Symmetry energy investigation with pion production from Sn+Sn systems

TL;DR

This study measures pion production in Sn+Sn heavy ion collisions to probe the nuclear symmetry energy, but discrepancies among transport models hinder definitive constraints, highlighting the need for improved models and observables.

Contribution

The paper provides high-precision measurements of pion multiplicities and ratios in Sn+Sn collisions and compares them with multiple transport models, revealing significant model discrepancies.

Findings

Transport models qualitatively reproduce multiplicity trends.

Model predictions vary too much for reliable symmetry energy constraints.

Results highlight the need for better models and more sensitive observables.

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 $^{132}$Sn+$^{124}$Sn, $^{112}$Sn+$^{124}$Sn, and $^{108}$Sn+$^{112}$Sn collisions at $E/A=270~\mathrm{MeV}$ with the S$\pi$RIT Time Projection Chamber. While the uncertainties of individual pion multiplicities are measured to 4\%, those of the charged pion multiplicity ratios are measured to 2\%. 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 to 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 better understanding of the differences among transport codes, and new observables that are more sensitive to the density dependence of the symmetry energy.