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

G0 experiment at Jefferson Lab.
IPNO Participation: L. Bimbot, S.Ong, J. Van de Wiele et J. Arvieux+
Collaboration : http://www.npl.uiuc.edu/exp/G0/admin/g0-phone-book.html
L’expérience G0 est une expérience de mesure de violation de parité dans la diffusion d’électrons polarisés
sur les nucléons. Le but principal est la détermination d’une éventuelle contribution du quark étrange aux
propriétés électrique et magnétique du proton. L’ensemble expérimental a servi pour les mesures aux angles
avants et aux angles arrières. Ainsi ont pu être mesurées les asymétries non seulement pour les électrons
élastiquement diffusés sur le proton mais aussi les quasi élastiques après interaction avec une cible
de deutérium, les électrons diffusés inélastiquement et les pions issus des mêmes interactions. Une grande
moisson de résultats a été analysée qui apporte de nouveaux éléments sur notre connaissance des facteurs
de formes étrange et axiaux des nucléons.
Motivation
Three asymmetry measurements are necessary to
extract G z E, G z M and G e A - from which the strange
form-factors can be deduced [1]. The asymmetry
of the reaction for the two helicity states of the
beam can be expressed as:
A
GFQ
2
GEG
Z
E
Z
GMGM
(
2
( GE)
1
(
4sin
4 2
G
M
2
)
2
w
)
' G
M
G
e
A
NUPPAC [2] and are shown in Fig.1.
where:
Q 2 is the squared fourmomentum
transfer (Q 2 > 0), G F and the usual
weak and electromagnetic couplings, M p the proton
mass and the laboratory electron scattering angle.
G E and G M are the electromagnetic formfactors,
G z E and G z M are the electroweak formfactors
and G e A is the effective axial form-factor of
the proton seen in parity-violating electron scattering.
From the measurements of G z E, G z M and G e A
and the knowledge of the electromagnetic formfactors
of the proton and the neutron, it is possible
to extract the s quark contribution (G s E and G s M ) to
the nucleon structure. This decomposition only
relies on charge symmetry of the nucleon and on
the assumption that only the light quark flavours
contribute to these form factors. Finally the measured
asymmetry can be expressed in terms of
strange form factors:
A = + G s E + G s M + G e A
where is the asymmetry known from neutron and
proton form factors (for the G0 experiment varies
between -1 and -35 x10 -6 ).
Experimental setup
To undertake the experiment a specialized instrumentation
apparatus has been set up in Hall C of
Jefferson Laboratory, Newport News, Virginia,
USA. Both forward and backward angle configurations
were described in detail in contributions to
Figure 1 Experimental setup
Data taking
The total charge of incident beam on different targets
is given in Table 1, for the different energies.
A long serie of additionnal measurements have
also been recorded for detailed studies of accuracy,
false asymmetries, backgrounds and dilutions
factors.
362 MeV 687 MeV 3.30 GeV
Hydrogen ~90C ~100C ~101C
Deuterium ~70C ~45C
Table 1 Total charge on target
Analysis and results
The analysis procedure was similar for both configurations;
blinding factors were introduced to prevent
distortion in the analysis from any expectation.
After rejecting all quartets including bad
events, the raw asymmetries were calculated from
which we could verify the data quality. Corrections
coming from beam quality and instrumental effects
were applied, followed by all the corrections coming
from background contamination. Then the results
were unblinded to allow combination with
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