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New data on quasifree polarized neutron-proton scattering, in the region of the recently observed d * resonance structure, have been obtained by exclusive and kinematically complete high-statistics measurements with WASA at COSY. This paper details the determination of the beam polarization, 2 checks of the quasifree character of the scattering process, on all obtained Ay angular distributions and on the new partial-wave analysis, which includes the new data producing a resonance pole in the 3 D3-3 G3 coupled partial waves at (2380±10−i40±5) MeV -in accordance with the d * dibaryon resonance hypothesis. The effect of the new partial-wave solution on the description of total and differential cross section data as well as specific combinations of spin-correlation and spin-transfer observables available from COSY-ANKE measurements at T d = 2.27 GeV is discussed.

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The presently world largest data sample for π 0 → γe + e − decays studies containing nearly 5×10 5 events was collected using the WASA detector at COSY. A search for a dark photon U produced in the π 0 → γU → γe + e − decay from the pp → ppπ 0 reaction was carried out. An upper limit on the square of the U − γ mixing strength parameter ǫ 2 of 5 × 10 −6 at 90% CL was obtained for the mass range 20 MeV < M U < 100 MeV. This result together with other recent experimental limits significantly reduces the M U vs. ǫ 2 parameter space which could explain the presently seen deviation between the Standard Model prediction and the direct measurement of the anomalous magnetic moment of the muon.

The double-polarization observable E and the helicity-dependent cross sections σ 1/2 and σ 3/2 have been measured for the first time for single π 0 photoproduction from protons and neutrons bound in the deuteron at the electron accelerator facility MAMI in Mainz, Germany. The experiment used a circularly polarized photon beam and a longitudinally polarized deuterated butanol target. The reaction products, recoil nucleons and decay photons from the π 0 meson were detected with the Crystal Ball and TAPS electromagnetic calorimeters. Effects from nuclear Fermi motion were removed by a kinematic reconstruction of the π 0 N final state. A comparison to data measured with a free proton target showed that the absolute scale of the cross sections is significantly modified by nuclear final-state interaction (FSI) effects. However, there is no significant effect on the asymmetry E since the σ 1/2 and σ 3/2 components appear to be influenced in a similar way. Thus, the best approximation of the two helicity-dependent cross sections for the free neutron is obtained by combining the asymmetry E measured with quasi-free neutrons and the unpolarized cross section corrected for FSI effects under the assumption that the FSI effects are similar for neutrons and protons.

Exclusive and kinematically complete measurements of the double pionic fusion to 3He have been performed in the energy region of the so-called ABC effect, which denotes a pronounced low-mass enhancement in the 7T7r-invariant mass spectrum. The experiments were carried out with the WASA detector setup at COSY (the cooler synchrotron at Forschungszentrum Jiilich). Similar to the observations in the basic pn -* d n 07t° reaction and in the dd 4 He n ajr0 reaction, the data reveal a correlation between the ABC effect and a resonance-like energy dependence in the total cross section. Differential cross sections are well described by the hypothesis of d* resonance formation during the reaction process in addition to the conventional /-channel AA mechanism. The deduced d* resonance width can be understood from collision broadening due to Fermi motion of the nucleons in initial and final nuclei.

The largest, at the moment, statistics of 7 × 10 6 η → 3π 0 decays, based on 6.2 × 10 7 η mesons produced in the γp → ηp reaction, has been accumulated by the A2 Collaboration at the Mainz Microtron, MAMI. It allowed a detailed study of the η → 3π 0 dynamics beyond its conventional parametrization with just the quadratic slope parameter α and enabled, for the first time, a measurement of the second-order term and a better understanding of the cusp structure in the neutral decay. The present data are also compared to recent theoretical calculations that predict a nonlinear dependence along the quadratic distance from the Dalitz-plot center. =-0.0318+/-0.0015) α PDG ( FIG. 2: Comparison of the experimental data [9][10][11][24][25][26][27][28][29][30] (plotted by black points), which were used in the RPP [23] to obtain the averaged value (shown by the vertical lines) for the η → 3π 0 quadratic slope parameter α, to each other and to various calculations [2-4, 16, 17, 19, 21, 22, 31] (colored points). Ref. [2] gives the magnitude of α for the analysis made in Ref.[1], in which its value was not given.

The Compton double-polarization observable 2z has been measured for the first time in the (1232) resonance region using a circularly polarized photon beam incident on a longitudinally polarized proton target at the Mainz Microtron. This paper reports these results, together with the model-dependent extraction of four proton spin polarizabilities from fits to additional asymmetry data using either a dispersion relation calculation or a baryon chiral perturbation theory calculations with the weighted average of these two fits resulting in γ E 1E 1 = −2.87 ± 0.52, γ M1M1 = 2.70 ± 0.43, γ E 1M2 = −0.85 ± 0.72, and γ M1E 2 = 2.04 ± 0.43, in units of 10 −4 fm 4 .