerprints of symmetry energy on π−/π+ and v2n/v2p can be either enhanced or reduced when different initializations are utilized.
Highlights
Nuclear symmetry energy, which describes the energy difference between pure neutron matter and isospin symmetric nuclear matter, is one of the crucial quantities for studying the structures and the properties of nuclei and neutron stars, the dynamics of heavy-ion collision, supernovae explosions, as well as neutron star mergers [1–13]
Model, the symmetry potential is derived from the Skyrme potential energy density function in the same manner as the improved quantum molecular dynamics (ImQMD) model, see e.g., [48,49]
It is found that for pion yield, the directed and elliptic flows of nucleons calculated with different initializations are different, because of the different density distribution in the compressed region, which is the result of the tiny difference in density distribution at the initial time
Summary
Nuclear symmetry energy, which describes the energy difference between pure neutron matter and isospin symmetric (with equal numbers of protons and neutrons) nuclear matter, is one of the crucial quantities for studying the structures and the properties of nuclei and neutron stars, the dynamics of heavy-ion collision, supernovae explosions, as well as neutron star mergers [1–13]. Exploring nuclear symmetry energy at various densities is one of the important scientific goals for intermediate-energy heavy-ion collision (HIC) studies in terrestrial laboratories [14–23]. Extracting Esym (ρ) with HIC should rely on both transport model simulations and experimental measurements, while various transport models have different philosophies/assumptions/parameters, and as a result constraints on Esym (ρ) with different transport models are usually different to some extent. The initial fluctuations (the preparation of initial nuclei, i.e., target and projectile) and dynamical fluctuations (i.e., stochastic nucleon collisions) might wash the effects of Esym (ρ) on observables, which make the constraint of
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