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We report on an exclusive and kinematically complete high-statistics measurement of the basic doublepionic fusion reaction pn ! d 0 0 over the full energy region of the ABC effect, a pronounced low-mass enhancement in the -invariant mass spectrum. The measurements, which cover also the transition region to the conventional t-channel ÁÁ process, were performed with the upgraded WASA detector setup at COSY. The data reveal the Abashian-Booth-Crowe effect to be uniquely correlated with a Lorentzian energy dependence in the integral cross section. The observables are consistent with a narrow resonance PRL

The COMPASS Collaboration at CERN has measured the transverse spin azimuthal asymmetry of\ud charged hadrons produced in semi-inclusive deep inelastic scattering using a 160 GeV μ+ beam and a\ud transversely polarised NH3 target. The Sivers asymmetry of the proton has been extracted in the Bjorken\ud x range 0.003 < x < 0.7. The new measurements have small statistical and systematic uncertainties of\ud a few percent and confirm with considerably better accuracy the previous COMPASS measurement. The\ud Sivers asymmetry is found to be compatible with zero for negative hadrons and positive for positive\ud hadrons, a clear indication of a spin–orbit coupling of quarks in a transversely polarised proton. As\ud compared to measurements at lower energy, a smaller Sivers asymmetry for positive hadrons is found\ud in the region x > 0.03. The asymmetry is different from zero and positive also in the low x region,\ud where sea-quarks dominate. The kinematic dependence of the asymmetry has also been investigated and\ud results are given for various intervals of hadron and virtual photon fractional energy. In contrast to the\ud case of the Collins asymmetry, the results on the Sivers asymmetry suggest a strong dependence on the\ud four-momentum transfer to the nucleon, in agreement with the most recent calculations

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Measurements of the Collins and Sivers asymmetries for charged pions and charged and neutral kaons produced in semi-inclusive deep-inelastic scattering of high energy muons off transversely polarised protons are presented. The results were obtained using all the available COMPASS proton data, which were taken in the years 2007 and 2010. The Collins asymmetries exhibit in the valence region a non-zero signal for pions and there are hints of non-zero signal also for kaons. The Sivers asymmetries are found to be positive for positive pions and kaons and compatible with zero otherwise

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The COMPASS collaboration has collected the currently largest data set on diffractively produced π − π − π þ final states using a negative pion beam of 190 GeV=c momentum impinging on a stationary proton target. This data set allows for a systematic partial-wave analysis in 100 bins of three-pion mass, 0.5 < m 3π < 2.5 GeV=c 2 , and in 11 bins of the reduced four-momentum transfer squared, 0.1 < t 0 < 1.0 ðGeV=cÞ 2. This two-dimensional analysis offers sensitivity to genuine one-step resonance production, i.e. the production of a state followed by its decay, as well as to more complex dynamical effects in nonresonant 3π production. In this paper, we present detailed studies on selected 3π partial waves with J PC ¼ 0 −þ , 1 þþ , 2 −þ , 2 þþ , and 4 þþ. In these waves, we observe the well-known groundstate mesons as well as a new narrow axial-vector meson a 1 ð1420Þ decaying into f 0 ð980Þπ. In addition, we present the results of a novel method to extract the amplitude of the π − π þ subsystem with I G J PC ¼ 0 þ 0 þþ in various partial waves from the π − π − π þ data. Evidence is found for correlation of the f 0 ð980Þ and f 0 ð1500Þ appearing as intermediate π − π þ isobars in the decay of the known πð1800Þ and π 2 ð1880Þ.