High-precision measurements of flow coefficients
v
n
(
n
=
1
-
4
) for protons, deuterons and tritons relative to the first-order spectator plane have been performed in Au+Au collisions at
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
t
and rapidity
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
,
d
v
1
/
d
y
′
|
y
′
=
0
where
y
′
is the scaled rapidity, and for
v
2
at mid-rapidity. Scaling with the number of nucleons is observed for the
p
t
dependence of
v
2
and
v
4
at mid-rapidity, which is indicative for nuclear coalescence as the main process responsible for light nuclei formation.
v
2
is found to scale with the initial eccentricity
⟨
ϵ
2
⟩
, while
v
4
scales with
⟨
ϵ
2
⟩
2
and
⟨
ϵ
4
⟩
. The multi-differential high-precision data on
v
1
,
v
2
,
v
3
, and
v
4
provides important constraints on the equation-of-state of compressed baryonic matter.
We investigate identical pion HBT intensity interferometry in central Au+Au collisions at 1.23A GeV. High-statistics π−π− and π+π+ data are measured with HADES at SIS18/GSI. The radius parameters, ...derived from the correlation function depending on relative momenta in the longitudinally comoving system and parametrized as three-dimensional Gaussian distribution, are studied as function of transverse momentum. A substantial charge-sign difference of the source radii is found, particularly pronounced at low transverse momentum. The extracted source parameters agree well with a smooth extrapolation of the center-of-mass energy dependence established at higher energies, extending the corresponding excitation functions down towards a very low energy.
In this letter we report the first multi-differential measurement of correlated pion-proton pairs from 2 billion Au+Au collisions at sNN=2.42 GeV collected with HADES. In this energy regime the ...population of Δ(1232) resonances plays an important role in the way energy is distributed between intrinsic excitation energy and kinetic energy of the hadrons in the fireball. The triple differential d3N/dMπ±pdpTdy distributions of correlated π±p pairs have been determined by subtracting the πp combinatorial background using an iterative method. The invariant-mass distributions in the Δ(1232) mass region show strong deviations from a Breit-Wigner function with vacuum width and mass. The yield of correlated pion-proton pairs exhibits a complex isospin, rapidity and transverse-momentum dependence. In the invariant mass range 1.1<Minv(GeV/c2)<1.4, the yield is found to be similar for π+p and π−p pairs, and to follow a power law 〈Apart〉α, where 〈Apart〉 is the mean number of participating nucleons. The exponent α depends strongly on the pair transverse momentum (pT) while its pT-integrated and charge-averaged value is α=1.5±0.08st±0.2sy.
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.
.
The combination of a production target for secondary beams, an optimized ion optical beam line setting, in-beam detectors for minimum ionizing particles with high rate capability, and an efficient ...large acceptance spectrometer around the reaction target constitutes an experimental opportunity to study in detail hadronic interactions utilizing pion beams impinging on nucleons and nuclei. For the 0.4-2.0GeV/c pion momentum regime such a facility is located at the heavy ion synchrotron accelerator SIS18 in Darmstadt (Germany). The layout of the apparatus, performance of its components and encouraging results from a first commissioning run are presented.
We present first data on sub-threshold production of Ks0 mesons and Λ hyperons in Au+Au collisions at sNN=2.4 GeV. We observe an universal 〈Apart〉 scaling of hadrons containing strangeness, ...independent of their corresponding production thresholds. Comparing the yields, their 〈Apart〉 scaling, and the shapes of the rapidity and the pt spectra to state-of-the-art transport model (UrQMD, HSD, IQMD) predictions, we find that none of them can simultaneously describe these observables with reasonable χ2 values.
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