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

Etude des Distributions de Partons Généralisées
du nucléon à CLAS
IPNO Participation: A. Fradi, M. Guidal, B. Moreno, M. Mac Cormick, S. Niccolai, S. Pisano
Collaboration : SPhN Saclay
The group is studying the Generalized Parton Distributions of the nucleon by measuring a series of exclusive
meson and photon electroproduction processes at the Jefferson Laboratory. We summarize below our
activities and results of these past two years.
Generalized Parton Distributions (GPDs) have
emerged this past decade as a powerful concept
and tool to study nucleon structure. Among many
aspects, the GPDs describe the (correlated) spatial
and momentum distributions of the quarks in the
nucleon, a feature unknown to this day. As a
consequence of these (longitudinal) momentum-
(transverse) space correlations, there is the possibility
to access the contribution of quarks to the
orbital momentum of the nucleon. This is of great
interest for instance for the "spin puzzle" of the
nucleon, a long-standing issue in nucleon structure
studies.
GPDs are the structure functions which are accessed
in hard exclusive leptoproduction of a photon
( ) or a meson (M) on the nucleon (N), such as:
*N->N(M, ). Jefferson Lab, with its 6 GeV continous
electron beam (to be upgraded to 12 GeV in
a couple of years), is a unique facility to begin to
explore these exclusive processes in a systematic
and precise fashion and extract some first insights
on GPDs.
Our group has played a leading role for several
years in this GPD experimental program within the
JLab Hall B collaboration, which uses the large
acceptance CLAS spectrometer. We review in this
short report our activity of these past two years.
Early 2009, we have published the largest set ever
of total and differential cross sections for the exclusive
electroproduction of the
0 meson (see Ref.
[1]). Fig.1 (which made the cover page of the first
EPJA issue of 2009) shows the (longitudinal) differential
cross section d /dt for this process for different
(x B, Q 2 ) bins. The Q 2 variable represents the
"hardness" of the collision. If it is sufficiently large,
the whole *p ->p 0 process can be interpreted in
terms of a collision between the virtual photon *
and a quark inside the nucleon. The x B variable
can then be associated, in a first approximation, to
the momentum fraction of the quark which is hit by
this virtual photon. The t variable, which represents
the momentum transfer between the final and initial
state proton, can be associated to the size of
the target nucleon. Precisely, the slope of
d /dt reflects, under certain conditions, the size of
the target nucleon. Thus, under the reserve that
the experimental data are at sufficiently large Q 2
and that thus a quark-level GPD interpretation is
possible, Fig.1 shows how the size of the nucleon
tends to increase (i.e. the t slope sharpens) as the
probed momentum fraction of the quarks decrease
(i.e. x B decreases). In other words, this suggests
that high-momentum quarks (x B close to 1), i.e. the
valence region) would tend to be located in the
core of the nucleon while low-momentum quarks
(x B close to 0), i.e. the ``sea" region of quarkantiquark
pairs, would extend up to the periphery,
in accordance with a traditional image of a meson
cloud surrounding the nucleon core.
Fig.1: Longitudinal cross section d /dt (in b/GeV 2 )
for the *p ->p 0 reaction for all our (x B, Q 2 ) bins as
a function of t (in GeV 2 ).
The GPDs are universal objects. This means that
the same quantities (weighted by different kinematic
or charge coefficients) can be accessed in other
exclusive processes. It is therefore important to
validate and confirm the preliminary conclusions
that are drawn from the
0 channel by studying
other channels. We have therefore also explored
the exclusive electroproduction of the
+ meson.
Fig.2 shows the (longitudinal) differential cross
section d /dt of the *p ->n + process for different
(x B, Q 2 ) bins. The same x B evolution of the t slope
as in Fig.1 is observed, providing further ground
and constraints for GPD interpretation. This is the
world first ever measurement of this reaction,
which has been the PhD subject of A. Fradi who
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