# Experimental Determination of $\eta$/$s$ for Finite Nuclear Matter

## TL;DR

This study measures shear viscosity and entropy density in nuclear matter, revealing their temperature dependence and universality, with implications for understanding strongly interacting quantum systems.

## Contribution

First simultaneous determination of shear viscosity and entropy density in finite nuclear systems across various sizes and temperatures.

## Key findings

- Both $ta$ and $s$ increase with temperature.
- The ratio $ta/s$ decreases mildly with temperature.
- $ta/s$ is independent of neutron-proton asymmetry.

## Abstract

We present, for the first time, simultaneous determination of shear viscosity ($\eta$) and entropy density ($s$) and thus, $\eta/s$ for equilibrated nuclear systems from $A$ $\sim$ 30 to $A$ $\sim$ 208 at different temperatures. At finite temperature, $\eta$ is estimated by utilizing the $\gamma$ decay of the isovector giant dipole resonance populated via fusion evaporation reaction, while $s$ is evaluated from the nuclear level density parameter (${a}$) and nuclear temperature ($T$), determined precisely by the simultaneous measurements of the evaporated neutron energy spectra and the compound nuclear angular momenta. The transport parameter $\eta$ and the thermodynamic parameter $s$ both increase with temperature resulting in a mild decrease of $\eta$/$s$ with temperature. The extracted $\eta$/$s$ is also found to be independent of the neutron-proton asymmetry at a given temperature. Interestingly, the measured $\eta$/$s$ values are comparable to that of the high-temperature quark-gluon plasma, pointing towards the fact that strong fluidity may be the universal feature of the strong interaction of many-body quantum systems.