Description
The most interesting but also most challenging task :
I. Study of the transition between hadronic and partonic dominated matter in heavy-ion reactions in energy range of $\sqrt{s}=3$ and $\sqrt{s}=7.7$ GeV as indicated by experimental data (e.g. the deviation from the quark scaling of the elliptic flow $v_2$, the ‚horn‘ in the $K/\pi$ ratio etc).
A consistent description of such a matter from a microscopic point of view is by far not achieved. This requires the development of advanced microscopic transport approaches, which have the capacity to deal with such problems. To achieve this a common effort from other theoretical fields – as lattice QCD, field theoretical models, phenomenological models – is needed.
- The first key question for the microscopic description of heavy-ion collisions is: what are the properties of the degrees-of-freedom and their interactions? This is relevant for the partonic as well as for the hadronic degrees of freedom. Up to now lQCD provides mainly the thermal properties (pressure, energy density, entropy etc.) of the QGP in equilibrium, while one needs to know how such „observables“ are created on a microscopic level by interactions of quarks and gluons in the hot and dense strongly interacting QGP.
A similar statement holds for the understanding of the hadronic properties in the medium. There are chiral models and G-matrix approaches, which indicate a strong modification of the interactions of hadrons (e.g. vector and strange mesons) in the medium at moderate densities, however, one has to overcome the limitations of such models and extend this knowledge to high densities and temperatures.
- One also needs an understanding of the hadronization of a Quark Gluon Plasma beyond the assumption of equilibrium.
- The transport approaches must be able to describe strongly interacting matter, i.e. to be based on a field theoretical description, which allows propagating the particles (hadrons and partons) with broad spectral functions. The step towards this direction is related to the use of Kananoff-Baym theory, which, however, is still limited to the mean-field description. The full quantum description of the dynamics was not achievable so far, partially due to the computational power, partially due to conceptual problems. The development of quantum computing might open a new era in this field, too.
- The novel transport model must be able to deal with the description of a first (or second) order phase transition (as predicted by effective theories), which might happen at these intermediate energies. It is not fully understood up to now if such thermostatically well-defined situation can be realized in the expanding system of heavy-ion collisions, which is far from equilibrium. A realization of such a 1st order phase transition between different degrees-of-freedom on a microscopic level in an expanding medium is a complicated theoretical task.
- Such transport models must be able to deal with fluctuations as a signal for a first-order phase transition. Since the fluctuations have a quantum nature, it is not trivial to propagate them in a consistent way during the time evolution of heavy-ion collisions.
- One of the main problems for attaching the intermediate energies is related to the absence of a consistent modeling of elementary reactions for multi-particle production.
- Of special interest is the production of multi-strange hadrons, which is considered as a prominent signal of the phase transition.
- The study of the sub-threshold open and hidden charm is sensitive to the
initial state of the heavy ion reaction and therefore of special interest.
II. Hard probes (jets, heavy-flavours) at high collision energies
On the high energy frontier a lot of progress has been made to understand jets and open heavy-flavour propagation in a strongly interacting environment. This has to be further pursued and should at the same time find its way in those approaches, which model the jet/heavy quark production.
The key words are here:
- To treat hidden heavy mesons as an open quantum system, an approach which turned out to be very promising
- To understand the interaction of jets with the medium and how the medium properties are modified by jets.
This study can help to understand the properties of the degrees-of -freedom and their interactions: where and how a transition from a perturbative description of QCD to a non-perturbative regime occurs.
- To transform this knowledge in simulation programs, which can be compared with experimental data.
III. Search of dark matter production in heavy-ion collisions
Developing of the theoretical models (including microscopic transport approaches) for dark matter particle production in heavy-ion collisions. This will help to guide experimental search of particles beyond the standard model.