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energies so far did not find indications for non-statistical
fluctuations, except for the measured kaon-to-pion ratio
at low SPS energies. Future progress in the search for the
critical point and a first order phase transition requires
a careful beam energy scan in the region of low SPS/
FAIR energies together with a systematic measurement
of fluctuations of various observables event by event.
Hadron properties in dense matter – One of the most
important goals of heavy ion collision experiments is to
search for signatures of chiral symmetry restoration,
which is expected to happen at very high baryon densities
and/or temperatures. An observable consequence
of chiral symmetry restoration would be a modification
of hadron properties, as nuclear matter approaches
the phase boundary. Indications for in-medium hadron
modifications have been found for kaons and for vector
mesons.
The yields and anisotropic flow of charged kaons show
strong effects of in-medium modifications in heavy ion
collisions at threshold beam energies as measured by
KaoS, FOPI and recently by HADES. The measured
results can be described under the assumption of a
repulsive potential between K + mesons and nucleons,
and an attractive potential between K - mesons and
nucleons. This is in qualitative agreement with calculations
based on the Chiral Lagrangian. The necessary
detailed transport calculations, which need to include,
e.g., off-shell dynamics, are only in a preliminary state
and are not yet able to achieve satisfactory agreement
Figure 8. Dimuon excess mass spectrum measured by NA60 in
In+In collisions at 160 A GeV (full symbols) compared to model
calculations. (Courtesy of R. Rapp et al.)
with the experimental data. From the current investigations
it is apparent that these modifications do not
allow a direct conclusion on chiral symmetry restoration.
Nevertheless, it is a theoretical challenge to systematically
explore chiral symmetry in nuclear many-body
systems with kaons.
Dilepton decays provide direct access to the properties
of light vector mesons in dense and/or hot nuclear
matter. In heavy ion collisions at the SPS, CERES and
NA60 found a significantly enhanced yield of lepton pairs
in the invariant mass range between 200 and 700 MeV/c 2 .
Figure 8 depicts the dimuon excess yield measured by
NA60 for In+In collisions at 158 A GeV. The excess yield
is defined relative to dimuon yields from known hadronic
decays, including the ω and the φ meson. According to
microscopic calculations, the excess dilepton yield is
dominated by ππ annihilation, which proceeds through
the ρ vector meson due to vector dominance. The shape
and the magnitude of the excess can be explained
assuming that the ρ-meson mass distribution is substantially
broadened. The calculations indicate that the
coupling to baryonic resonances plays a crucial role.
A central objective of dilepton measurements is to find
signatures of chiral symmetry restoration in the quarkhadron
transition. To this end a connection between
observables and chiral order parameters must be established.
According to the calculations shown in Figure 8
the vector spectral function, which is dominated by the
ρ-meson at invariant masses below 1 GeV, broadens to
such an extent that it smoothly goes over to the quark
rate in the plasma phase. Since chiral symmetry is
restored in this phase the broadening of the ρ-meson
could be viewed as a consequence of chiral symmetry
restoration. A more direct measure of chiral symmetry
restoration is the degeneracy of the vector and axial vector
spectral functions. An important step forward would
be the systematic derivation of both in-medium vector
and axial vector spectral functions based on the Chiral
Lagrangian, together with accurate dilepton measurements
to constrain the vector channel. An additional
bonus would be experimental information on the inmedium
axial vector spectral function, possibly through
the π ± γ channel.
The HADES collaboration has performed precision
measurements of dilepton invariant mass spectra in
nuclear collisions at beam energies of 1-2 A GeV. The
HADES data confirm the results of the DLS collaboration
for C+C collisions. Moreover, it has been experimentally
proven that the dilepton spectra from C+C collisions correspond
to a superposition of lepton pairs from p+p and
p+n collisions. In heavier systems like Ar+KCl, however,
a dilepton excess yield relative to the nucleon-nucleon
reference data was observed. This effect is illustrated
Perspectives of Nuclear Physics in Europe – NuPECC Long Range Plan 2010 | 91