exotic nuclei structure and reaction noyaux exotiques ... - IPN - IN2P3

Latest Results from GRAAL
IPNO Participation: J.-P. Didelez, M. Guidal, E. Hourany, R. Kunne
Collaboration : GRAAL
Depuis l’arrêt de l’expérience GRAAL à l’ESRF de Grenoble en 2008, plusieurs publications sont apparues.
Elles résultent de l’analyse des états finals complexes menée à bien par les différentes équipes de la Collaboration
GRAAL. Il s’agit de la photo-production simultanée de π° et de η, la mesure des observables de
polarisation correspondant au canal KΛ, la photo-production de π° et de η sur le neutron et enfin,
l’observation des effets du milieu nucléaire affectant la photo-production de mésons sur les noyaux.
Simultaneous photoproduction of π° and
ηmesons on the proton
The two baryonic resonances S11(1535) and Δ
(1700) are expected to be dynamically generated
in this channel, as quasi-bound states of a meson
and a baryon. It is remarkable that there is almost
no excitation of the S11(1535), but exclusive
excitation of the Δ(1700), decaying to η Δ(1232)
and then to η π° p. This decay mode for the Δ
(1700) was not known before. Therefore, the Δ
(1700) could be considered as a Δ(1232) + η in the
same way as the S11(1535) can be described as a
N + η system (Ref. 1).
Polarization observables for the γp → KΛ
reaction
The KΛ channel has been presented since the
beginning of the GRAAL venture, as the most
promising one, because it allows to measure
double polarization observables, the Λ polarization
in the final state being accessible through its decay
products. The double polarization (beam-recoil)
observables Ox and Oz have been measured
from threshold to 1500 MeV. Values for the target
asymmetry T could also be extracted, despite the
use of an unpolarized target. Those new data
confirm the complexity of this channel, by requiring
inclusion of poorly known resonances in theoretical
models (Ref. 2).
Photoproductionon of π° or η meson on the
neutron
The beam asymmetry Σ for π° has been measured
from quasi-free nucleons in a deuterium target.
Results are very similar to those obtained with a
free proton target below 0.82 GeV and significantly
different at higher energies (Ref. 3). The values
are fairly well reproduced by recent partial wave
analysis.
Σ beam asymmetries and cross sections for η have
been deduced from the photoproduction on quasifree
nucleons in a deuterium target. The analysis
has been completed by our late colleague E. Hourany
using an original method similar to the one
used for the double π° photoproduction on the
neutron (Ref 4). The GRAAL detector allows com
Figure 1 : In part (a), various data on the total
cross section for η photoproduction on the proton.
In (b), the ratio of the cross section on the neutron
to the one on the proton is shown: by solid squares
(this work), thick line from the MAID code and thin
line from Zhao model. In (c), the total cross section
on the free proton deduced from (a): empty circles
and the one scaled for the free neutron: dots.
Predictions from MAID and Zhao: thick and thin
lines respectively (continuous for the neutron and
dotted for the proton).
plete kinematical reconstruction of the final state
and therefore, determination of the momentum of
the so called « spectator nucleon ». This in turn
permits the selection of quasi-free events and corrections
for any underlying background.
The analysis compares quasi-free nucleon data
(neutron and proton) to previous free proton data
to access « free » neutron observables.
As shown on Fig. 1, the total cross section on the
neutron is found to be higher than the proton one
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