My path of life is determined by the quest for uncovering basic laws and principles governing the phenomena of matter appearance. In particular, to make predictions about phase transitions in the system
which might occur in response to extreme conditions of, e.g., temperatures, densities and strong fields
provided in laboratory experiments or in the Cosmos.
During my studies at Rostock University I got acquainted with methods of thermodynamic Green
functions in equilibrium and nonequilibrium quantum statistics, applied to the physics of strong correlations in nonideal plasmas. My Diploma thesis was devoted to the formulation of a Path Integral approach
to collective modes (plasmons) and bound states in strongly correlated Fermion systems. Under the supervision of my teacher, Gerd R¨opke, the dissolution of bound states under the influence of density and
temperature (Mott effect) came into the focus of my scientific research.
My Ph.D. studies were partly performed at the JINR Dubna where I came in contact with leading
physicists of the Bogoliubov and Landau schools, e.g., Dmitri Zubarev and Evgenii Lifshitz. I studied the
quark model of hadrons and investigated the idea that quark deconfinement in hadronic matter under
high compression could be described as a Mott effect, driven by the screening of the interaction and the
Pauli exclusion principle. In my Ph.D. thesis on “The role of Pauli blocking effects in the equation of
state for strongly interacting matter”, I developed the corresponding approach (string-flip model) for hot
and dense quark matter by postulating a saturation of confining interactions within a nearest neighbors
using the thermodynamic Green functions technique.
After my Ph.D. in 1987, I developed further applications of the string-flip model approach to the study
of the deconfinement transition in heavy-ion collisions, in cosmology and in neutron stars. Particularly
interesting became the study of the J/ψ suppression effect in heavy-ion collisions, which I performed still
before the first experimental results of the CERN NA38 collaboration appeared. Within the string-flip
model I could derive in-medium cross sections for charmonium and bottomonium breakup and formulate
a unified approach to heavy quarkonia suppression which suggested an increase of the suppression effect
related to the quark deconfinement, later observed in the NA50 experiment as “anomalous” J/ψ suppression. A deeper understanding of the string-flip process in hadronic matter required the analysis of
quark exchange processes between hadronic bound states, which I performed for the case of meson-meson
interactions. My idea to apply this approach to quark exchange processes for a study of charmonium
dissociation cross sections in hadronic matter led to a now well-known paper (above 100 citations) which
stimulated further studies by several other groups.
After 1989, I could develop new scientific contacts and widen my scientific horizon. I count J¨org H¨ufner
(Heidelberg) and Helmut Satz (Bielefeld/CERN) as my postdoctoral sponsors. During my postdoctoral
fellowship in the CERN Theory group (1991-92), I profited from contacts with leading scientists, in
particular with John Ellis, Rolf Hagedorn and Helmut Satz. I was challenged by the necessity to develop
my ideas for the QCD phase transition from the nonrelativistic Green function approach to a quantum
field theoretical formulation.
The next stage of my scientific life was an appointment with the temporary research group for “Theoretical Many-Particle Physics” of the Max-Planck-Gesellschaft, led by Gerd R¨opke in Rostock, where I
prepared my habilitation thesis on “Quantum statistics for effective quark models of hadronic matter”.
I developed methods of finite-temperature quantum field theory for the study of strong correlations and
phase transitions in Quantum Chromodynamics (QCD) at nonzero density and/or temperature. I used
the path-integral approach for QCD motivated model field theories with a chiral quark sector of the
Nambu–Jona-Lasinio type and generalized it to a nonlocal, separable 4-fermion coupling. The step to
master chiral quark models was instrumental in describing the double nature of the pion as a quarkantiquark bound state and as a Goldstone boson of the broken chiral symmetry. With this approach I
studied not only the modification of meson properties at finite temperatures and densities but also their
Mott transition driven by the chiral phase transition.
After my habilitation in 1995, I started working on understanding aspects of deconfinement in strongly
interacting matter on the basis of the QCD Schwinger-Dyson equation approach. I developed applications
of this approach for finite temperatures and chemical potentials using different gluon propagator models
and addressing the problem of the confinement-deconfinement transition. These works formed the basis
for studies of the equation of state for quark matter and its applications for quark stars.
In 1998 I won the competition for a five-year assistant professorship on “Particle and Astrophysics”
at the University of Rostock which gave a boost to my career: I developed my teaching skills, formed a
research group and established international collaborations as well as contacts to scientific organizations.
