Nils Paar received his Ph.D. in theoretical nuclear physics at Technical University Munich in 2003 (with Prof. Peter Ring), and held a postdoctoral position at Technical University Darmstadt, until 2006 when he moved at the Faculty of Science University of Zagreb and worked as assistant, associate (2009-), and full professor (2013-present). From 2014-2015 he was Marie Curie researh fellow at Universitaet Basel, Switzerland. His research interests include exotic nuclear structure and dynamics, neutrino-induced reactions, and nuclear astrophysics. His achievements are published in more than 108 publications in international scientific journals and conference proceedings, with more than 3600 citations (WoS). Contributions at international conferences and workshops include more than 60 talks (including 31 invited talks). In 2017 he received the Annual State Award for Science from the Croatian Parliament, for significant scientific achievement in natural sciences. He served as head of the Division for Theoretical Physics, head of the Department of Physics, Faculty of Science and as member of the Senate of the University of Zagreb.

Modern Theory of Nuclear Structure
and Nuclear Astrophysics


Nuclear structure theory is rapidly evolving from macroscopic and microscopic models of stable nuclei towards vast regions of short-lived and exotic nuclei far from the valley of beta-stability. The main goal is a comprehensive description of nuclei and their excited states based on a fundamental understanding of the effective nucleon-nucleon interactions. Many nuclear structure issues are relevant for processes in astrophysical environments, and accurate global microscopic calculations are necessary input in astrophysical applications.

Modern accelerators of radioactive nuclear beams, e.g. at GANIL(France), RIKEN(Japan), ORNL(USA), GSI(Germany), Louvain-la-Neuve(Belgium), CERN, etc. and especially the next generation of these facilities (e.g. Facility for Antiproton and Ion Research (FAIR) at GSI, Germany), will provide valuable insight into the structure and reactions of nuclei away from the valley of beta-stability. For understanding of these nuclear systems and their role in astrophysical processes which produce the most of the visible matter in universe, modern theoretical models of nuclear structure play a crucial role. One of the Eleven Science Questions for the New Century is: How were the elements from iron to uranium made? With a good chance of answering this question in the near future, nuclear structure and astrophysics find themselves on the threshold of the most exciting era in decades.

Exotic Modes of Nuclear Excitations

Pygmy mode

The extreme isospin of nuclei far from stability and their weak binding reveal unique structure phenomena such as neutron halo and skin, which play a crucial role in understanding of the nuclear many-body problem at the limits of stability. The multipole response of nuclei far from the beta-stability line and the possible occurrence of new exotic modes of excitation presents a rapidly growing field of research. Several theoretical studies suggested possible existence of the pygmy dipole resonance (PDR) in medium-mass and heavy nuclei, i.e. the resonant oscillation of nucleons from weakly bound orbits against the isospin saturated proton-neutron core, and on the other side the experimental studies provide some evidence on the low-lying dipole excitations, mainly based on resonant photon scattering or Coulomb excitation of fission fragments. The properties of observed low-energy dipole modes in nuclei with a pronounced neutron excess is currently very much under discussion and further theoretical studies are of outmost importance, not only to disclose the nature of these novel physical phenomena but also to create new ideas for experiments to be performed. Facing these challenges, recently a new collective mode has been suggested by theory calculations to appear in nuclei close to the proton drip-line: proton pygmy dipole resonance whose dynamics is governed by vibrations of protons from weakly bound orbits against the core composed of other nucleons. This exotic mode is currently awaiting for its experimental confirmation.

The exotic modes of excitation are interesting not only as new physical phenomena, but also because they could play an important role in astrophysically relevant processes. The description of multipole spin-flip and isospin-flip excitations is essential in calculations of beta-decay, electron capture and neutrino-nucleus interaction rates. The low-energy dipole transition strength and PDR in neutron-rich nuclei have a pronounced effect on the calculated r-process abundances, i.e. on the production of chemical elements and on the propagation of ultra-high energy cosmic rays. On the proton-rich side the proton pygmy dipole resonance could contribute to the nucleosynthesis in rapid proton capture processes, as well as in the two-proton capture in astrophysical conditions characteristic for explosive hydrogen burning in novae and x-ray bursts.

Neutrino-Nucleus Reactions and Weak Interaction Rates

Image of the Globe

Neutrino-nucleus reactions are of central importance in astrophysics where the transport of neutrinos determines the rate of cooling of many stellar objects, and their detection provides a unique way of looking at such fascinating astrophysical phenomena as the interior processes in our sun, supernovae explosions, and stellar nucleosynthesis. These reactions are also essentially connected to another of the Eleven Science Questions for the New Century: do neutrinos have mass? Since nuclei are used as neutrino detectors in solar experiments, supernova observatories and in neutrino oscillation measurements, it is of outmost importance to achieve a quantitative description of neutrino-nucleus reactions in a fully microscopic theory.

Description of a neutrino-nucleus cross section becomes increasingly complicated as the target mass number increases and accurate self-consistent mean-field approaches must be developed and applied in calculations for all relevant neutrino-induced reactions. More generally, microscopic nuclear structure theory must be integrated into various astrophysical models of nucleosynthesis processes, supernova dynamics, and neutrino-induced reactions, by providing accurate global predictions for bulk nuclear properties and nuclear excitations.

Understanding of the nucleosynthesis of heavier elements during the r-process necessitates accurate knowledge on the neutrino-nucleus cross sections not only in stable nuclei but also in nuclei away from the valley of beta stability. In addition, quantitative description of neutrino-nucleus reactions will provide the relevant information for feasibility studies and simulations of low-energy beta beam facility which could produce neutrino beams of interest for particle physics, nuclear physics and astrophysics.

Nils Paar

The key Methods

for the studies of nuclear structure, exotic excitations, and weak interaction rates...

Relativistic density functional theory

based on modern energy density functionals, it allows not only a consistent, universal and quantitative description of nuclei along the valley of beta-stability but also extrapolations to the unknown regions of nuclide chart towards the particle drip lines.

Relativistic quasi particle random phase approximation

derived from the small amplitude limit of the time dependent relativistic Hartree Bogoliubov model, it provides a consistent insight into relevant excitations for the weak interaction rates and exotic collective modes of excitation.

Advanced scientific computing

is essential method in solving the nuclear many body problem based on coupled systems of differential equations and eigenvalue problems with huge matrices, in evaluation of large number of matrix elements, and in large scale calculations across the nuclide chart.

More details are available in the listed publications and references therein.

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University of Zagreb

The University of Zagreb is one of the oldest universities in Europe, it was officially founded on 23 September 1669 by Emperor and King Leopold I Habsburg who issued a decree granting the status and privileges of a university to the Jesuit Academy of the Royal Free City of Zagreb. In naturalist field the teaching started in 1896, with first lectures in mineralogy and geology, and then in botanic, physics, mathematics, chemistry, zoology and geography. Today, the University of Zagreb is the largest university in Croatia with more than 50000 full-time students.

Department of Physics

The Department of Physics at the Faculty of Science has a long tradition in teaching and in scientific research. Since 1991, after moving to the new building on Horvatovac, the Department had its renaissance. The new equipment considerably improved the practical side of the education process and experimental research. Studies of the Departments theoretical physicists have led to new discoveries in the process of neleptonic decay, research of cosmology of neutrino, nuclear structures, theory of classic and quantum chaos, low dimensional systems, and electron dynamics on the surfaces of conductor and isolators. Since 2005, the Physics department consolidated its transition towards Bologna system of study programmes. Within the academic year around 700 students are pursuing one of the various theoretical, experimental, computational and educational physics programmes.