Many QCD based and phenomenological models predict changes of hadron properties in a strongly interacting environment. The results of these models differ significantly and the experimental ...determination of hadron properties in nuclear matter is essential. In this paper we present a review of selected physics results obtained at GSI Helmholtzzentrum fur Schwerionenforschung GmbH by HADES (High-Acceptance Di-Electron Spectrometer). The e super(+)e super(-) pair emission measured for proton and heavy-ion induced collisions is reported together with results on strangeness production. The future HADES activities at the planned FAIR facility are also discussed.
.
The HADES data from p + Nb collisions at a center-of-mass energy of
GeV are analyzed employing a statistical hadronization model. The model can successfully describe the production yields of the ...identified hadrons
,
,
,
K
0
s
,
with parameters
MeV and
MeV, which fit well into the chemical freeze-out systematics found in heavy-ion collisions. In addition, we reanalyze our previous HADES data from Ar + KCl collisions at
GeV with an updated version of the model. We address equilibration in heavy-ion collisions by testing two aspects: the description of yields and the regularity of freeze-out parameters from a statistical model fit as a function of colliding energy and system size. Despite its success, the model fails to describe the observed
yields in both, p + Nb and Ar + KCl . Special emphasis is put on feed-down contributions from higher-lying resonance states as a possible explanation for the observed excess.
The High Acceptance DiElectron Spectrometer HADES 1 is installed at the Helmholtzzentrum fur Schwerionenforschung (GSI) accelerator facility in Darmstadt. It investigates dielectron emission and ...strangeness production in the 1-3 AGeV regime. A recent experiment series focusses on medium-modifications of light vector mesons in cold nuclear matter. In two runs, p+p and p+Nb reactions were investigated at 3.5 GeV beam energy; about 9 times 10 super(9) events have been registered. In contrast to other experiments the high acceptance of the HADES allows for a detailed analysis of electron pairs with low momenta relative to nuclear matter, where modifications of the spectral functions of vector mesons are predicted to be most prominent. Comparing these low momentum electron pairs to the reference measurement in the elementary p+p reaction, we find in fact a strong modification of the spectral distribution in the whole vector meson region.
Recent results obtained with the HADES experimental set-up at GSI are presented with a focus on dielectron production and strangeness in pp and quasi-free np reactions. Perspectives related to the ...very recent experiment using the pion beam at GSI are also discussed.
High-statistics
π
-
π
-
and
π
+
π
+
femtoscopy data are presented for Au + Au collisions at
s
NN
=
2.4
GeV
, measured with HADES at SIS18/GSI. The experimental correlation functions allow the ...determination of the space-time extent of the corresponding emission sources via a comparison to models. The emission source, parametrized as three-dimensional Gaussian distribution, is studied in dependence on pair transverse momentum, azimuthal emission angle with respect to the reaction plane, collision centrality and beam energy. For all centralities and transverse momenta, a geometrical distribution of ellipsoidal shape is found in the plane perpendicular to the beam direction with the larger extension perpendicular to the reaction plane. For large transverse momenta, the corresponding eccentricity approaches the initial eccentricity. The eccentricity is smallest for most central collisions, where the shape is almost circular. The magnitude of the tilt angle of the emission ellipsoid in the reaction plane decreases with increasing centrality and increasing transverse momentum. All source radii increase with centrality, largely exhibiting a linear rise with the cube root of the number of participants. A substantial charge-sign difference of the source radii is found, appearing most pronounced at low transverse momentum. The extracted source parameters are consistent with the extrapolation of their energy dependence down from higher energies.
Abstract
High-precision measurements of flow coefficients
$$v_{n}$$
v
n
(
$$n = 1 - 4$$
n
=
1
-
4
) for protons, deuterons and tritons relative to the first-order spectator plane have been performed ...in Au+Au collisions at
$$\sqrt{s_{_{{\text {NN}}}= 2.4$$
s
NN
=
2.4
GeV with the High-Acceptance Di-Electron Spectrometer (HADES) at the SIS18/GSI. Flow coefficients are studied as a function of transverse momentum
$$p_{{\text {t}}}$$
p
t
and rapidity
$$y_{{\text {cm}}}$$
y
cm
over a large region of phase-space and for several classes of collision centrality. A clear mass hierarchy, as expected by relativistic hydrodynamics, is found for the slope of
$$v_{1}$$
v
1
,
$$d v_{1}/d y^{\prime }|_{y^{\prime } = 0}$$
d
v
1
/
d
y
′
|
y
′
=
0
where
$$y^{\prime }$$
y
′
is the scaled rapidity, and for
$$v_{2}$$
v
2
at mid-rapidity. Scaling with the number of nucleons is observed for the
$$p_{{\text {t}}}$$
p
t
dependence of
$$v_{2}$$
v
2
and
$$v_{4}$$
v
4
at mid-rapidity, which is indicative for nuclear coalescence as the main process responsible for light nuclei formation.
$$v_{2}$$
v
2
is found to scale with the initial eccentricity
$$\langle \epsilon _{2} \rangle $$
⟨
ϵ
2
⟩
, while
$$v_{4}$$
v
4
scales with
$$\langle \epsilon _{2} \rangle ^{2}$$
⟨
ϵ
2
⟩
2
and
$$\langle \epsilon _{4} \rangle $$
⟨
ϵ
4
⟩
. The multi-differential high-precision data on
$$v_{1}$$
v
1
,
$$v_{2}$$
v
2
,
$$v_{3}$$
v
3
, and
$$v_{4}$$
v
4
provides important constraints on the equation-of-state of compressed baryonic matter.
The tagged quasi-free np→npπ+π− reaction has been studied experimentally with the High Acceptance Di-Electron Spectrometer (HADES) at GSI at a deuteron incident beam energy of 1.25 GeV/nucleon ...(s∼2.42 GeV/c for the quasi-free collision). For the first time, differential distributions of solid statistics for π+π− production in np collisions have been collected in the region corresponding to the large transverse momenta of the secondary particles. The invariant mass and angular distributions for the np→npπ+π− reaction are compared with different models. This comparison confirms the dominance of the t-channel with ΔΔ contribution. It also validates the changes previously introduced in the Valencia model to describe two-pion production data in other isospin channels, although some deviations are observed, especially for the π+π− invariant mass spectrum. The extracted total cross section is also in much better agreement with this model. Our new measurement puts useful constraints for the existence of the conjectured dibaryon resonance at mass M∼2.38 GeV and with width Γ∼70 MeV.
New results on the differential cross section in deuteron-proton elastic scattering are obtained at the deuteron kinetic energy of 2.5 GeV with the HADES spectrometer. The angular range of 69° – 125° ...in the center of mass system is covered. The obtained results are compared with the relativistic multiple scattering model calculation using the CD-Bonn deuteron wave function. The data at fixed scattering angles in the c.m. are in qualitative agreement with the constituent counting rules prediction.
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