My closest collaborators were Hovik Grigorian (Yerevan), Yuri Kalinovsky (Dubna), Craig Roberts (Ar-
gonne) and Sebastian Schmidt (Rostock/ T¨ubingen). In this stage three major directions of my research
were formed: (1) signals of QCD phase transitions in heavy-ion collisions, (2) quark matter in compact
star interiors, and (3) particle production in strong fields, with the following achievements.
My main contribution to the still ongoing debate about the interpretation of the anomalous J/ψ
suppression effect as a quark-gluon plasma signal is a quantum kinetic formulation of the charmonium
lifetime. I applied the Kadanoff-Baym formalism of non-equilibrium Green functions to the treatment of
rearrangement scattering between mesonic correlations (bound states or resonances) and could demonstrate that the spectral broadening of D-meson states due to their Mott effect at the chiral phase transition
gives rise to a threshold-like enhancement of the J/ψ dissociation rate, thus explaining the anomalous
suppression. I plan to study further consequences of the Mott transition for the explanation of strong
coupling phenomena in the quark gluon plasma (sQGP), relevant for RHIC Brookhaven, CERN-LHC
and upcoming GSI-FAIR and JINR-NICA experiments.
The second major research topic concerns the study of neutron stars with quark matter cores on the
basis of microscopic approaches to the equation of state. In studies of the rotational evolution of accreting
neutron stars in low-mass X-ray binaries, I developed the concept of a phase diagram for compact stars
as a heuristic tool for identifying phase transitions in the neutron star interior by a population statistics.
This could in future play a similar role as the well-known Hertzsprung-Russell diagram for ordinary
stars. When studies of color superconducting quark matter were revived for nonperturbative dense
QCD in 1998, I was among the first to discuss its role for magnetic fields and cooling of compact stars.
Since these first works, I developed the description of structure and cooling of compact stars with color
superconducting quark matter phases and corresponding observational tests to the international stateof-the-art. This expertise in the astrophysical constraints on the properties of matter at highest densities
was acknowledged by the community and earned me invitations to conferences and symposia. Since 2004
I acted as a coordinator of the European initiative for the “Physics of Compact Stars” (CompStar) which
2008-2013 received funding as a Research Networking Programme of the European Science Fundation. In
this position I contributed to the development networking structures for studying and solving problems
in the overlapping fields of neutron star-, heavy-ion collision- and gravitational wave physics with their
respective strong experimental backgrounds. This network got further supported as European COST
Action ”NewCompStar” (2013-2017) and ”PHAROS” (2017-22).
The third main field in my spectrum of scientific research interests concerns the problem of vacuum
pair production in strong fields which, more than 70 years after the exact QED prediction by Schwinger
has not yet been seen in experiment. Starting from a quantum field theoretic formulation, I have developed
a kinetic approach to particle production under nonequilibrium conditions and applied it to situations
in heavy ion collisions, in cosmology and for modern high-intensity lasers. This latter application has
entered to proposals for experimental verification in modern optical laser experiments.
In 2001 I was elected by the Scientific Council of the JINR Dubna as vice director of the Bogoliubov
Laboratory for Theoretical Physics. Using this position (until 2007), I fostered international contacts
and developed summer school programs for the training of young scientists in the field “Structure of
Matter” with substantial financial support from German Institutions. In 2003, I became the first speaker
of the Virtual Institute on “Dense hadronic matter and QCD phase transitions”, joining six University
groups with the GSI theory group. In this frame, I was appointed as a guest professor/scientist at the
University Bielefeld (2004-2005) and the GSI Darmstadt (2005-2006) and could develop interdisciplinary
collaborations on quarkonium dissociation using QCD lattice data and on the constraints for the nuclear
equation of state from astrophysics and heavy-ion collisions.
In summer 2006, I won the competition for a full professorship at the University of Wroclaw and
since October 2006 I hold this position. This first permanent position, international collaborations and
major research grants provided a solid basis to attack problems which deserve serious devotion and
need sufficient experience with advanced methods of strong correlations in finite temperature quantum
field theory. To this class belong the problems of Mott dissociation, e.g., of nuclear clusters into their
nucleonic constituents and of nucleons into their quark constituents in hot and dense matter. The
implementation of both effects within a new equation of state resulted in major progress for simulations
of both, upcoming heavy-ion collision experiments at high baryon densities (CBM @ FAIR and NICA @
JINR) and astrophysical processes such as compact star mergers and supernova explosions. I am glad
that my educational path in science provided me with a spectrum of expertise that is well-suited for
meeting the modern challenges in the physics of strong interactions and international collaborations, in
particular with partner Institutes in Germany