The atomic nucleus is not a solid ball but a drop of liquid that can be squeezed. To understand how elastic it is, physicists accelerate gold nuclei and smash them into each other. On impact, the matter compresses to densities like those in a neutron star, then flies apart. The shape of this debris tells us about the properties of the matter inside.
In the new study, scientists used data from collisions at an energy of 1.23 gigaelectronvolts per nucleon. They modeled the collisions with a computer program that accounts for how particles repel and collide in a dense medium. The program varied two key parameters: the stiffness of nuclear matter (how strongly it resists compression) and the strength of particle interactions within it. The calculations were then compared with actual measurements of proton debris.
It turned out that the data are best described if the matter is fairly soft and the collisions of particles inside it are slightly suppressed. This means that at high density, protons and neutrons collide with each other less often than in free space. Such a picture is important for understanding neutron stars — the densest objects in the universe.
The study also showed that if the model does not account for how particle interactions depend on their velocity, then to agree with experiment one has to make the matter artificially stiffer. This underscores that the details of interactions in a dense medium are critical for accurate predictions.
as written by the authors arXiv:2602.13991
We perform a comprehensive Bayesian analyses of Au + Au collision data at 1.23 GeV/nucleon using an isospin-dependent Boltzmann-Uehling-Uhlenbeck transport model that incorporates a momentum-dependent mean field and medium-modified baryon-baryon cross sections. The model parameters are calibrated to empirical properties of nuclear matter at saturation density, with particular attention to variations in the incompressibility $$K_0$$. Within a Bayesian statistical framework and using a Gaussian Process emulator, we simultaneously extract constraints on the incompressibility $$K_0$$ and the in-medium baryon-baryon scattering modification factor $$X$$ by systematically comparing model predictions with HADES measurements of proton collective flow, including the slopes ($$F_1$$ and $$F_3$$) of directed and triangular flow, as well as elliptic ($$v_2$$) and quadrupole ($$v_4$$) flow observables. We find that the extracted incompressibility favors relatively small values, indicating a soft nuclear equation of state, while the inferred average $$X$$ values fall at $$0.9$$-$$1.0$$, suggesting mild suppression of baryon-baryon cross sections in the medium. Furthermore, we demonstrate that transport models employing momentum-independent mean fields require stiffer equations of state and stronger in-medium corrections to reproduce the same observables. These results highlight the critical role of momentum dependence in the mean field and its interplay with in-medium scattering in constraining the properties of dense nuclear matter from heavy-ion collisions.
The density of matter at the center of a neutron star is about one hundred trillion times the density of water — a teaspoon of such matter would weigh as much as a mountain.
In Liu Cixin's novel 'The Three-Body Problem,' aliens create superdense 'droplets' capable of piercing ships — a distant echo of the physics of dense nuclear matter.