Exploration of hot and dense nuclear matter with virtual photons

Real and virtual photons emitted from the hot and dense collision zone formed in heavy-ion reactions are appropriate probes for investigating in-medium properties of hadrons and, in general, the properties of hadronic and partonic bulk matter under extreme conditions.

Electromagnetic probes may also be utilized to identify and/or characterize possible new phases of the strong interaction. The electromagnetic current-current correlator, which enters into the (virtual) photon emission rate, depends on the properties of the strongly interacting medium and its constituents.

The key objective of using dileptons as probes is to identify and study modifications of hadrons in the QCD medium, search for the onset of deconfinement and chiral symmetry restoration, and possibly to discover unconventional states of matter.

Electromagnetic probes (dileptons and photons) carry important information about the decaying objects to the detectors without being affected by strong final-state interaction while traversing the medium.

Establishing the hadronic cocktail from energies of 1 GeV/u up to 45 GeV/u

Our group aims to explore strongly interacting matter at high baryon densities and moderate temperatures using penetrating lepton pairs. The long-term goal of this research is to establish a complete excitation function of dilepton production from energies of 1 GeV/u up to 45 GeV/u.

The systematic investigation of the multi-differential emission probability of dileptons provides unparalleled inside into the microscopic properties of matter under extreme conditions and can signal chiral and/or deconfinement phase transitions.

Measurement of di-electrons with HADES and CBM

Within our group we intend to pursue a di-electron research program starting at the GSI facility with the High Acceptance Di-Electron Spectrometer (HADES). We are involved in the further development of the capability of HADES and participate in the Compressed Baryonic Matter (CBM) experiments at future FAIR facility.

The HADES spectrometer installed at SIS18 at the GSI Helmholtzzentrum für Schwerionenforschung is used to study in a systematic way electron pairs and also hadron production in heavy-ion collisions in the energy regime of 1-2 GeV/u. The search for significant medium modifications of hadron properties, based on an established N+N reference spectrum, is proposed and will focus on the heaviest collision systems at the highest available beam energies.

Development of realistic event generators

The most important task of theoretical heavy-ion physics in this context is to link experimental observables to the fundamental physics and the microscopic structure of the different phases of strongly interacting matter.

Model calculations and predictions need to be tested against data, which is critical for deducing mechanisms underlying the observed spectral modifications. The proposed observables have to be implemented in realistic event generators and the sensitivity of the respective experiments has to be evaluated by means of Monte Carlo simulations.

Experiments with pion beam

The coupling of virtual massive photons to baryon resonances can experimentally be probed by means of the πN→R→e+e-N process for which neither experimental data nor reliable theoretical predictions exist. In view of these uncertainties in the theoretical description of the elementary cross section, it is necessary that the exclusive cross sections for dilepton production on the nucleon are measured. Our group also studies pion induced reactions. The production beam time took place in August 2014.

Optimization of the read-out electronics for calorimeters

Apart of physics analysis our group is working on the optimization of the read-out electronics for calorimeters (FPGA-TDC based readout electronics) to achieve individually optimized time and energy measurements including the interfacing to HADES and/or CBM read-out is needed in both cases.

Main topics of interest:

  • Reconstruction of real and virtual photons with HADES and CBM
  • Modeling of the experimental results and development of phenomenological event generators
  • Experiments with pion beam, with particular emphasis on π N → R → e+ e- N reactions
  • Establishing the hadronic cocktail from 2–45 GeV/u
  • Optimization of general readout scheme for Calorimeter type detectors (ECAL for HADES, PSD for CBM)
  • Design and implementation of slow control systems for Calorimeter

Publication list of the HADES collaboration on inSPIRE.

Topics for PhD, Master and Bachelor thesis are available.

Funding provided by

Helmholtz Association , GSI , EMMI and BMBF