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4.2 Phases of Strongly Interacting Matter
Figure 9. Di-electron invariant mass spectra measured by HADES.
The Ar+KCl data (symbols) are compared to a superposition of p+p
and p+n data, represented by the shaded area.
in Figure 9, which depicts the di-electron invariant mass
spectra for Ar+KCl collisions (symbols), and for a superposition
of p+p and p+n collisions (shaded area), both
normalised to the measured pion yields. HADES will
systematically study the origin of the dilepton excess in
collisions of heavy systems up to Au+Au.
Outlook
Strange particles and dileptons are the most promising
diagnostic probes of nuclear matter at two to three times
saturation density as created in nucleus-nucleus collisions
at 1 - 2 A GeV. Strangeness production in nuclear
collisions is being systematically investigated by the
FOPI collaboration at GSI. Within this study, experimental
evidence for the existence of a strange dibaryon decaying
into a Lambda and a proton was found. The HADES
collaboration has also started a strangeness programme,
and identified double-strange Ξ hyperons at deep subthreshold
beam energies. HADES identified for the first
time ω mesons via the dilepton channel in collisions
between light nuclei at low energies. These measurements
should be continued and extended to heavy
collision systems, both for strangeness and dileptons.
The theoretical conjecture of a fi rst-order deconfinement
phase transition and a QCD critical endpoint
existing at large baryon-chemical potentials, together
with the intriguing observations made in heavy ion
collisions at low SPS energies, triggered new experimental
activities at the major heavy ion laboratories:
the beam energy scan programme at RHIC, the fixedtarget
NA61/SHINE experiment at CERN-SPS, the NICA
collider project at JINR in Dubna, and the proposed
fixed-target Compressed Baryonic Matter (CBM) experiment
at FAIR.
The collider experiments at RHIC and NICA have
the advantage of a constant acceptance as function
of beam energy. On the other hand, when running at
low beam energies, collider experiments are restricted
to the measurement of abundantly produced particles
due to limitations in luminosity. The same is true for
the experiment NA61/SHINE at the SPS, which operates
at max. 80 Hz (although SPS could deliver much
higher intensities). In contrast, the experiments at FAIR
are designed for extremely high luminosities, enabling
the systematic measurement of multi-differential cross
sections with unprecedented statistics even for rare
diagnostic probes like multi-strange hyperons, lepton
pairs, charmonium and open charm.
The SIS-100 accelerator at FAIR will deliver heavy ion
beams with energies up to 14 A GeV to the HADES and
CBM experimental setups. This energy range is ideally
suited to produce and to investigate net baryon densities
as they exist in the cores of neutron stars. For the first
time, penetrating probes like dileptons and multi-strange
particles such as Ω-hyperons will be used to study systematically
the properties of baryonic matter in this beam
energy range. The 30 GeV proton beams from SIS-100
will allow pioneering measurements to be performed on
(open) charm production at threshold energies, as well as
the detailed study of charm propagation in cold nuclear
matter. The SIS-300 accelerator will deliver high-intensity
heavy ion beams with energies up to 45 A GeV to the
high-rate CBM experiment providing excellent conditions
for the investigation of the QCD phase diagram at large
baryon-chemical potentials.
92 | Perspectives of Nuclear Physics in Europe – NuPECC Long Range Plan 2010