Production of Eta-Mesons in Collisions of Nucleons and Delta-Resonances

We calculate the cross section for the production of $\eta$-mesons via \mbox{$\Delta N \to N N \eta$} in a relativistic One-Boson-Exchange-Model. Using this cross section we then determine the probability for the production of an $\eta$-meson by a $\Delta$-resonance moving in nuclear matter. The result is compared to prescriptions in BUU-calculations in which \et-production proceeds both through a direct channel and through the sequential process $\Delta \to N \pi ; \pi N \to N \eta$.Comment: revised version: large parts have been rewritten, there are two new figures. A discusion of effects of multi-body reactions of the intermediate pion on our results has been adde

Two-loop corrections to the decay rate of parapositronium

Order $\alpha^2$ corrections to the decay rate of parapositronium are calculated. A QED scattering calculation of the amplitude for electron-positron annihilation into two photons at threshold is combined with the technique of effective field theory to determine an NRQED Hamiltonian, which is then used in a bound state calculation to determine the decay rate. Our result for the two-loop correction is $5.1243(33)$ in units of $(\alpha/\pi)^2$ times the lowest order rate. This is consistent with but more precise than the result $5.1(3)$ of a previous calculation.Comment: 26 pages, 7 figure

New test of modulated electron capture decay of hydrogen-like 142Pm ions: Precision measurement of purely exponential decay

An experiment addressing electron capture (EC) decay of hydrogen-like 142Pm60+ions has been conducted at the experimental storage ring (ESR) at GSI. The decay appears to be purely exponential and no modulations were observed. Decay times for about 9000 individual EC decays have been measured by applying the single-ion decay spectroscopy method. Both visually and automatically analysed data can be described by a single exponential decay with decay constants of 0.0126(7)s−1for automatic analysis and 0.0141(7)s−1for manual analysis. If a modulation superimposed on the exponential decay curve is assumed, the best fit gives a modulation amplitude of merely 0.019(15), which is compatible with zero and by 4.9 standard deviations smaller than in the original observation which had an amplitude of 0.23(4)