38 research outputs found
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
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 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 at 1.03 and 1.35 GeV. The measurements, which
have been carried out at CELSIUS-WASA, reveal the ABC effect to be a
channel phenomenon associated with both a resonance-like
energy dependence in the integral cross section and the formation of a
system in the intermediate state. A corresponding simple
s-channel resonance ansatz provides a surprisingly good description of the
data
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
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
Exclusive measurement of two-pion production in the dd --> 4Hepipi reaction
The results from the first kinematically complete measurement of the dd -->
4Hepipi reaction are reported. The aim was to investigate a long standing
puzzle regarding the origin of the peculiar pipi-invariant mass distributions
appearing in double pion production in light ion collisions, the so-called ABC
effect. The measurements were performed at the incident deuteron energies of
712 MeV and 1029 MeV, with the WASA detector assembly at CELSIUS in Uppsala,
Sweden. We report the observation of a characteristic enhancement at low
pipi-invariant mass at 712 MeV, the lowest energy yet. At the higher energy, in
addition to confirming previous experimental observations, our results reveal a
strong angular dependence of the pions in the overall centre of mass system.
The results are qualitatively reproduced by a theoretical model, according to
which the ABC effect is described as resulting from a kinematical enhancement
in the production of the pion pairs from two parallel and independent NN--> dpi
sub-processes.Comment: 20 pages including 18 figure
Nuclear astrophysics with radioactive ions at FAIR
The nucleosynthesis of elements beyond iron is dominated by neutron captures in the s and r processes. However, 32 stable, proton-rich isotopes cannot be formed during those processes, because they are shielded from the s-process flow and r-process, β-decay chains. These nuclei are attributed to the p and rp process. For all those processes, current research in nuclear astrophysics addresses the need for more precise reaction data involving radioactive isotopes. Depending on the particular reaction, direct or inverse kinematics, forward or time-reversed direction are investigated to determine or at least to constrain the desired reaction cross sections. The Facility for Antiproton and Ion Research (FAIR) will offer unique, unprecedented opportunities to investigate many of the important reactions. The high yield of radioactive isotopes, even far away from the valley of stability, allows the investigation of isotopes involved in processes as exotic as the r or rp processes
Coexisting oblate and strongly triaxial structures in Pt
Excited states in Pt have been populated using the reaction W + B at 85MeV beam energy. Gamma-ray coincidences were measured using the EUROGAM-II spectrometer array. The level scheme has been extended considerably. Evidence for favoured structures of highly triaxial shape with -90° coexisting with oblate structures has been obtained by comparison with total Routhian surface and cranked shell model calculations. The presence of proton excitations in both positive- and negative-parity states has been confirmed