The global polarization of Λ hyperons along the total orbital angular momentum of a relativistic heavy-ion collision is presented based on the high statistics data samples collected in Au+Au ...collisions at sNN=2.4 GeV and Ag+Ag at 2.55 GeV with the High-Acceptance Di-Electron Spectrometer (HADES) at GSI, Darmstadt. This is the first measurement below the strangeness production threshold in nucleon-nucleon collisions. Results are reported as a function of the collision centrality as well as a function of the hyperon's transverse momentum (pT) and rapidity (yCM) for the range of centrality 0–40%. We observe a strong centrality dependence of the polarization with an increasing signal towards peripheral collisions. For mid-central (20 – 40%) collisions the polarization magnitudes are 〈PΛ〉(%)=6.8±1.3(stat.)±2.1(syst.) for Au+Au and 〈PΛ〉(%)=6.2±0.4(stat.)±0.6(syst.) for Ag+Ag, which are the largest values observed so far. This observation thus provides a continuation of the increasing trend previously observed by STAR and contrasts expectations from recent theoretical calculations predicting a maximum in the region of collision energies about 3 GeV. The observed polarization is of a similar magnitude as predicted by 3D-fluid-dynamics and the UrQMD plus thermal vorticity model and significantly above results from the AMPT model.
Abstract
The production of
$$\Sigma ^{0}$$
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
$${\sigma (pK^{+}\Sigma ^{0}) = 17.7 \pm 1.7 (stat) \pm 1.6 (syst)}$$
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
$${pp \rightarrow pK^{+}\Sigma ^{0}}$$
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
$$\textrm{K}^{+}\Sigma ^{0}$$
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
$${\textrm{GeV}/\textrm{c}^{2}}$$
GeV
/
c
2
(
$${\textrm{N}^{*}(1710)}$$
N
∗
(
1710
)
) and 1.900
$${\textrm{GeV}/\textrm{c}^{2}}$$
GeV
/
c
2
(
$${\textrm{N}^{*}(1900)}$$
N
∗
(
1900
)
or
$${\Delta ^{*}(1900)}$$
Δ
∗
(
1900
)
) are preferred by the fit.
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.
Inclusive e$^+$e$^-$ production has been studied with HADES in $\pi^-$ + p,
$\pi^-$ + C and $\pi^- + \mathrm{CH}_2$ reactions, using the GSI pion beam at
$\sqrt{s_{\pi p}}$ = 1.49 GeV. Invariant mass ...and transverse momentum
distributions have been measured and reveal contributions from Dalitz decays of
$\pi^0$, $\eta$ mesons and baryon resonances. The transverse momentum
distributions are very sensitive to the underlying kinematics of the various
processes. The baryon contribution exhibits a deviation up to a factor seven
from the QED reference expected for the dielectron decay of a hypothetical
point-like baryon with the production cross section constrained from the
inverse $\gamma$ n$\rightarrow \pi^-$ p reaction. The enhancement is attributed
to a strong four-momentum squared dependence of the time-like electromagnetic
transition form factors as suggested by Vector Meson Dominance (VMD). Two
versions of the VMD, that differ in the photon-baryon coupling, have been
applied in simulations and compared to data. VMD1 (or two-component VMD)
assumes a coupling via the $\rho$ meson and a direct coupling of the photon,
while in VMD2 (or strict VMD) the coupling is only mediated via the $\rho$
meson. The VMD2 model, frequently used in transport calculations for dilepton
decays, is found to overestimate the measured dielectron yields, while a good
description of the data can be obtained with the VMD1 model assuming no phase
difference between the two amplitudes. Similar descriptions have also been
obtained using a time-like baryon transition form factor model where the pion
cloud plays the major role.
Hadron production (\(\pi^\pm\), proton, \(\Lambda\), \(K_S^0\), \(K^\pm\)) in \(\pi^- + \mathrm{C}\) and \(\pi^- + \mathrm{W}\) collisions is investigated at an incident pion beam momentum of ...\(1.7~\mathrm{GeV}/c\). This comprehensive set of data measured with HADES at SIS18/GSI significantly extends the existing world data on hadron production in pion induced reactions and provides a new reference for models that are commonly used for the interpretation of heavy-ion collisions. The measured inclusive differential production cross-sections are compared with state-of-the-art transport model (GiBUU, SMASH) calculations. The (semi-) exclusive channel \(\pi^- + A \rightarrow \Lambda + K_S^0 +X\), in which the kinematics of the strange hadrons are correlated, is also investigated and compared to a model calculation. Agreement and remaining tensions between data and the current version of the considered transport models are discussed.
