Exclusive and kinematically complete high-statistics measurements of quasifree polarized np scattering have been performed in the energy region of the narrow resonancelike structure d* with I(JP) = ...0(3 super(+)), M approximately 2380 MeV, and Gamma approximately 70 MeV observed recently in the double-pionic fusion channels pn arrow right dpi super(0)pi super(0) and pn arrow right dpi super(+)pi super(-). 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 arrow right np + p sub(spectator). This allowed the np analyzing power, A sub(y) to be measured over a broad angular range. The obtained A sub(y)angular distributions deviate systematically from the current SAID SP07 NN partial-wave solution. Incorporating the new A sub(y)data into the SAID analysis produces a pole in the super(3)D sub(3) - super(3)G sub(3) waves in support of the d* resonance hypothesis.
We report on an exclusive and kinematically complete high-statistics measurement of the basic double-pionic fusion reaction pn→dπ(0)π(0) over the full energy region of the ABC effect, a pronounced ...low-mass enhancement in the ππ-invariant mass spectrum. The measurements, which cover also the transition region to the conventional t-channel ΔΔ 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 with m=2.37 GeV, Γ≈70 MeV and I(J(P))=0(3(+)) in both pn and ΔΔ systems. Necessary further tests of the resonance interpretation are discussed.
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.
Differential cross sections have been extracted from exclusive and kinematically complete high-statistics measurements of quasifree polarized $\vec n$p scattering performed in the energy region of ...the d* (2380) dibaryon resonance covering the range of beam energies Tn = 0.98 - 1.29 GeV (√ s = 2.32 - 2.44 GeV). The experiment was carried out with the WASA-at-COSY setup having a polarized deuteron beam impinged on the hydrogen pellet target and utilizing the quasifree process dp → np + pspectator. In this way the np differential cross section σ (Θ) was measured over a large angular range. The obtained angular distributions complement the corresponding analyzing power Ay (Θ) measurements published previously. A SAID partial-wave analysis incorporating the new data strengthens the finding of a resonance pole in the coupled 3D3 - 3G3 waves.
The WASA-at-COSY experiment has collected 3 x 10(7) events with eta mesons produced via the reaction pd -> He-3 eta at T = 1.0 GeV. Using this data set, we evaluate the branching ratios of the ...decays eta -> pi(+)pi(-)gamma, eta -> e(+)e(-)gamma, eta -> pi(+)pi(-)e(+)e(-), and eta -> e(+)e(-)e(+)e(-). The branching ratios are normalized to the eta -> pi(+)pi(-) pi(0) decay. In addition an upper limit on a CP-violating asymmetry in eta -> pi(+)pi(-)e(+)e(-) is extracted.
Exclusive measurements of the quasi-free pn→ppπ− and pp→ppπ0 reactions have been performed by means of pd collisions at Tp=1.2 GeV using the WASA detector setup at COSY. Total and differential cross ...sections have been obtained covering the energy region Tp=0.95–1.3 GeV (s=2.3–2.46 GeV), which includes the regions of Δ(1232), N⁎(1440) and d⁎(2380) resonance excitations. From these measurements the isoscalar single-pion production has been extracted, for which data existed so far only below Tp=1 GeV. We observe a substantial increase of this cross section around 1 GeV, which can be related to the Roper resonance N⁎(1440), the strength of which shows up isolated from the Δ resonance in the isoscalar (Nπ)I=0 invariant-mass spectrum. No evidence for a decay of the dibaryon resonance d⁎(2380) into the isoscalar (NNπ)I=0 channel is found. An upper limit of 180 μb (90% C.L.) corresponding to a branching ratio of 9% has been deduced.
The differential cross section for the H-1(d, pp)n breakup reaction at deuteron beam energy of 340 MeV has been measured with the use of the WASA detector at COSY-Jillich. The set of proton-proton ...coincidences registered at the Forward Detector has been analyzed on a dense grid of kinematic variables, giving in total around 5600 data points. The cross-section data are compared to theoretical predictions based on the state-ofthe-art nucleon-nucleon potentials, combined with a three-nucleon force or the Coulomb interaction or carried out in a relativistic regime.
New data on both total and differential cross sections of the production of η mesons in proton–deuteron fusion to He3η in the excess energy region 13.6MeV≤Qη≤80.9MeV are presented. These data have ...been obtained with the WASA-at-COSY detector setup located at the Forschungszentrum Jülich, using a proton beam at 15 different beam momenta between pp=1.60GeV/c and pp=1.74GeV/c. While significant structure of the total cross section is observed in the energy region 20MeV≲Qη≲60MeV, a previously reported sharp variation around Qη≈50MeV cannot be confirmed. Angular distributions show the typical forward-peaking that was noted earlier. For the first time, it is possible to study the development of these angular distributions with rising excess energy over a wide interval.
An exclusive measurement of the decay η→π+π−γ has been performed at the WASA facility at COSY. The η mesons were produced in the fusion reaction pd→He3 X at a proton beam momentum of 1.7 GeV/c. ...Efficiency corrected differential distributions have been extracted based on 13960±140 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 η→π+π−γ. 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 π+π− system.
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