Anisotropy in the pion angular distribution of the reaction pp -> pp pi0 at 400 MeV

The reaction pp -> pp pi0 was studied with the WASA detector at the CELSIUS storage ring. The center of mass angular distribution of the pi0 was obtained by detection of the gamma decay products together with the two outgoing protons, and found to be anisotropic with a negative second derivative slope, in agreement with the theoretical predictions from a microscopic calculation.Comment: Revtex 4 style, 5 pages 7 figures, PACS numbers:13.60.Le, 13.75.Cs, 21.45.+v, 25.10.+

Study of the p p -> p p pi+ pi- Reaction in the Low-Energy Tail of the Roper Resonance

Exclusive measurements of the p p -> p p pi+ pi- reaction have been carried out at Tp = 775 MeV at CELSIUS using the PROMICE/WASA setup. Together with data obtained at lower energy they point to a dominance of the Roper excitation in this process. From the observed interference of its decay routes N* -> N sigma and N* -> Delta pi -> N sigma their energy-dependent relative branching ratio is determined

Double-Pionic Fusion of Nuclear Systems and the ABCEffect -- Aproaching a Puzzle by Exclusive and Kinematically Complete Measurements

The ABC effect - a puzzling low-mass enhancement in the $\pi\pi$ invariant mass spectrum - is well-known from inclusive measurements of two-pion production in nuclear fusion reactions. Here we report on first exclusive and kinematically complete measurements of the most basic double pionic fusion reaction $pn \to d \pi^0\pi^0$ at 1.03 and 1.35 GeV. The measurements, which have been carried out at CELSIUS-WASA, reveal the ABC effect to be a $(\pi\pi)_{I=L=0}$ channel phenomenon associated with both a resonance-like energy dependence in the integral cross section and the formation of a $\Delta\Delta$ system in the intermediate state. A corresponding simple s-channel resonance ansatz provides a surprisingly good description of the data

Measurement of the Slope Parameter for the eta->3pi0 Decay in the pp->pp eta Reaction

The CELSIUS/WASA setup is used to measure the 3pi0 decay of eta mesons produced in pp interactions with beam kinetic energies of 1.36 and 1.45 GeV. The efficiency-corrected Dalitz plot and density distributions for this decay are shown, together with a fit of the quadratic slope parameter alpha yielding alpha = -0.026 +/- 0.010(stat) +/- 0.010(syst). This value is compared to recent experimental results and theoretical predictions.Comment: 4 pages, 7 Postscript figures, uses revtex4.st

Measurement of the eta->pi+pi-e+e- decay branching ratio

The reaction pd->3He eta at threshold was used to provide a clean source of eta mesons for decay studies with the WASA detector at CELSIUS. The branching ratio of the decay eta->pi+pi-e+e- is measured to be (4.3+/-1.3+/-0.4)x10^-4.Comment: 10 pages,6 figures revised versio

Polarisation of the omega meson in the pd-->3He+omega reaction at 1360 and 1450 MeV

The tensor polarisation of omega mesons produced in the pd-->3He+omega reaction has been studied at two energies near threshold. The 3He nuclei were detected in coincidence with the pi0pi+pi- or pi0gamma decay products of the omega. In contrast to the case of phi meson production, the omega mesons are found to be unpolarised. This brings into question the applicability of the Okubo-Zweig-Iizuka rule when comparing the production of vector mesons in low energy hadronic reactions.Comment: 11 pages, 4 figure

The pd --> ^3He eta pi0 reaction at T_p = 1450 MeV

The cross section for the pd --> ^3He eta pi0 reaction has been measured at a beam energy of 1450 MeV using the WASA detector at the CELSIUS storage ring and detecting one ^3He and four photons from the decays of the two photons. The data indicate that the production mechanism involves the formation of the Delta(1232) isobar. Although the beam energy does not allow the full peak of this resonance to be seen, the invariant masses of all three pairs of final state particles are well reproduced by a phase space Monte Carlo simulation weighted with the p-wave factor of the square of the pi^0 momentum in the ^3Hepi^0 system.Comment: 10 pages, 5 figure

Search for Narrow NNpi Resonances in Exclusive p p -> p p pi+ pi- Measurements

Narrow structures in the range of a few MeV have been searched for in p p pi+ and p p pi- invariant mass spectra obtained from exclusive measurements of the p p -> p p pi+ pi- reaction at Tp = 725, 750 and 775 MeV using the PROMICE/WASA detector at CELSIUS. The selected reaction is particularily well suited for the search for NN and / or N Delta decoupled dibaryon resonances. Except for a possible fluctuation at 2087 MeV/c^2 in Mpppi- no narrow structures could be identified neither in Mpppi+ nor in Mpppi- on the 3 sigma level of statistical significance, giving an upper limit (95% C.L.) for dibaryon production in this reaction of sigma < 20 nb for 2020 MeV/c^2 < m(dibaryon) < 2085 MeV/c^2Comment: 3 pages, 4 figure

Exclusive Measurements of pd→3Heππpd\to ^3He \pi\pi : the ABCABC Effect Revisited

Exclusive measurements of the reactions $pd\to$ $^3He \pi^+ \pi^-$ and $pd\to$ $^3He \pi^0\pi^0$ have been carried out at $T_p=0.895$ GeV at the CELSIUS storage ring using the WASA detector. The $\pi^+\pi^-$ channel evidences a pronounced enhancement at low invariant $\pi\pi$ masses - as anticipated from previous inclusive measurements of the ABC effect. This enhancement is seen to be even much larger in the isoscalar $\pi^0\pi^0$ channel. The differential distributions prove this enhancement to be of scalar-isoscalar nature. $\Delta\Delta$ calculations give a good description of the data, if a boundstate condition is imposed for the intermediate $\Delta\Delta$ system.Comment: extended version, 8 pages, 7 figures, theoretical model calculations adde

Production of eta and 3pi mesons in the pd->3HeX reaction at 1360 and 1450 MeV

The cross sections of the pd -> 3He eta, pd -> 3He pi0 pi0 pi0 and pd -> 3He pi+ pi- pi0 reactions have been measured at beam kinetic energies T_p= 1360 MeV and T_p= 1450 MeV using the CELSIUS/WASA detector setup. At both energies, the differential cross section dsigma/dOmega of the eta meson in the pd -> 3He eta reaction shows a strong forward-backward asymmetry in the CMS. The ratio between the pd -> 3He pi+ pi- pi0 and the pd -> 3He pi0 pi0 pi0 cross sections has been analysed in terms of isospin amplitudes. The reconstructed invariant mass distributions of the pi-pi, 3He-pi and 3He-2pi systems provide hints on the role of nucleon resonances in the 3pi production process.Comment: Shorter version accepted to EPJA 10 pages 14 figure