Experimental Determination of η / s for Finite Nuclear Matter

We present, for the first time, simultaneous determination of shear viscosity ($\ensuremath{\eta}$) and entropy density ($s$) and thus, $\ensuremath{\eta}/s$ for equilibrated nuclear systems from $A\ensuremath{\sim}30$ to $A\ensuremath{\sim}208$ at different temperatures. At finite temperature, $\ensuremath{\eta}$ is estimated by utilizing the $\ensuremath{\gamma}$ 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 $\ensuremath{\eta}$ and the thermodynamic parameter $s$ both increase with temperature, resulting in a mild decrease of $\ensuremath{\eta}/s$ with temperature. The extracted $\ensuremath{\eta}/s$ is also found to be independent of the neutron-proton asymmetry at a given temperature. Interestingly, the measured $\ensuremath{\eta}/s$ 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.