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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

Exclusive and kinematically complete high-statistics measurements of quasifree polarized np scattering have been performed in the energy region of the narrow resonance-like structure d * with 2 I(J P ) = 0(3 + ), M ≈ 2380 MeV and Γ ≈ 70 MeV observed recently in the double-pionic fusion channels pn → dπ 0 π 0 and pn → dπ + π − . The experiment was carried out with the WASA detector setup at COSY having a polarized deuteron beam impinged on the hydrogen pellet target and utilizing the quasifree process dp → np + pspectator. This allowed the np analyzing power, Ay, to be measured over a broad angular range. The obtained Ay angular distributions deviate systematically from the current SAID SP07 NN partial-wave solution. Incorporating the new Ay data into the SAID analysis produces a pole in the 3 D3 − 3 G3 waves in support of the d * resonance hypothesis.

A new method to determine the spin tune is described and tested. In an ideal planar magnetic ring, the spin tune-defined as the number of spin precessions per turn-is given by ν s ¼ γG (γ is the Lorentz factor, G the gyromagnetic anomaly). At 970 MeV=c, the deuteron spins coherently precess at a frequency of ≈120 kHz in the Cooler Synchrotron COSY. The spin tune is deduced from the up-down asymmetry of deuteron-carbon scattering. In a time interval of 2.6 s, the spin tune was determined with a precision of the PRL 115, 094801 (2015) P H Y S I C A L

Exclusive measurements of the quasi-free np → npπ 0 π 0 reaction have been performed by means of dp collisions at T d = 2.27 GeV using the WASA detector setup at COSY. Total and differential cross sections have been obtained covering the energy region √ s = (2.35-2.46) GeV, which includes the region of the ABC effect and its associated d * (2380) resonance. Adding the d * resonance amplitude to that for the conventional processes leads to a reasonable description of the data. The observed resonance effect in the total cross section is in agreement with the predictions of Fäldt and Wilkin as well with those of Albadajedo and Oset. The ABC effect, i.e. the low-mass enhancement in the π 0 π 0 -invariant mass spectrum, is found to be very modest -if present at all, which might pose a problem to some of its interpretations.