In-medium $\Delta(1232)$ potential, pion production in heavy-ion collisions and the symmetry energy

TL;DR

This study uses the dcQMD model to analyze the in-medium $(1232)$ potential and its impact on pion production in heavy-ion collisions, providing new constraints on the symmetry energy and challenging common assumptions about $$ and nucleon potentials.

Contribution

It offers the first detailed extraction of the isoscalar and isovector in-medium $(1232)$ potentials from experimental data, revealing their differences from nucleon potentials.

Findings

The isoscalar potential is moderately more attractive than the nucleon optical potential.

The isovector potential constraints depend on the symmetry energy slope and neutron-proton effective mass difference.

Current models and data uncertainties limit precise constraints on the symmetry energy density dependence.

Abstract

Using the dcQMD transport model, the isoscalar and isovector in-medium potentials of the $\Delta$(1232) baryon are studied and information regarding their effective strength is obtained from a comparison to experimental pion production data in heavy-ion collisions below 800 MeV/nucleon impact energy. The best description is achieved for an isoscalar potential moderately more attractive than the nucleon optical potential and a rather small isoscalar relative effective mass m$^*_\Delta \approx$ 0.45. For the isovector component only a constraint between the potential's strength at saturation and the isovector effective mass difference can be extracted, which depends on quantities such as the slope of the symmetry energy and the neutron-proton effective mass difference. These results are incompatible with the usual assumption, in transport models, that the $\Delta$(1232) and nucleon potentials are equal. The density dependence of symmetry energy can be studied using the high transverse momentum tail of pion multiplicity ratio spectra. Results are however correlated with the value of neutron-proton effective mass difference. This region of spectra is shown to be affected by uncertain model ingredients such as the pion potential or in-medium correction to inelastic scattering cross-sections at levels smaller than 10$\%$. Extraction of precise constraints for the density dependence of symmetry energy above saturation will require experimental data for pion production in heavy-ion collisions below 800 MeV/nucleon impact energy and experimental values for the high transverse momentum tail of pion multiplicity ratio spectra accurate to better than 5$\%$.