In the very heavy collision system Au197+197Au the K+ production process was studied as a function of impact parameter at 1 GeV/nucleon, a beam energy well below the free N-N threshold. The K+ multiplicity increases more than linearly with the number of participant nucleons and the K+/π+ ratio rises significantly when going from peripheral to central collisions. The measured K+ double differential cross section is enhanced by a factor of 6 compared to microscopic transport calculations if secondary processes (ΔN→KΛN and ΔΔ→KΛN) are ignored.
The cross section for the production of $\omega$ mesons in proton-proton collisions has been measured in a previously unexplored region of incident energies. Cross sections were extracted at 92 MeV and 173 MeV excess energy, respectively. The angular distribution of the $\omega$ at $\epsilon$=173 MeV is strongly anisotropic, demonstrating the importance of partial waves beyond pure s-wave production at this energy.
Results from the HELIOS External Spectrometer on kaon production in 200 GeV/ A S + W and p + W collisions are presented. The K π ratios are compared with results from a lower beam energy and are found to be remarkably similar. Evidence for secondary production of K + by meson-baryon rescattering is reviewed. Our results at y = 1.0–1.5 are compared with neutral strange particle results at midrapidity.
The production of eta mesons has been measured in the proton-proton interaction close to the reaction threshold using the COSY-11 internal facility at the cooler synchrotron COSY. Total cross sections were determined for eight different excess energies in the range from 0.5 MeV to 5.4 MeV. The energy dependence of the total cross section is well described by the available phase-space volume weighted by FSI factors for the proton-proton and proton-eta pairs.
We report on a sample of Jψ mesons coming from secondary vertices, a characteristic of heavyquark decay, detected in the Fermilab Meson West spectrometer. Based on eight signal events in which a Jψ emerges from a secondary vertex occurring in an air-gap region, we obtain an inclusive bb¯ cross section of 75 ± 31 ± 26 nb/nucleon. This result is compared to recent QCD predictions. We have also observed several events in the exclusive decay modes B±→Jψ+K± and B0→Jψ+K0* in which the B mass is fully reconstructed.
Sigma+ hyperon production was measured at the COSY-11 spectrometer via the p p --> n K+ Sigma+ reaction at excess energies of Q = 13 MeV and Q = 60 MeV. These measurements continue systematic hyperon production studies via the p p --> p K+ Lambda/Sigma0 reactions where a strong decrease of the cross section ratio close-to-threshold was observed. In order to verify models developed for the description of the Lambda and Sigma0 production we have performed the measurement on the Sigma+ hyperon and found unexpectedly that the total cross section is by more than one order of magnitude larger than predicted by all anticipated models. After the reconstruction of the kaon and neutron four momenta, the Sigma+ is identified via the missing mass technique. Details of the method and the measurement will be given and discussed in view of theoretical models.
Threshold measurements of the associated strangeness production reactions pp --> p K(+) Lambda and pp --> p K(+) Sigma(0) are presented. Although slight differences in the shapes of the excitation functions are observed, the most remarkable feature of the data is that at the same excess energy the total cross section for the Sigma(0) production appears to be about a factor of 28 smaller than the one for the Lambda particle. It is concluded that strong Sigma(0)-p final state interactions, and in particular the Sigma-N --> Lambda-p conversion reaction, are the likely cause of the depletion for the yield in the Sigma signal. This hypothesis is in line with other experimental evidence in the literature.
K+ meson production in pA (A = C, Cu, Au) collisions has been studied using the ANKE spectrometer at an internal target position of the COSY-Juelich accelerator. The complete momentum spectrum of kaons emitted at forward angles, theta < 12 degrees, has been measured for a beam energy of T(p)=1.0 GeV, far below the free NN threshold of 1.58 GeV. The spectrum does not follow a thermal distribution at low kaon momenta and the larger momenta reflect a high degree of collectivity in the target nucleus.
Measurements of charged pion and kaon production in central Pb+Pb collisions at 40, 80 and 158 AGeV are presented. These are compared with data at lower and higher energies as well as with results from p+p interactions. The mean pion multiplicity per wounded nucleon increases approximately linearly with s_NN^1/4 with a change of slope starting in the region 15-40 AGeV. The change from pion suppression with respect to p+p interactions, as observed at low collision energies, to pion enhancement at high energies occurs at about 40 AGeV. A non-monotonic energy dependence of the ratio of K^+ to pi^+ yields is observed, with a maximum close to 40 AGeV and an indication of a nearly constant value at higher energies.The measured dependences may be related to an increase of the entropy production and a decrease of the strangeness to entropy ratio in central Pb+Pb collisions in the low SPS energy range, which is consistent with the hypothesis that a transient state of deconfined matter is created above these energies. Other interpretations of the data are also discussed.
The reaction $ pp\to pp\bf \omega$ was investigated with the TOF spectrometer, which is an external experiment at the accelerator COSY (Forschungszentrum Julich, Germany). Total as well as differential cross sections were determined at an excess energy of $93 MeV$ ($p_{beam}=2950 MeV/c$). Using the total cross section of $(9.0\pm 0.7 \pm1.1) \mu b$ for the reaction $ pp\to pp\omega$ determined here and existing data for the reaction $pp\to pp\bf \phi$, the ratio $\mathcal{R}_{\phi/\omega}=\sigma_\phi/\sigma_\omega$ turns out to be significantly larger than expected by the Okubo-Zweig-Iizuka (OZI) rule. The uncertainty of this ratio is considerably smaller than in previous determinations. The differential distributions show that the $\omega$ production is still dominated by S-wave production at this excess energy, however higher partial waves clearly contribute. A comparison of the measured angular distributions for $\omega$ production to published distributions for $\phi$ production at $83 MeV$ shows that the data are consistent with an identical production mechanism for both vector mesons.
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