A new upper limit for the probability of spontaneous muonium to antimuonium conversion was established at P MM ≤ 8.2 · 10 −11 (90%C.L.) in 0.1 T magnetic field, which implies consequences for speculative extensions to the standard model. Coupling parameters in R-parity violating supersymmetry and the mass of a flavour diagonal bileptonic gauge boson can be significantly restricted. A Z 8 model with radiative mass generation through heavy lepton seed and the minimal version of 331-GUT models are ruled out.

A new precision measurement of the branching ratio of the rare pion decay into a positron and a neutrino Or-* ev) has been completed. A beam of positive pions was stopped in an active target of plastic scintillator surrounded by a An BGO calorimeter. 3xl0 5 rare decays and 1.2xl0 6 normal pion decays (/r-*/xv) were recorded. The branching ratio was finally calculated from 1.2 xlO 5 rare decays after various cuts in the time window from 7.5 to 200 ns after pion stop. The errors of the result (1.235 ±0.005) x 10 " 4 are 0.28% statistical and 0.29% systematical.PACS numbers: 13.20.Cz An experiment was performed to measure the branching ratio R for the rare decay /r-> ev(y) (i.e., K-+ ev including K-* evy), normalized to the normal pion decay K-* jnv(y) with high precision, testing the fie universality of the weak interaction. This rare decay has been treated theoretically many times and values for R have been published by several authors during the development of the theory of the weak interaction [1-3] and also later inthe framework of gauge theories. The most recent predictions for R are as follows: Marciano and Sirlin [4], (1.233 ±0.004) x 10 " 4 ; Goldman and Wilson [5], (1.239 ± 0.001) x 10 " 4 ; Marciano [6], (1.2345 ± 0.0010) x 10 " 4 .

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New measurements of the pionic double charge exchange on SaFe exhibit a pronounced energy dependence of this reaction at low energies in accordance with similar observations on other nuclei. We show that this peculiar energy dependence finds its natural explanation by a resonance in the 7rNN subsystem with JP = 0-,T = 0 and a mass of 2.065 GeV. PACS: 14.20 P, 24.30, 25.80 The substructure of the nucleon should lead to nontrivial, i.e. non-nucleonic resonances in the B = 2 system. Although such dibaryon resonances have been predicted [1,2] since long, no unambiguous evidence for their existence has been found up to now, despite a vast number of dedicated experiments. In this short note we present first evidence for the existence of a low-lying isoscalar 0-dibaryon resonance embedded in the nuclear medium. Due to its quantum numbers this resonance does not couple to the nucleon-nucleon (NN) channel, where most of the searches have been done, but nearly exclusively to the 7rNN-channel [1,2]. Therefore an ideal reaction to look for resonances in this channel is the pionic double charge exchange (DCX) reaction (Tr+, ~r -) or (Tr-, ~r +) on nuclei [3]. Also, since charge conservation ensures that the reaction takes place on at least two nucleons, the DCX is basically a 2-step process with correspondingly very small cross sections, which, however, are extraordinarily sensitive to small NN distances [4][5][6]. The largest DCX cross sections are expected for transitions to double isobaric analog states (DIAT), since in this case there is maximum overlap between initial and final states. On the other hand ground state transitions (GST) on N = Z nuclei are strongly suppressed [4] in a conventional purely nucleonic mechanism.Experimentally the situation is quite different, in particular for GSTs on nuclei having N = Z in initial or final states. Fig. 1 shows the energy dependence of the forward angle cross sections of the DCX measurements on 12C This work has been funded by the German Federal Minister for Research and Technology (BMFT) under the contract numbers 06 TU 656 and 06 KA 266 and by the DFG (Mu 704/3, Graduiertenkolteg)[7] and 56Fe [8,9]. For the latter reaction, both GST leading to the doubly closed shell nucleus 56Ni and DIAT are presented; the data for 35 and 61 MeV are from this work, for details see ref. [8]. The data on 12C exhibit a broad structure near the energy of the A, and then again a strong increase in cross section towards T,~ = 50 MeV. For the GST on 56Fe the energy dependence is markedly different. The cross section in the A region is lower by an order of magnitude relative to the 12C results, whereas at low energies the cross section exhibits a clear resonancelike energy dependence, reaching at T,~ ~ 50 MeV a cross section a~ large as in case of 12C. The behaviour in the A region is well understood as a consequence of strong pion absorption and successive double A mechanism [10]. The

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