The pp-->pp pi(+) pi(-) reaction has been measured exclusively near threshold at CELSIUS. The total cross sections are nearly an order of magnitude lower than expected from previous inclusive measurements. The differential cross sections reveal pp-->pp(*)(1440)-->pp sigma = pp(pi(+)pi(-))(I = l = 0) as the dominant process as well as significant contributions from p(*)-->Delta(++)pi(-)-->psigma. The observed anisotropy in the proton angular dependence is consistent with heavy-meson exchange. In the invariant mass spectra, no narrow structures of statistical relevance (3sigma) are found.

The WASA 4π multidetector system, aimed at investigating light meson production in light ion collisions and η meson rare decays at the CELSIUS storage ring in Uppsala is presented. A detailed description of the design, together with the anticipated and achieved performance parameters are given.

Analyzing powers of pion-proton elastic scattering have been measured at PSI with the Low Energy Pion Spectrometer LEPS as well as a novel polarized scintillator target. Angular distributions between 40 and 120 deg (c.m.) were taken at 45.2, 51.2, 57.2, 68.5, 77.2, and 87.2 MeV incoming pion kinetic energy for π + p scattering, and at 67.3 and 87.2 MeV for π − p scattering. These new measurements constitute a substantial extension of the polarization data base at low energies. Predictions from phase shift analyses are compared with the experimental results, and deviations are observed at low energies.

Recently, it has been proposed that the observed anomaly in the time distribution of neutrino induced reactions, reported by the KARMEN collaboration, can be interpreted as a signal from an exotic muon decay branch µ + → e + X. It has been shown that this hypothesis gives an acceptable fit to the KARMEN data if the boson X has a mass of m X = 103.9 MeV/c 2 , close to the kinematical limit. We have performed a search for the X particle by studying for the first time the very low energy part of the Michel spectrum in µ + decays. Using a HPGe detector setup at the µE4 beamline at PSI we find branching ratios BR(µ + → e + X) < 5.7 · 10 −4 (90% C.L.) for most of the region 103 MeV/c 2 < m X < 105 MeV/c 2 .

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