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

Exotic low mass narrow baryons from charge exchange
reactions and one pion electroproduction at JLAB
IPNO Participation: B, Tatischeff and E. Tomasi-Gustafsson
Les résultats des mesures des réactions d’échange de charge utilisant des hadrons incidents, sont réanalysés
dans la région du Δ et en dessous. L’objectif de cette étude est de vérifier si les maxima des oscillations
observées dans les spectres de sections efficaces, correspondent aux masses des baryons étroits,
faiblement excités, observés précédemment. Les fonctions de structure des sections efficaces d’ électroproduction
d’ un pion, mesurées à JLAB, sont réanalysées. Les sections efficaces des fonctions σ TT , σ TL et
σ T +ε σ L , sont décrites à l’aide du modèle phènomènologique MAID2007. Les différences entre les spectres
expérimentaux et les résultats du modèle, sont ajustées par des gaussiennes, de largeur constante, ajustées
sur les masses des résonances baryoniques exotiques, déterminées précédemment par d’autres expériences.
On obtient une nette amélioration entre les valeurs expérimentales et les descriptions, après
introduction des structures baryoniques étroites aux masses fixées auparavant.
Narrow baryonic structures were previously extracted
at stable masses, from missing masses and
invariant masses of the following reactions:
p+p→p+π+X, p+p→p+p+X, and α+p→α’+X. These
three reactions, as well as the reactions discussed
now, were studied at the beam lines SPES3 and
principally SPES4 at Saturne.
Charge exchange reactions
A large number of charge-exchange reactions were
investigated with the aim to study isospin-spin
excitations. The corresponding missing mass range
from nucleon up to Δ(1232), therefore are
convenient for the present study. The performed
reanalysis concerns the following reactions [1]:
pp →nΔ++, pd →2pΔ 0 , d 12 C →2pX, p 3 He →tΔ++,
d 3 He →tX, 12 C 3 He →tX, 12 Cp →Δ 0 12 N;
20 Ne p →Δ ++20 F, 12 Ne p,→Δ 0 20 Na, 12 C( 12 C → 12 C)X
12 C( 14 N → 14 C)X; p( 16 O → 16 N)X, 12 C( 16 O → 16 N)X,
27 Al( 20 Ne → 20 Na)X, and 27 Al( 20 Ne → 20 F)X.
These reactions were measured at different angles
and different incident energies. The data are read,
as well as the calculated cross-sections, when given,
which included the quasi-elastic contribution
and the final state interaction (Δ-N interaction and
ΔN→NN process). The result of this calculation is
taken as background. Namely we fit the difference
between data and the calculation with the function
summing the gaussians corresponding to the
narrow structures. In the following figures, the positions
of the narrow structure masses are identified
by greek letters as follows: M=1004 MeV(α), 1044
(β), 1094(γ), 1136(δ), 1173(ε), 1210(λ), 1249(η),
1277(φ), 1339(ν), and 1384 MeV.
The oscillating behaviour in all two figures shown,
one and two, is well fitted by the masses of the
narrow baryonic structures. A large improvement is
obtained in the description of the data. The mean
value of the statistical significance χ 2 of the data,
shown in figure 1, is improved from 21.7 up to 1.9.
These structures were associated with quark clusters.
In order to reproduce the masses of exotic,
narrow, and weakly excited baryons, we use a
phenomenological relation which describe them by
Fig.1 Cross-section of the p(d,2p)Δ 0 reaction, versus Mx at
Td=2 GeV, θ=0.5 0 in insert (a), and θ=4.3 0 in insert (b) [2].
a configuration adding one (or more) q-qbar pairs
to the classical configuration. Two parameters are
adjusted to reproduce the masses, spins and isospins
of the nucleon and of the Roper N * (1440) resonance,
M 0 = 838.2 MeV and M 1 = 100.3 MeV,
allow to reproduce the masses of most of the ten
excited states existing between them.
Figure 2 shows the forward cross-section of 12 C
charge exchange reaction. We observe in all figures,
a very good agreement, between the maxima
of the experimental structures and their masses,
located by vertical arrows.
One pion electro and photoproduction at JLAB
The structure functions of one pion electroproduction
cross-sections, measured at JLAB, are studied
to add information on the possible existence of
narrow exotic baryonic resonances.
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