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

Fig. 2 Cross-section of the p(12C,12N) reaction at T p =1100
MeV/N, θ=0 0 [3].
The σ TT , σ TL and σ T +ε.σ L functions are described with
the help of the phenomenological model MAID2-
007 [4]. The differences between data and model
are fitted by single width gaussians, at the masses
of these resonances, previously determined on the
basis of different experiments. No attempt is done
to adjust any mass. Such procedure assumes no
interference between background and resonances.
The γp→π 0 p reaction
The backward cross-sections of the structure functions
of the γp→π 0 p reaction, were measured by
the Hall A collaboration at four angles θ and at
Q 2 =1 (GeV/c) 2 [5]. The angular variations of all
three structure functions display continuous shapes
for nearly all seven narrow structure masses,
namely: M=1136 MeV, 1210 MeV, 1277 MeV,
1339 MeV, 1384 MeV, 1480 MeV and 1540 MeV.
Only one distribution, corresponding to ζ TL function
for M=1136 MeV, displays a not smooth shape.
The improvement between data and final fits is
noteworthy; indeed before the introduction of the
narrow structure contributions, the experimental
data have to be compared to the baseline. The
mean χ 2 is reduced from 24 to 3.1.
The γp→π + n reaction
The γp→π + n reaction was measured at JLAB in
Hall B, by the CLAS collaboration [6]. The crosssection
of the three structure functions were extracted
at four values of the momentum transfers:
Q 2 =0.3, 0.4, 0.5, and 0.6 (GeV/c) 2 and ten angles
from θ=7.5 0 up to 142.5 0 . Figure 3 shows a reduced
sample of structure function data, after substraction
of the corresponding MAID2007 calculated
results[4]. No attempt to get a better adjustment
by shifting the masses is done on the fits. A
large improvement between data and theory is
observed. Quantitatively the mean χ 2 , obtained
on a large number of structure functions, is reduced
from 55 until 9.3. The ζ TT structure functions of
the γp→π + n reaction, contribute to a large part of
the uncertainty on the previous χ 2 value. The extracted
structure surfaces, exhibit rather smooth
angular variations.
Although the two discussed experiments, namely
one pion photoproduction reaction on proton, were
performed with another aim, therefore with a resolution
not appropriate for the present study, we
have shown that their analysis is essentially
Figure 3. The reaction γp→π + n [6]. σ T +ε.σ L structure function,
at Q 2 =0.3 (GeV/c) 2 for θ=52.5 0 insert (a), and
θ=127.5 0 insert (b), σ TT structure function at Q 2 =0.3
(GeV /c) 2 and θ=97.5 0 , insert (c), σ TL structure function at
Q 2 =0.4 (GeV/c) 2 and θ=67.5 0 insert (d) .The red lines are
the final fit, the gaussians are shown as black lines.
improved after introduction of narrow low mass
baryonic structures. Indeed the description with
MAID is sometimes more qualitative than quantitative.
The difference between data and results from
MAID calculations, exhibit narrow peaks. The entire
range discussed here, can be described by
structures at the masses extracted from previous
experiments performed with hadrons and previous
analysis.
In conclusion, this work increases the experimental
signatures for the excitation of low mass baryons,
previously observed in different reactions. A large
improvement is obtained in the description of the
data by a fit which takes narrow structures into account
by gaussians with pre-determined masses
and with widths given by the experimental resolution.
References:
[1] B.Tatischeff and E. Tomasi-Gustafsson,
arXiv:1002.2059v1[nucl-ex].
[2]C. Ellegaarde et al., Phys. Rev. Lett. 59, 974
(1987)]
[3] M. Roy-Stephan et al. Nucl. Phys. A488, 187c
(1988).
[4] D. Drechsel, S.S. Kamalov, L. Tiator
Unitary Isobar Model - MAID2007, Eur. Phys. J.
A34 (2007) 69.
[5] G. Laveissière et al., Phys. Rev. C69
(2004)
045203.
[6] H. Egiyan et al. Phys. Rev. C73
(2006) 025204
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