Exclusive and kinematically complete high-statistics measurements of the basic double-pionic fusion reactions pn→dπ0π0, pn→dπ+π− and pp→dπ+π0 have been carried out simultaneously over the energy ...region of the ABC effect using the WASA detector setup at COSY. Whereas the isoscalar reaction part given by the dπ0π0 channel exhibits the ABC effect, i.e. a low-mass enhancement in the ππ-invariant mass distribution, as well as the associated resonance structure in the total cross section, the isovector part given by the dπ+π0 channel shows a smooth behavior consistent with the conventional t-channel ΔΔ process. The dπ+π− data are very well reproduced by combining the data for isovector and isoscalar contributions, if the kinematical consequences of the isospin violation due to different masses for charged and neutral pions are taken into account.
An exclusive measurement of the decay eta arrow right pi super(+) pi super(-) gamma has been performed at the WASA facility at COSY. The eta mesons were produced in the fusion reaction inline image X ...at a proton beam momentum of 1.7 GeV/c. Efficiency corrected differential distributions have been extracted based on inline image events after background subtraction. The measured pion angular distribution is consistent with a relative p-wave of the two-pion system, whereas the measured photon energy spectrum was found at variance with the simplest gauge invariant matrix element of eta arrow right pi super(+) pi super(-) gamma . A parameterization of the data can be achieved by the additional inclusion of the empirical pion vector form factor multiplied by a first-order polynomial in the squared invariant mass of the pi super(+) pi super(-) system.
The reaction pp arrow right pp pi super(0) pi super(0) has been investigated at a beam energy of 1.4 GeV using the WASA-at-COSY facility. The total cross section is found to be (324+/-systematic ...sub(21)+/-normalization sub(58)) mu b. In order to study the production mechanism, differential kinematic distributions have been evaluated. The differential distributions indicate that both initial state protons are excited into intermediate Delta (1232) resonances, each decaying into a proton and a single pion, thereby producing the pion pair in the final state. No significant contribution of the Roper resonance N super( square )(1440) via its decay into a proton and two pions is found.
Exclusive and kinematically complete measurements of the double pionic fusion to He-3 have been performed in the energy region of the so-called ABC effect, which denotes a pronounced low-mass ...enhancement in the pi pi-invariant mass spectrum. The experiments were carried out with the WASA detector setup at COSY (the cooler synchrotron at Forschungszentrum Julich). Similar to the observations in the basic pn -> d pi(0)pi(0) reaction and in the dd -> He-4 pi(0)pi(0) 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 t-channel Delta Delta mechanism. The deduced d* resonance width can be understood from collision broadening due to Fermi motion of the nucleons in initial and final nuclei.
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