157 research outputs found
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
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
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
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
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
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
The pp -> pp pi pi pi reaction channels in the threshold region
The cross section for prompt neutral and charged three pion production in pp
interactions was measured at excess energies in the range 160 - 217 MeV. That
comprises the first measurement of the pp->pp pi0pi0pi0 reaction and the
comparison with the pp->pp pi+pi-pi0 reaction, in a very direct way. The
experiment was performed above the eta meson production threshold and the cross
section normalization was obtained from a concurrent measurement of the
reaction pp->pp eta with the eta decaying into 3 pions. Since the same final
states are selected, the measurement has a low systematical error. The measured
cross section ratio sigma(pp->pp pi+pi-pi0)/sigma(pp->pp pi0\pi0\pi0) is
compared to predictions of dominance of different isobars in the intermediate
state.Comment: 12 pages, 4 figures New discussion on the pp->pp3pi reaction
mechanis
Exclusive Measurements of : the Effect Revisited
Exclusive measurements of the reactions and
have been carried out at GeV at the
CELSIUS storage ring using the WASA detector. The channel
evidences a pronounced enhancement at low invariant masses - as
anticipated from previous inclusive measurements of the ABC effect. This
enhancement is seen to be even much larger in the isoscalar
channel. The differential distributions prove this enhancement to be of
scalar-isoscalar nature. calculations give a good description of
the data, if a boundstate condition is imposed for the intermediate
system.Comment: extended version, 8 pages, 7 figures, theoretical model calculations
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