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# Pawel Danielewicz

My research is in nuclear reaction theory. Primarily I am interested in the central energetic reactions of heavy nuclei. These reactions are a testing ground for bulk nuclear properties such as the Equation of State (EoS) of nuclear matter. Particular focus of the field is now on determining the nuclear Symmetry Energy that describes changes in the matter with changes in neutron-proton imbalance. Insights into the Symmetry Energy would allow to extrapolate from large nuclei explored in the laboratory, with low neutron-over-proton excess, to neutron stars, with large excess. I am interested in the many-body theory tied to the central reactions, such as the semiclassical transport theory and the quantum nonequilibrium Green’s function theory. I use the theory to understand what happens in the reactions and to learn about the bulk nuclear properties, by analyzing reaction measurements. When more information can be gained from data in an adjacent field or any methods are not there for my use, I often dive into the adjacent fields and tackle challenges there, to move the overall efforts forward.

Current research projects I am engaged in include the use of the so-called charge-exchange reactions to learn about the Symmetry Energy at the densities characteristic for nuclei, the use of the charged pion yields from central reactions to learn about the Symmetry Energy at higher densities than in the nuclei, and the use of central reactions to create transient rings out of nuclear matter. Moreover I am engaged in the development of quantum transport theory suitable for studying collisions of heavy nuclei and of other systems. Charge exchange reactions, where proton incident on a nucleus turns into a neutron, probe the relative distributions of neutrons vs protons in a nucleus. The latter distribution is largely governed by the Symmetry Energy. The rings out of nuclear matter form in the transport theory simulations of collisions at modest incident energy, in effect of a low-density instability in the nuclear EoS, while the matter expands, mimicking the formation of smoke rings. Experimental observation of such rings would validate the current understanding of the low-density EoS and give insight into phenomena in the crusts of neutron stars.

Possible incoming student projects, of different difficulty, include:

• Exploring impacts of a medium-modification of pion production and absorption rates on pion yields from central collisions of heavy nuclei

• Generalizing error analysis to produce faithful errors on the conclusions drawn from charge-exchange and elastic scttering reactions. The traditional analysis falters due to a serious difference in the accuracy of measuring charged protons and neutral neutrons

• Constructing and implementing modern energy functional for exploring EoS in central reactions

• Advancing quantum transport theory for central reactions, based on nonequilibrium Green’s functions

### Selected Publications

Shear viscosity from nuclear stopping, Brent Barker and Pawel Danielewicz, Phys. Rev. C 99, 034607 (2019)Symmetry Energy III: Isovector Skins, Pawel Danielewicz, Pardeep Singh and Jenny Lee, Nucl. Phys. A 958, 147 (2017)

Subthreshold pion production within a transport description of central Au+Au collisions, Jun Hong and Pawel Danielewicz, Phys. Rev. C 90, 024605 (2014)

Symmetry Energy II: Isobaric Analog States, Pawel Danielewicz and Jenny Lee, Nucl. Phys. A 922, 1 (2014)

Towards a nonequilibrium Green’s function description of nuclear reactions: one-dimensional mean-field dynamics, A. Rios, B. Barker, M. Buchler and P. Danielewicz, Ann. Phys. (N.Y.) 326, 1274 (2011)