Abstract
We present high-statistic data on charged-pion emission from Au + Au collisions at
$$\sqrt{s_{\mathrm{NN}}} = 2.4~\hbox {GeV}$$
s
NN
=
2.4
GeV
(corresponding to
$$E_{beam} = 1.23~\hbox {A ...GeV}$$
E
beam
=
1.23
A GeV
) in four centrality classes in the range 0–40% of the most central collisions. The data are analyzed as a function of transverse momentum, transverse mass, rapidity, and polar angle. Pion multiplicity per participating nucleon decreases moderately with increasing centrality. The polar angular distributions are found to be non-isotropic even for the most central event class. Our results on pion multiplicity fit well into the general trend of the available world data, but undershoot by
$$2.5~\sigma $$
2.5
σ
data from the FOPI experiment measured at slightly lower beam energy. We compare our data to state-of-the-art transport model calculations (PHSD, IQMD, PHQMD, GiBUU and SMASH) and find substantial differences between the measurement and the results of these calculations.
At energies below sNN≈2.55GeV, strange quarks cannot be produced in binary nucleon-nucleon collisions because of the higher production threshold of the lightest hadrons carrying strangeness. Hence, ...the investigation of sub-threshold strangeness production in heavy-ion collisions is one of the most promising probes, to access the properties of the created system, as the missing energy must be provided by the latter one. For the first time, a nearly complete set of strange particles has been reconstructed in the 40% most central Au+Au collisions at sNN=2.42GeV. The data sample includes multi-differential representations of charged and neutral particles containing strangeness (K±, Ks0, ϕ, Λ). To achieve a better understanding of strangeness production the properties of the short-lived resonances have to be investigated. The first steps in this direction are presented here, including the reconstruction of baryon resonances using a new iterative technique, comparison to microscopic transport model calculations and interpretation of the pion transverse momentum distribution.
The production of Σ0 baryons in the nuclear reaction p (3.5 GeV) + Nb (corresponding to sNN=3.18 GeV) is studied with the detector set-up HADES at GSI, Darmstadt. Σ0s were identified via the decay ...Σ0→Λγ with subsequent decays Λ→pπ− in coincidence with a e+e− pair from either external (γ→e+e−) or internal (Dalitz decay γ⁎→e+e−) gamma conversions. The differential Σ0 cross section integrated over the detector acceptance, i.e. the rapidity interval 0.5<y<1.1, has been extracted as ΔσΣ0=2.3±(0.2)stat±(−0.6+0.6)sys±(0.2)norm mb, yielding the inclusive production cross section in full phase space σΣ0total=5.8±(0.5)stat±(−1.4+1.4)sys±(0.6)norm±(1.7)extrapol mb by averaging over different extrapolation methods. The Λall/Σ0 ratio within the HADES acceptance is equal to 2.3±(0.2)stat±(−0.6+0.6)sys. The obtained rapidity and momentum distributions are compared to transport model calculations. The Σ0 yield agrees with the statistical model of particle production in nuclear reactions.
Abstract
High-statistics
$$\pi ^-\pi ^-$$
π
-
π
-
and
$$\pi ^+\pi ^+$$
π
+
π
+
femtoscopy data are presented for Au + Au collisions at
$$\sqrt{s_\mathrm{NN}} = 2.4~\hbox {GeV}$$
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.
We present high-statistic data on charged-pion emission from Au + Au collisions at
s
NN
=
2.4
GeV
(corresponding to
E
beam
=
1.23
A GeV
) in four centrality classes in the range 0–40% of the most ...central collisions. The data are analyzed as a function of transverse momentum, transverse mass, rapidity, and polar angle. Pion multiplicity per participating nucleon decreases moderately with increasing centrality. The polar angular distributions are found to be non-isotropic even for the most central event class. Our results on pion multiplicity fit well into the general trend of the available world data, but undershoot by
2.5
σ
data from the FOPI experiment measured at slightly lower beam energy. We compare our data to state-of-the-art transport model calculations (PHSD, IQMD, PHQMD, GiBUU and SMASH) and find substantial differences between the measurement and the results of these calculations.
.
We report on the investigation of dielectron production in tagged quasi-free neutron-proton collisions by using a deuteron beam of kinetic energy 1.25GeV/u impinging on a liquid hydrogen target. ...Our measurements with HADES confirm a significant excess of
e
+
e
-
pairs above the
π
0
mass in the exclusive channel
d
p
→
n
p
e
+
e
-
(
p
s
p
e
c
t
)
as compared to the exclusive channel
p
p
e
+
e
-
measured in proton-proton collisions at the same energy. That excess points to different bremsstrahlung production mechanisms. Two models were evaluated for the role of the charged pion exchange between nucleons and double-
Δ
excitation combined with intermediate
ρ
-meson production. Differential cross sections as a function of the
e
+
e
-
invariant mass and of the angles of the virtual photon, proton and electrons provide valuable constraints and encourage further investigations on both experimental and theoretical sides.
We present and discuss recent experimental activities of the HADES collaboration on open and hidden strangeness production close or below the elementary NN threshold. Special emphasis is put on the ...feed-down from φ mesons to antikaons, the presence of the Ξ- excess in cold nuclear matter and the comparison of statistical model rates to elementary p+p data. The implications for the interpretation of heavy-ion data are discussed as well.
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