Narrow Results

We present preliminary results on the observation of the η→π⁺π^-e⁺e^- decay in the CELSIUS/WASA experiment. The η mesons were produced from proton-proton interactions at 1360 MeV incident proton energy.

The experimental program on meson production in light-ion collisions at the CELSIUS cooler/storage ring of the The Svedberg Laboratory in Uppsala, is reviewed. Production of pions and eta-mesons have been studied in pp and pd collisions using the PROMICE/WASA detector. This program is now continued at CELSIUS with the WASA4π detector together with studies of meson decays.

The isobar models η-MAID and η′-MAID have been used to analyze new data on quasi-free η photoproduction on the deuteron from Bonn and recent η′ data on the proton from Jlab. In η photoproduction on the neutron a bump around W = 1700 MeV was observed which could possibly arise from a narrow P₁₁ state that is discussed as a non-strange member of the Θ⁺ antidecuplett. In η′ photoproduction on the proton resonance contributions are found that can be attributed to missing resonances in the energy region around W = 1900 MeV.

The production of η mesons at an excess energy of 72 MeV has been studied in the reaction pp → pp(η)γγ. It is shown that a simple model with Pp final states included reproduces observed differential distributions better than the same model restricted to Ss, Sd and Ds final states. The strong influence of the Pp states could be taken as an indication of ρ dominance within an one boson exchange model for the excitation of N*(1535).

Differential cross sections for η production in proton-proton collisions at 40 and 72 MeV above threshold show that higher partial waves are necessary to describe the reaction. A simple model assuming constant production amplitudes and including most partial waves up to two units of angular momentum reproduces the data rather well, and also describes the invariant mass distributions found at 15.5 MeV excess energy.