Abstract
In the past two decades, pions created in the high density regions of heavy ion collisions have been predicted to be sensitive at high densities to the symmetry energy term in the nuclear equation of state, a property that is key to our understanding of neutron stars. In a new experiment designed to study the symmetry energy, the multiplicities of negatively and positively charged pions have been measured with high accuracy for central 132Sn+124Sn, 112Sn+124Sn, and 108Sn+112Sn collisions at E/A=270 MeV with the SπRIT Time Projection Chamber. While individual pion multiplicities are measured to 4% accuracy, those of the charged pion multiplicity ratios are measured to 2% accuracy. We compare these data to predictions from seven major transport models. The calculations reproduce qualitatively the dependence of the multiplicities and their ratios on the total neutron and proton number in the colliding systems. However, the predictions of the transport models from different codes differ too much to allow extraction of reliable constraints on the symmetry energy from the data. This finding may explain previous contradictory conclusions on symmetry energy constraints obtained from pion data in Au+Au system. These new results call for still better understanding of the differences among transport codes, and new observables that are more sensitive to the density dependence of the symmetry energy.
Highlights
Gravitational waves (GW) from the first binary neutron star merger event GW170817 observed by the LIGO-VIRGO collaboration have provided a glimpse into the properties of asymmetric compact nuclear objects with large imbalance of protons and neutrons under extreme conditions [1, 2]
In the following we show seven transport model predictions made without knowledge of the present experimental data, These seven widely used transport codes are: (i) AMD+JAM [31, 32, 40], (ii) IQMD-BNU [41, 42, 43], (iii) pBUU [30, 44], (iv) SMASH [45], (v) TuQMD [46, 47], (vi) UrQMD [48, 49] and (vii) χBUU [50] which is a variant of RVUU [51, 52] using the Skχm∗ energy functional [53]
Detailed description of experimental setup and performance of the SπRIT Time Projection Chamber (TPC) can be found in Refs. [54, 58]
Summary
Gravitational waves (GW) from the first binary neutron star merger event GW170817 observed by the LIGO-VIRGO collaboration have provided a glimpse into the properties of asymmetric compact nuclear objects with large imbalance of protons and neutrons under extreme conditions [1, 2]. The eventual fate of such merged objects as giant neutron stars or as transient neutron stars that later collapse into black holes are currently not known [3] It depends on the equation of state (EoS) of very neutron-rich nuclear matter that is of great interest to astronomy and astrophysics [4, 5]. Measurements of nucleus-nucleus collisions and their interpretations via transport models have provided independent and consistent constraints on the EoS of symmetric matter [10, 11, 12] which has equal numbers of neutrons and protons By combining such laboratory constraints with the GW results, the density dependence of the symmetry pressure, which contributes when neutron and proton densities differ, has been obtained with large uncertainties for densities above the saturation density, i.e. ρ0 = 1.74 × 1014 g/cm3 [8]
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