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.

The differential and total cross sections for kaon pair production in the pp->ppK+K- reaction have been measured at three beam energies of 2.65, 2.70, and 2.83 GeV using the ANKE magnetic spectrometer at the COSY-Juelich accelerator. These near-threshold data are separated into pairs arising from the decay of the phi-meson and the remainder. For the non-phi selection, the ratio of the differential cross sections in terms of the K-p and K+p invariant masses is strongly peaked towards low masses. This effect can be described quantitatively by using a simple ansatz for the K-p final state interaction, where it is seen that the data are sensitive to the magnitude of an effective K-p scattering length. When allowance is made for a small number of phi events where the K- rescatters from the proton, the phi region is equally well described at all three energies. A very similar phenomenon is discovered in the ratio of the cross sections as functions of the K-pp and K+pp invariant masses and the identical final state interaction model is also very successful here. The world data on the energy dependence of the non-phi total cross section is also reproduced, except possibly for the results closest to threshold.

We analyze a sample of W + jet events collected with the Collider Detector at Fermilab (CDF) in ppbar collisions at sqrt(s) = 1.8 TeV to study ttbar production. We employ a simple kinematical variable "H", defined as the scalar sum of the transverse energies of the lepton, neutrino and jets. For events with a W boson and four or more jets, the shape of the "H" distribution deviates by 3.8 standard deviations from that expected from known backgrounds to ttbar production. However this distribution agrees well with a linear combination of background and ttbar events, the agreement being best for a top mass of 180 GeV/c^2.

We present the results of a search for neutral Higgs bosons produced in association with $b$ quarks in $p\bar{p}\to b\bar{b} \phi\to b\bar{b}b\bar{b}$ final states with $91 \pm 7$ pb$^{-1}$ of $p\bar{p}$ collisions at $\sqrt{s}=1.8$ TeV recorded by the Collider Detector at Fermilab. We find no evidence of such a signal and the data is interpreted in the context of the neutral Higgs sector of the Minimal Supersymmetric extension of the Standard Model. With basic parameter choices for the supersymmetric scale and the stop quark mixing, we derive 95% C.L. lower mass limits for neutral Higgs bosons for $\tb$ values in excess of 35.

Using a secondary pion beam from the Argonne Zero Gradient Synchrotron we have studied the process π−p→φn in the region of the cross-section enhancement near kinematic threshold. For incident momenta between 1.6 and 2 GeV/c, we have determined production and decay angular distributions and extrapolated total cross sections from a sample of about 160 φ's above background. The production and decay distributions are consistent with isotropy over this entire incident-momentum range. The extrapolated total cross section varies between 19 and 25 μb.

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.

We present the results of a search for third generation leptoquark (LQ) pairs in 110±8pb−1of p¯p collisions at s=1.8TeV recorded by the Collider Detector at Fermilab. We assume third generation leptoquarks decay to a τ lepton and a b quark with branching ratio β. We observe one candidate event, consistent with standard model background expectations. We place upper limits on σ(p¯p→LQLQ¯)̇β2 as a function of the leptoquark mass MLQ. We exclude at 95% confidence level scalar leptoquarks with MLQ<99GeV/c2, gauge vector leptoquarks with MLQ<225GeV/c2, and nongauge vector leptoquarks with MLQ<170GeV/c2 for β=1.

We use 106 $\ipb$ of data collected with the Collider Detector at Fermilab to search for narrow-width, vector particles decaying to a top and an anti-top quark. Model independent upper limits on the cross section for narrow, vector resonances decaying to $\ttbar$ are presented. At the 95% confidence level, we exclude the existence of a leptophobic $\zpr$ boson in a model of topcolor-assisted technicolor with mass $M_{\zpr}$ $<$ 480 $\gev$ for natural width $\Gamma$ = 0.012 $M_{\zpr}$, and $M_{\zpr}$ $<$ 780 $\gev$ for $\Gamma$ = 0.04 $M_{\zpr}$.

The cross section for inclusive multipion production in the pp->ppX reaction was measured at COSY-ANKE at four beam energies, 0.8, 1.1, 1.4, and 2.0 GeV, for low excitation energy in the final pp system, such that the diproton quasi-particle is in the 1S0 state. At the three higher energies the missing mass Mx spectra show a strong enhancement at low Mx, corresponding to an ABC effect that moves steadily to larger values as the energy is increased. Despite the missing-mass structure looking very different at 0.8 GeV, the variation with Mx and beam energy are consistent with two-pion production being mediated through the excitation of two Delta(1232) isobars, coupled to S-- and D-- states of the initial pp system.

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.