.
On the basis of the generalized eikonal approximation we study the exclusive reactions
p
¯
d
→
J
/
ψ
n
and
p
¯
d
→
ψ
n
in the vicinity of the thresholds for charmonium production on a free proton ...target. It is shown that the rescattering of the incoming antiproton and outgoing charmonium on the spectator neutron leads to a depletion of the charmonium production at low- and to an enhancement at high-transverse momenta. This is in qualitative agreement with previous studies of hard proton knockout in proton-deuteron collisions. We analyze different physical sources of uncertainty which may influence the extraction of the total charmonium-neutron cross section. The color transparency effect for the incoming
p
¯
largely compensates the influence of charmonium rescattering both at low and high transverse momenta. Different choices of the deuteron wave function lead to significant uncertainties at high transverse momenta. As an outcome of the calculations of charmonium production, we also provide predictions on the production of open charm hadrons due to the dissociation of the charmonium on the neutron. It is shown that the open charm production cross section is proportional to the total charmonium-nucleon cross section and quite stable with respect to the variation of other parameters of the model. We thus suggest that open charm channels are most suited for future studies of charmonium-nucleon interactions at PANDA with a deuteron target.
The proton–proton elastic differential cross section at very small four momentum transfer squared has been measured at three different incident proton momenta in the range of 2.5 to 3.2 GeV/c by ...detecting the recoil proton at polar angles close to 90∘. The measurement was performed at COSY with the KOALA detector covering the Coulomb–nuclear interference region. The total cross section σtot, which has been determined precisely, is consistent with previous measurements. The values of the slope parameter B and the relative real amplitude ratio ρ determined in this experiment alleviate the lack of data in the relevant energy region. These precise data on ρ might be an important check for a new dispersion analysis.
We present the first determination of the energy-dependent amplitudes of N⁎ resonances extracted from their decay in KΛ pairs in p+p→pK+Λ reactions. A combined Partial Wave Analysis of seven data ...samples with exclusively reconstructed p+p→pK+Λ events measured by the COSY-TOF, DISTO, FOPI and HADES Collaborations in fixed target experiments at kinetic energies between 2.14 to 3.5 GeV is used to determine the amplitude of the resonant and non-resonant contributions into the associated strangeness final state. The contribution of seven N⁎ resonances with masses between 1650 MeV/c2 and 1900 MeV/c2 for an excess energy between 0 and 600 MeV has been considered. The Σ–p cusp and final state interactions for the p–Λ channel are also included as coherent contributions in the PWA. The N⁎ contribution is found to be dominant with respect to the phase space emission of the pKΛ+ final state at all energies demonstrating the important role played by both N⁎ and interference effects in hadron–hadron collisions.
.
The
Λ
polarization, the analyzing power, and the
Λ
spin transfer coefficient of the reaction
pp
→
pK
+
Λ
were measured at beam momenta of 2.70GeV/c and 2.95GeV/c corresponding to excess energies of ...122MeV and 204MeV. While the analyzing power and the spin transfer coefficient do not change significantly with the excess energy, the
Λ
polarization varies strongly and changes its sign. As this is the first measurement of polarization observables below an excess energy of 200MeV, the change of the sign of the
Λ
polarization was not observed before. The high statistics of the data (
≈
200 k events for each momentum) enables detailed studies of the dependence of the
Λ
polarization and the analyzing power on the center-of-mass momentum of the particles. The results of the spin transfer coefficient are in qualitative agreement with the DISTO experiment. The
Λ
polarization data of 2.95GeV/c are only conform with the DISTO experiment, while both the 2.70GeV/c and 2.95GeV/c data differ strongly from all previous measurements, whether exclusive or inclusive.
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.
The objective of this contribution is to review the present status of experimental information on the properties of pions and kaons in nuclear matter. Medium effects on pions are discussed in the ...context of the experimental results on deeply bound pionic states. The repulsive in-medium potential for kaons has been determined in proton-nucleus collsions in line with results from studies of nucleus-nucleus collisions. The deduction of the depth of the potential for antikaons from heavy ion data is still affected by a large degree of model dependence. An experimental search for -nuclear bound states may clarify this question in the future.
Mid-rapidity spectra and yields of K
− and K
+ have been measured for Au+Au collisions at 4, 6, 8, and 10.7 AGeV. The K
− yield increases faster with beam energy than for K
+ and hence the K
−/K
+ ...ratio increases with beam energy. This ratio is studied as a function of both
s
and
s
−
s
th
which allows the direct comparison of the kaon yields with respect to the production threshold in p+p reactions. For equal
s
−
s
th
the measured ratio K
−/K
+=0.2 at energies above threshold in contrast to the K
−/K
+ ratio of near unity observed at energies below threshold. The use of the K
−/K
+ ratio to test the predicted changes of kaon properties in dense nuclear matter is discussed.
An excitation function of proton rapidity distributions for different centralities is reported from AGS Experiment E917 for Au+Au collisions at 6, 8, and 10.8 GeV/nucleon. The rapidity distributions ...from peripheral collisions have a valley at midrapidity which smoothly change to distributions that display a broad peak at midrapidity for central collisions. The mean rapidity loss increases with increasing beam energy, whereas the fraction of protons consistent with isotropic emission from a stationary source at midrapidity decreases with increasing beam energy. The data suggest that the stopping is substantially less than complete at these energies.
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