The production of $\Sigma^0$ 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 $\mathrm{\sigma (pK^{+}\Sigma^{0}) \mu b = 17.7
\pm 1.7 (stat) \pm 1.6 (syst)}$. Differential cross section distributions of
the exclusive channel $\mathrm{pp \rightarrow pK^{+}\Sigma^{0}}$ 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
$\mathrm{K^{+}\Sigma^{0}}$. 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 $\mathrm{GeV/c^{2}}$ ($\mathrm{N^{*}(1710)}$) and
1.900 $\mathrm{GeV/c^{2}}$ ($\mathrm{N^{*}(1900)}$ or
$\mathrm{\Delta^{*}(1900)}$) are preferred by the fit.
The double differential production cross sections, \(d^2\sigma/d\Omega dE\), for hydrogen isotopes and charged pions in the reaction of p + Nb at 3.5 GeV proton beam energy have been measured by the ...High Acceptance DiElectron Spectrometer (HADES). Thanks to the high acceptance of HADES at forward emission angles and usage of its magnetic field, the measured energy range of hydrogen isotopes could be significantly extended in comparison to the relatively scarce experimental data available in the literature. The data provide information about the development of the intranuclear cascade in the proton-nucleus collisions. They can as well be utilized to study the rate of energy/momentum dissipation in the nuclear systems and the mechanism of elementary and composite particle production in excited nuclear matter at normal density. Data of this type are important also for technological and medical applications. Our results are compared to models developed to describe the processes relevant to nuclear spallation (INCL++) or oriented to probe either the elementary hadronic processes in nuclear matter or the behavior of compressed nuclear matter (GiBUU).
The global polarization of {\Lambda} hyperons along the total orbital angular momentum of a relativistic heavy-ion collision is presented based on the high statistics data samples collected in Au+Au ...collisions at \sqrt{s_{NN}} = 2.4 GeV and Ag+Ag at 2.55 GeV with the High-Acceptance Di-Electron Spectrometer (HADES) at GSI, Darmstadt. This is the first measurement below the strangeness production threshold in nucleon-nucleon collisions. Results are reported as a function of the collision centrality as well as a function of the hyperon transverse momentum (p_T) and rapidity (y_{CM}) for the range of centrality 0--40%. We observe a strong centrality dependence of the polarization with an increasing signal towards peripheral collisions. For mid-central (20--40%) collisions the polarization magnitudes are <P_{\Lambda}>(%) = 6.8 \pm 1.3 (stat.) \pm 2.1 (syst.) for Au+Au and <P_{\Lambda}>(%) = 6.2 \pm 0.4 (stat.) \pm 0.6 (syst.) for Ag+Ag, which are the largest values observed so far. This observation thus provides a continuation of the increasing trend previously observed by STAR and contrasts expectations from recent theoretical calculations predicting a maximum in the region of collision energies about 3 GeV. The observed polarization is of a similar magnitude as predicted by 3D fluid dynamics and the UrQMD plus thermal vorticity model and significantly above results from the AMPT model.
First information on the timelike electromagnetic structure of baryons in the second resonance region has been obtained from measurements of invariant mass and angular distributions in the quasi-free ...reaction \(\pi^- p \to nee\) at \(\sqrt{s_{\pi^- p}}\) = 1.49 GeV with the High Acceptance Di-Electron Spectrometer (HADES) detector at GSI using the pion beam impinging on a CH\(_2\) target. We find a total cross section \(\sigma (\pi^- p \to nee) = 2.97 \pm 0.07^{data} \pm 0.21^{acc} \pm 0.31^{\rm{Z}_{\rm{eff}}} \mu\)b. Combined with the Partial Wave Analysis of the concurrently measured two-pion channel, these data sets provide a crucial test of Vector Meson Dominance (VMD) inspired models. The commonly used "strict VMD" approach strongly overestimates the \(e^+e^-\) yield. Instead, approaches based on a VMD amplitude vanishing at small \(e^+e^-\) invariant masses supplemented coherently by a direct photon amplitude provide a better agreement. A good description of the data is also obtained using a calculation of electromagnetic timelike baryon transition form factors in a covariant spectator-quark model, demonstrating the dominance of meson cloud effects. The angular distributions of \(e^+e^-\) pairs demonstrate the contributions of virtual photons with longitudinal polarization, in contrast to real photons. The virtual photon angular dependence supports the dominance of J=3/2, I=1/2 contributions observed in both the \(\gamma^{\star} n\) and the \(\pi \pi n\) channels.
