We report on the investigation of dielectron production in tagged quasi-free neutron-proton collisions by using a deuteron beam of kinetic energy 1.25 GeV/u inpinging on a liquid hydrogen target. Our ...measurements with HADES confirm a significant excess of \(e^+e^-\) pairs above the \(\pi^{0}\) mass in the exclusive channel \(dp \to npe^{+}e^{-}(p_{spect})\) as compared to the exclusive channel \(ppe^{+}e^{-}\) 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-\(\Delta\) excitation combined with intermediate \(\rho\)-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 side.
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.
The inclusive production of {\Lambda} hyperons in proton-proton collisions at \(\sqrt{s}\) = 3.18 GeV was measured with HADES at the GSI Helmholtzzentrum f\"ur Schwerionenforschung in Darmstadt. The ...experimental data are compared to a data-based model for individual exclusive {\Lambda} production channels in the same reaction. The contributions of intermediate resonances such as {\Sigma}(1385), {\Delta}++ or N* are considered in detail. In particular, the result of a partial wave analysis is accounted for the abundant pK\(^+\){\Lambda} final state. Model and data show a reasonable agreement at mid rapidities, while a difference is found for larger rapidities. A total {\Lambda} production cross section in p+p collisions at \(\sqrt{s}\) = 3.18 GeV of {\sigma}(pp \(\to\) {\Lambda} + X) = 207.3 \(\pm\) 1.3 +6.0 -7.3 (stat.) \(\pm\) 8.4 (syst.) +0.4 -0.5 (model) {\mu}b is found.
Correlations between p and pbar's at transverse momenta typical of enhanced baryon production in Au+Au collisions are reported. The PHENIX experiment measures same and opposite sign baryon pairs in ...Au+Au collisions at sqrt(s_NN) = 200 GeV. Correlated production of p and p^bar with the trigger particle from the range 2.5 < p_T < 4.0 GeV/c and the associated particle with 1.8 < p_T < 2.5 GeV/c is observed to be nearly independent of the centrality of the collisions. Same sign pairs show no correlation at any centrality. The conditional yield of mesons triggered by baryons (and anti-baryons) and mesons in the same pT range rises with increasing centrality, except for the most central collisions, where baryons show a significantly smaller number of associated mesons. These data are consistent with a picture in which hard scattered partons produce correlated p and p^bar in the p_T region of the baryon excess.
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.
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