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

defended in July 2009 (adviser M. Guidal). The
article is in preparation.
Fig.2: Longitudinal cross section d /dt (in b/GeV 2 )
for the *p ->n + reaction for all our (x B, Q 2 ) bins as
a function of t (in GeV 2 ).
Besides meson channels, we also have a strong
implication in the DVCS ( *p ->p program. DVCS
has a different sensitivity to GPDs than meson production
channels. In particular, its theoretical interpretation
is expected to suffer less corrections and
uncertainties which should provide a more direct
access to GPDs. We are still in the process of finalizing
the extraction of the DVCS cross section
from the data that we took a few years ago. However,
beam spin asymmetries (easier to extract as
no normalization is required) were published in
2008 (see Fig.3 and Ref. [2]). We recall that the
IPN detector and electronics groups built a dedicated
electromagnetic calorimeter based on PbWO 4
crystals in order to detect the final state photon of
See Ref.[2] for description of variables, curves and
other data points.
In this experiment, we also analyzed a sideproduct,
the DVCS process on the nucleon resonance:
*p -> with the decaying into a pion and
a nucleon. A first-ever beam spin asymmetry signal
for this channel has been observed. This promises
to give access to the space-momentum correlations
of quarks in the excited states of the nucleon
as well. This has been the PhD subject of B.
Moreno who defended in July 2009 (adviser M.
Mac Cormick).
Besides data analysis, we have taken data in 2009
during about three months for a new DVCS experiment
which used a longitudinally polarized target.
S. Niccolai is one of the spokespersons of this experiment.
Target spin asymmetries allow to disentangle
various GPDs and are a necessary complement
to beam spin asymmetries. Data are now
under analysis by S. Pisano with first preliminary
results expected by the end of 2010.
Finally, the group is also carrying phenomenological
studies of GPDs by modeling them and/or fitting
the experimental data in order to extract them.
In particular, these past two years, a fitting code
has been developed aimed at fitting the DVCS
world data. This work has been the subject of three
articles and has led to the first-ever modelindependent
extraction of several GPDs at particular
kinematics (see Refs [3]).
Concerning perspectives, the group is, in parallel
to these current activities, preparing the mid-term
future in relation with 12 GeV energy upgrade of
JLab. We have submitted a letter of intent in 2009
for an original study of the DVCS on the neutron,
which is a barely explored process. The aim is, on
the one hand, to carry out a flavor (u and d quarks)
separation of the GPDs by combining the neutron
and proton DVCS data and, on the other hand, to
have access to the particular E GPD, which is very
sensitive to the orbital momentum contribution of
the quarks to the nucleon spin. There is an associated
R&D project for building a neutron detector,
which is the subject of another contribution in this
yearbook.
References
[1] S. Morrow et al., Eur.Phys.J. A 39, 5 (2009)
[2] F.-X. Girod et al., Phys.Rev.Lett. 100, 162002
(2008).
[3] M. Guidal, Eur.Phys.J. A 37, 319 (2008); M.
Guidal and H. Moutarde, Eur.Phys.J. A 42, 71
(2009); M. Guidal, arXiv:1003.0307 [hep-ph].
the process.
Fig.3: Solid circles: DVCS beam spin asymmetry at
=90 o as a function of t for different (x B, Q 2 ) bins.
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