Research - Agnieszka Sorensen, PhD

You can find all my publications on InspireHEP.

My research focuses on the thermodynamic properties of dense, strongly interacting matter. These properties are encoded in the QCD equation of state (EOS). Understanding the EOS is important for describing behavior of nuclear matter in extreme environments, such as supernovae, neutron stars and their mergers, and relativistic collisions of heavy nuclei. It can also address key open questions about strongly-interacting systems:

Does QCD matter undergo a first-order phase transition at finite baryon density?
How does the isospin dependence of the EOS evolve with density and temperature?
How do nuclear matter degrees of freedom and their properties change as the thermodynamic variables are varied?

I study these questions using microscopic transport simulations of relativistic heavy-ion collisions. Much of my work focuses on developing flexible EOS meta-models that can be used as adjustable input to these simulations. Constraining the QCD EOS also requires comparisons to experimental data and extensive use of high-performance computing. More recently, I have begun using machine learning methods to reduce the computational cost of these analyses.

Currently, I am working on the following projects:

Locating the critical point for the hadron to quark-gluon plasma phase transition from finite-size scaling of proton cumulants in heavy-ion collisions
In this first analysis of experimental data to locate the QCD critical point in a range consistent with modern theoretical predictions, we study fluctuations of the number of protons produced in collisions of gold nuclei. By adjusting the portion of the collision we analyze (using proton fluctuations measured in different rapidity bin widths), we test how fluctuations scale with system size. At collision energies above \( \sqrt{s_{\rm{NN}}} = 7.7~\rm{GeV}\), we observe that the data follows Taylor’s law, which suggests scale-free behavior: something expected near a critical point. Our analysis indicates a critical point near a baryon chemical potential of \( \mu_B = 580 \pm 30~\rm{MeV} \).

You can read this work here.
Volume dependence of second-order baryon cumulant ratio in hadronic transport simulations
Using hadronic transport simulations of symmetric nuclear matter with mean-field interactions in a box with periodic boundary conditions, I study the dependence of the ratio of variance to mean on the considered subvolume and technical parameters of the simulation such as the number of test particles per particle, mean-field lattice spacing, and smearing radius for density calculation. I find that the results do not depend on the number of test particles (as long as it's large enough to warrant a reliable density calculation) or the smearing radius. Further, I show that the dependence on the microscopic scale -- the lattice spacing -- is suppressed as compared to the dependence on the subvolume, which follows a power law. Finally, I show that in the limit of an infinite subvolume and infinitesimal lattice spacing, the simulation results approach the value expected from the underlying mean-field EOS.

The manuscript is currently in preparation.

My past projects include:

Speed of sound from neutron stars to heavy-ion collisions
In this project, we explored the compatibility of several neutron-star EOSs with a large peak in the speed of sound squared as a function of density, favored by observational data, with heavy-ion collision measurements. To convert from a neutron-star EOS to an EOS applicable in heavy-ion collisions, we explored a range of possible symmetry energy expansion coefficients and rejected coefficient sets which led to acausal, unstable, or unphysical behavior of nuclear matter. This in turn put a constraint on the high-density behavior of the symmetry energy, as can be seen in the figure. Using a flexible parametrization of density-dependent mean-field interactions, we then compared results using the converted EOSs against experimental data, and we found that a neutron-star EOS with a peak in the speed of sounds squared around twice the saturation density of nuclear matter is the most favored.

You can read this work here.
Bayesian analysis of Beam Energy Scan FXT data
In this project, we used flexible density-dependent mean-field potentials in a Bayesian analysis of STAR measurements of golg-gold collisions at \( \sqrt{s_{\rm{NN}}} = \{3.0, ~4.5\}~\rm{GeV} \). The flexibility of the potentials allowed for a meaningful exploration of the behavior of the EOS at high baryon densities, leading to a Bayesian constraint that suggests a strong softening of the EOS -- and possibly even a first-order phase transition -- at densities between 3~and~4 times the nuclear saturation density (as indicated by the area with green vertical stripes in the figure). This nontrivial result at high densities was inaccessible in previous studies (see area with black horizontal stripes in the figure), which used potentials parametrized using only a single parameter: the incompressibility of nuclear matter at saturation, making it impossible to account for changes in the behavior of nuclear matter at high densities.

You can read this work here.
Speed of sound from baryon cumulants in heavy-ion collisions
In this work, I connected cumulants of baryon number to the isothermal speed of sound squared \(c_T^2 \) and its derivative with respect to the baryon density. Using experimental data (red triangles in the figure) compared to a model (purple stars), I showed that the density-derivative of \(c_T^2 \) plateaus as collision energy is decreased (within the collider range of the BES, \(\sqrt{s_{\rm{NN}}} \geq 7.7~\rm{GeV}\)), but then dramatically increases at the HADES collision energy of \(\sqrt{s_{\rm{NN}}} = 2.4~\rm{GeV}\). This suggests a dramatic change in the behavior of the speed sound between these two energy regimes.

You can read this work here.
Flexible density-dependent EOS for hadronic transport
In this work I developed flexible, relativistically-covariant vector density functional (VDF) model of mean-field interactions. The model has been designed to allow for high flexibility of the EOS. For applications to hadronic transport simulations of heavy-ion collisions, the EOS at low to moderate densities is parametrized to reproduce the known properties of nuclear matter (such as the saturation density, binding energy, and the location of the nuclear liquid-gas critical point), while at higher density the EOS are fit to produce a postulated first-order phase transition. In the figure, I show coexistence (solid lines) and spinodal (dashed lines) regions for the nuclear-liquid gas phase transition (black) and several postulated phase transitions at high density (colorful lines) with varying location of the critical point and/or width of the coexistence region.

You can read this work here.

You can find all my publications on InspireHEP.

Currently, I am working on the following projects:

Locating the critical point for the hadron to quark-gluon plasma phase transition from finite-size scaling of proton cumulants in heavy-ion collisions

Volume dependence of second-order baryon cumulant ratio in hadronic transport simulations

My past projects include:

Speed of sound from neutron stars to heavy-ion collisions

Bayesian analysis of Beam Energy Scan FXT data

Speed of sound from baryon cumulants in heavy-ion collisions

Flexible density-dependent EOS for hadronic transport