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

eversed with a frequency of 50 Hz. An independent
working half wave plate in the laser beam at
the electron source can flip the polarization as well.
In the forward configuration a liquid hydrogen target
with 10 cm length is used, while in the backward
setup the electrons are scattered off a 23 cm
length target. The luminosity is monitored by eight
water Cerenkov counters monitors placed between
4 and 10°.
Special care has been taken to minimize false
asymmetries coming from helicity correlated beam
differences in position, angle, energy or intensity.
Several feedback systems have been installed to
control and minimize such beam fluctuations.
doubt on this simple picture. To investigate if two
photon exchanges are more important than previously
assumed, the measurement of other polarization
observables might help. One such observable
is the transfer beam spin asymmetry A T in the
elastic scattering of transversely polarized electrons
of unpolarized protons, which is directly sensitive
to two photon effects by interference with the
Born amplitude. In parallel with the main experiment,
the collaboration performed two measurements
at momentum transfers of 0.11 (GeV/c) 2 and
0.23 (GeV/c) 2 , while data taking has started on a
third one at 0.60 (GeV/c) 2 .
The scattered electrons are detected in a calorimeter
that consists of 1022 PbF2 crystals to which the
IPN Orsay contributed. The crystals are placed in 7
rings of 146 crystals. For each accepted event the
signals of 3x3 adjacent crystals are summed, digitized
and stored in real time in histograms.
Fig.3: Separation of form factors at Q 2 = 0.23
(GeV/c) 2 . The solid line with its error band represents
the backward angle result. Also shown are
some theoretical predictions.
Published in PRL 102 (2009) 151803.
Fig.2: The detector illuminated by UV light,
used to fight aging of the crystals.
To reduce the background in the backward configuration
from photons originating from the decays
of inelastically produced π 0 's, a plastic scintillator
system is installed between the target chamber
and the detector. Events are stored in different histograms
depending on whether a signal from the
scintillator is detected in coincidence with a signal
in the PbF2 calorimeter or not.
Transverse beam spin asymmetry
The elastic scattering of electrons on protons has
always been approximated as a single photon exchange
process. Higher order processes like twophoton
exchange, are treated as small ‘’radiative’’
corrections. Recent polarization transfer measurements
at Jefferson Laboratory have put serious
Results and outlook
In 2008 and 2009 we took more than 1000 hours
of asymmetry data for the Q 2 = 0.23 (GeV/c) 2 backward
angle measurement with deuterium and proton
target. This completed our measurement program
at that particular momentum transfer and
allowed for the first time the disentanglement of
G s M and G s E using data from the same experiment.
The result
G s E = 0.050 ± 0.060
G s M = -0.14 ± 0.22
is shown in figure 3. The errors include statistics
and systematics, while the third error takes into
account the uncertainty in the electromagnetic
form factors. G s E and G s M are small and compatible
with the assumption of no strangeness. The analysis
of the deuterium data, and the separation of
G s M and the axial form factor G p A is under way.
The upgrade of MAMI increased the energy from
855 MeV to 1508 MeV, which allows data taking at
Q 2 = 0.62 (GeV/c) 2 . This forward angles measurement
corresponds to a backward angle measurement
performed by the experiment G0 at the same
momentum transfer. The technical challenge was
the adjustment and new installation of the beam
stabilization systems capable of managing the
doubling of the energy. It is expected that 700
hours of data taking will be sufficient. After a commissioning
run, a second run has allowed us to
collect already one third of the data.
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