K. Lindberg

Double-pionic fusion of nuclear systems and the "ABC" effect: approaching a puzzle by exclusive and kinematically complete measurements.

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

Exclusive measurement of two-pion production in the dd -g He-4 pi pi reaction

The results from the first kinematically complete measurement of the dd -> He-4 pi pi reaction are reported. The aim was to investigate a long standing puzzle regarding the origin of the peculia ...

Single- and double-pion production in nucleon collisions on the nucleon and on nuclei—the ABC effect and its origin in a dibaryonic resonance

Abstract The ABC effect–an intriguing low-mass enhancement in the π π invariant mass spectrum–is known from inclusive measurements of two-pion production in nuclear fusion reactions. First exclusive measurements carried out at CELSIUS-WASA for the fusion reactions leading to d or 3 He reveal this effect to be a σ -channel phenomenon associated with the formation of a Δ Δ system in the intermediate state and combined with a resonance-like behavior in the total cross-section. Together with the observation that the differential distributions do not change in shape over the resonance region the features fulfill the criteria of an isoscalar s-channel resonance in p n and N N π π systems, if the two emitted nucleons are bound. It obviously is robust enough to survive in nuclei as a dibaryonic resonance configuration. In this context also the phenomenon of N Δ resonances is reexamined.

Measurement of the eta -g pi(+)pi(-)e(+)e(-) decay branching ratio

The reaction at threshold was used to provide a clean source of η mesons for decay studies with the WASA detector at CELSIUS. The branching ratio of the decay η→π+π−e+e− is measured to be (4.3±1.3 ...

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

The tensor polarisation of omega mesons produced in the pd -> He-3 omega reaction has been studied at two energies near threshold. The 3 He nuclei were detected in coincidence with the pi(0)pi(+ ...

Production of the omega meson in the pd -g He-3 omega reaction at 1450 MeV and 1360 MeV

The production of omega mesons in the pd -> He-3 omega reaction has been studied at two energies near the kinematic threshold, T-p = 1450 MeV and T-p = 1360 MeV. The differential cross section w ...

Tagging of η mesons using the pd → 3Heη reaction near threshold

A zero-degree spectrometer for tagging η mesons on the CELSIUS/WASA facility is described, and its characteristics are presented. Tagging of η mesons is performed by detecting 3He ions produced by reaction pd → 3Heη close to the production threshold. The low background level (<2%) is in good agreement with the data obtained earlier on the SATURNE accelerator.

Measurement of the η→π+π−e+e− decay branching ratio

The reaction at threshold was used to provide a clean source of η mesons for decay studies with the WASA detector at CELSIUS. The branching ratio of the decay η→π+π−e+e− is measured to be (4.3±1.3 ...

A Search for Deeply Bound Pionic States of Xenon Produced in the {sup 136}Xe(d, {sup 3}He){sup 135}Xe{sub {pi}}{sub -bound} Reaction

A search for deeply bound pionic states of xenon produced in the 136Xe(d, 3He)135Xeπ-bound reaction at Ed = 500 MeV is reported. The population of the 1s pionic-atom state of 135Xe is observed on the predicted level of the cross section ∼ 40 μb/sr. The binding energy of the 1s state, B = 2.9 ± 0.5 MeV, however, is lower than theoretically predicted.

A search for deeply bound pionic states of xenon produced in the 136Xe(d, 3He)135Xeπ-bound reaction

A search for deeply bound pionic states of xenon produced in the 136Xe(d, 3He)135Xeπ-bound reaction at Ed = 500 MeV is reported. The population of the 1s pionic-atom state of 135Xe is observed on the predicted level of the cross section ∼ 40 μb/sr. The binding energy of the 1s state, B = 2.9 ± 0.5 MeV, however, is lower than theoretically predicted.