The production of
Σ
0
hyperons in proton proton collisions at a beam kinetic energy of 3.5 GeV impinging on a liquid hydrogen target was investigated using data collected with the HADES setup. The ...total production cross section is found to be
σ
(
p
K
+
Σ
0
)
=
17.7
±
1.7
(
s
t
a
t
)
±
1.6
(
s
y
s
t
)
µb. Differential cross section distributions of the exclusive channel
p
p
→
p
K
+
Σ
0
were analyzed in the center-of-mass, Gottfried–Jackson and helicity reference frames for the first time at the excess energy of 556 MeV. The data support the interplay between pion and kaon exchange mechanisms and clearly demonstrate the contribution of interfering nucleon resonances decaying to
K
+
Σ
0
. The Bonn–Gatchina partial wave analysis was employed to analyse the data. Due to the limited statistics, it was not possible to obtain an unambiguous determination of the relative contribution of intermediate nucleon resonances to the final state. However nucleon resonances with masses around 1.710
GeV
/
c
2
(
N
∗
(
1710
)
) and 1.900
GeV
/
c
2
(
N
∗
(
1900
)
or
Δ
∗
(
1900
)
) are preferred by the fit.
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.
In nuclear collisions the incident protons generate a Coulomb field which acts on produced charged particles. The impact of these interactions on charged-pion transverse-mass and rapidity spectra, as ...well as on pion–pion momentum correlations is investigated in Au + Au collisions at
s
NN
= 2.4 GeV. We show that the low-
m
t
region (
m
t
<
0.2
GeV/
c
2
) can be well described with a Coulomb-modified Boltzmann distribution that also takes changes of the Coulomb field during the expansion of the fireball into account. The observed centrality dependence of the fitted mean Coulomb potential energy deviates strongly from a
A
part
2
/
3
scaling, indicating that, next to the fireball, the non-interacting charged spectators have to be taken into account. For the most central collisions, the Coulomb modifications of the HBT source radii are found to be consistent with the potential extracted from the single-pion transverse-mass distributions. This finding suggests that the region of homogeneity obtained from two-pion correlations coincides with the region in which the pions freeze-out. Using the inferred mean-square radius of the charge distribution at freeze-out, we have deduced a baryon density, in fair agreement with values obtained from statistical hadronization model fits to the particle yields.
Results on the production of the double strange cascade hyperon Ξ^{-} are reported for collisions of p(3.5 GeV)+Nb, studied with the High Acceptance Di-Electron Spectrometer (HADES) at SIS18 at GSI ...Helmholtzzentrum for Heavy-Ion Research, Darmstadt. For the first time, subthreshold Ξ^{-} production is observed in proton-nucleus interactions. Assuming a Ξ^{-} phase-space distribution similar to that of Λ hyperons, the production probability amounts to P_{Ξ^{-}}=2.0±0.4(stat)±0.3(norm)±0.6(syst)×10^{-4} resulting in a Ξ^{-}/(Λ+Σ^{0}) ratio of P_{Ξ^{-}}/P_{Λ+Σ^{0}}=1.2±0.3(stat)±0.4(syst)×10^{-2}. Available model predictions are significantly lower than the measured Ξ^{-} yield.
We report measurements of electron pair production in elementary p+p and d+p reactions at 1.25 GeV/u with the HADES spectrometer. For the first time, the electron pairs were reconstructed for n+p ...reactions by detecting the proton spectator from the deuteron breakup. We find that the yield of electron pairs with invariant mass Me+e−>0.15 GeV/c2 is about an order of magnitude larger in n+p reactions as compared to p+p. A comparison to model calculations demonstrates that the production mechanism is not sufficiently described yet. The electron pair spectra measured in C+C reactions are compatible with a superposition of elementary n+p and p+p collisions, leaving little room for additional electron pair sources in such light collision systems.
All of the former transition economies
in Central, Eastern, and Southern Europe (CESE) inherited
from the era of central planning traditional defined-benefit
pension systems financed on a ...pay-as-you-go basis. Like many
pay-as-you-go public pension systems elsewhere in the world,
CESE pension systems were in need of reforms to address
short-term fiscal imbalances and longer-term issues relating
to population aging. Reforms were also needed to adjust
benefit and contribution structures to meet the challenges
of-as well as to take advantage of opportunities relating to
the transition to a market economy, including the widespread
adoption of multiplier designs with improved risk-sharing
across funded and unfunded pillars. By 2006, most countries
in Europe and Central Asia had introduced a voluntary
private pension scheme. By 2008, 14 countries roughly half
of all countries in the region had legislated mandatory
private pension schemes, and all but one of those schemes
(the one in Ukraine) had been introduced. These reforms
shared a number of common objectives, in particular putting
the systems on a sounder financial footing and better
aligning them with the (very different) incentives of a
market economy. This report is organized as follows. The
first section discusses the motivation for reform across the
eight countries included in the study against the backdrop
of the regional (and global) trend toward multiplier pension
arrangements. The second section summarizes the key
provisions of the reformed systems in the eight countries
within the World Bank's five-pillar framework for
pension system design. The third section summarizes pension
system performance against the two crucially important
dimensions of adequacy and sustainability. The last section
provides some policy recommendations for addressing gaps in
reforms and taking advantage of further opportunities.
Reactions of the halo systems Be-11 and Li-11 (at 460 and 280 MeV/nucleon) with a carbon target demonstrate that (n + Li-9) has an (unbound) l = 0 ground state very close to the threshold. The ...neutron halo of Li-11 has appreciable (1s(1/2))(2) and (0p(1/2))(2) components.
We present data on dielectron emission in proton induced reactions on a Nb target at 3.5 GeV kinetic beam energy measured with HADES installed at GSI. The data represent the first high statistics ...measurement of proton-induced dielectron radiation from cold nuclear matter in a kinematic regime, where strong medium effects are expected. Combined with the good mass resolution of 2%, it is the first measurement sensitive to changes of the spectral functions of vector mesons, as predicted by models for hadrons at rest or small relative momenta. Comparing the e+e− invariant mass spectra to elementary p+p data, we observe for e+e− momenta Pee<0.8 GeV/c a strong modification of the shape of the spectrum, which we attribute to an additional ρ-like contribution and a decrease of ω yield. These opposite trends are tentatively interpreted as a strong coupling of the ρ meson to baryonic resonances and an absorption of the ω meson, which are two aspects of in-medium modification of vector mesons.
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