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 present the inclusive invariant mass, transverse momentum and rapidity distributions of dielectrons (e+e- pairs) in p+p interactions at 3.5GeV beam kinetic energy. In the vector meson mass region, ...a distinct peak corresponding to direct ω decays is reconstructed with a 2% mass resolution. The data is compared to predictions from three model calculations. Due to the large acceptance of the HADES apparatus for e+e- invariant masses above 0.2 GeV/c 2 and for transverse pair momenta p t < 1 GeV/c, acceptance corrections are, to a large extent, model independent. This allows us to extract from dielectron data for the first time at this energy the inclusive production cross-sections for light vector mesons. Inclusive production cross-sections for π0 and η mesons are also reported. The obtained results will serve as an important reference for the study of vector meson production in proton-nucleus and heavy-ion collisions. Furthermore, using this data, an improved value for the upper bound of the branching ratio for direct η decays into the electron-positron channel is obtained.
.
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.
The tagged quasi-free np→npπ+π−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 (View the MathML sources∼2.42 GeV/c for the quasi-free collision). For the first time, differential distributions of solid statistics for π+π−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−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−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 GeVM∼2.38 GeV and with width Γ∼70 MeVΓ∼70 MeV.
We report on the first measurement of $p\Lambda$ and $pp$ correlations via the femtoscopy method in p+Nb reactions at $\mathrm{\sqrt{s_{NN}}=3.18} ~\mathrm{GeV}$, studied with the High Acceptance ...Di-Electron Spectrometer (HADES). By comparing the experimental correlation function to model calculations, a source size for $pp$ pairs of $r_{0,pp}=2.02 \pm 0.01(\mathrm{stat})^{+0.11}_{-0.12} (\mathrm{sys}) ~\mathrm{fm}$ and a slightly smaller value for $p\Lambda$ of $r_{0,\Lambda p}=1.62 \pm 0.02(\mathrm{stat})^{+0.19}_{-0.08}(\mathrm{sys}) ~\mathrm{fm}$ is extracted. Using the geometrical extent of the particle emitting region, determined experimentally with $pp$ correlations as reference together with a source function from a transport model, it is possible to study different sets of scattering parameters. The $p\Lambda$ correlation is proven sensitive to predicted scattering length values from chiral effective field theory. We demonstrate that the femtoscopy technique can be used as valid alternative to the analysis of scattering data to study the hyperon-nucleon interaction.
The HADES collaboration uses the e
+
e
−
production as a probe of the resonance matter produced in collisions at incident energies of 1-3.5 GeV/nucleon at GSI. Elementary reactions provide useful ...references for these studies and give information on resonance Dalitz decays (R→Ne
+
e
−
). Such processes are sensitive to the structure of time-like electromagnetic baryon transitions in a kinematic range where (off-shell) vector mesons play a crucial role. Results obtained in proton-proton reactions and in a commissioning pion-beam experiment are reported and prospects for future pion beam experiments and for first hyperon Dalitz decay measurements are described. The connection with the investigations of medium effects to be continued with HADES in the next years at SIS18 and SIS100 is also discussed.
Radiative transition of an excited baryon to a nucleon with emission of a virtual massive photon converting to dielectron pair (Dalitz decays) provides important information about baryon-photon ...coupling at low
q
2
in timelike region. A prominent enhancement in the respective electromagnetic transition Form Factors (etFF) at
q
2
near vector mesons
ρ/ω
poles has been predicted by various calculations reflecting strong baryon-vector meson couplings. The understanding of these couplings is also of primary importance for the interpretation of the emissivity of QCD matter studied in heavy ion collisions via dilepton emission. Dedicated measurements of baryon Dalitz decays in proton-proton and pion-proton scattering with HADES detector at GSI/FAIR are presented and discussed. The relevance of these studies for the interpretation of results obtained from heavy ion reactions is elucidated on the example of the HADES results.
Flow coefficients v_{n} of the orders n=1-6 are measured with the High-Acceptance DiElectron Spectrometer (HADES) at GSI for protons, deuterons, and tritons as a function of centrality, transverse ...momentum, and rapidity in Au+Au collisions at sqrts_{NN}=2.4 GeV. Combining the information from the flow coefficients of all orders allows us to construct for the first time, at collision energies of a few GeV, a multidifferential picture of the angular emission pattern of these particles. It reflects the complicated interplay between the effect of the central fireball pressure on the emission of particles and their subsequent interaction with spectator matter. The high precision information on higher order flow coefficients is a major step forward in constraining the equation of state of dense baryonic matter.
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