Probing the Symmetry Energy with the Spectral Pion Ratio

TL;DR

This study uses charged pion spectral ratios from tin isotope collisions to constrain the symmetry energy slope parameter, providing insights relevant for neutron star properties.

Contribution

It introduces a novel method of using high transverse momentum pion ratios to constrain the symmetry energy slope at supra-saturation densities.

Findings

The symmetry energy slope L is constrained between 42 and 117 MeV.

Results are consistent with neutron skin measurements of lead-208.

The method offers a new way to probe dense nuclear matter.

Abstract

Many neutron star (NS) properties, such as the proton fraction within a NS, reflect the symmetry energy contributions to the Equation of State that dominate when neutron and proton densities differ strongly. To constrain these contributions at supra-saturation densities, we measure the spectra of charged pions produced by colliding rare isotope tin (Sn) beams with isotopically enriched Sn targets. Using ratios of the charged pion spectra measured at high transverse momenta, we deduce the slope of the symmetry energy to be $42 < L < 117$ MeV. This value is slightly lower but consistent with the $L$ values deduced from a recent measurement of the neutron skin thickness of $^{208}$Pb.