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

Hadron physics in pbar-p p annihilation with the PANDA detector at
FAIR
IPNO Participation: J. Boucher, T. Hennino, R. Kunne, D. Marchand, S. Ong, B. Ramstein, M. Sudol,
E. Tomasi-Gustafsson, J. Van de Wiele.
Collaboration : Panda Collaboration
Le faisceau d’antiprotons de FAIR à Darmstadt, unique pour ses qualités d’énergie et d’intensité, sera un
outil privilégié pour l’étude de la Chromodynamique Quantique (QCD), théorie de l’interaction forte. Le thème
central de notre étude est la détermination des facteurs de forme électromagnétiques du genre temps
du proton qui sont des observables caractérisant la structure interne du nucléon. Le conseil scientifique de
l’IN2P3, en donnant un avis favorable à ce projet d’expérience en juin 2009, a reconnu la pertinence du
sujet et la démonstration argumentée de la faisabilité de cette mesure. Ce soutien ouvre de nouvelles perspectives
pour l’étude d’autres canaux hadroniques à deux corps et électromagnétiques à trois corps dans
l’état final, dans un domaine cinématique totalement inexploré.
Introduction
The physics program of the PANDA experiment at
the upcoming FAIR facility is concentrated on exploring
the hadron physics from the nonperturbative
down to the perturbative regime of
QCD, via the study of hadronic and electromagnetic
final states in pbar-p annihilation. The high luminosity
at the HESR antiproton storage ring of
2x10 32 cm -2 s -1 and the values of the total energy
square up to 30 (GeV/c) 2 provide the optimal
conditions for a sensitive search of glueballs, hybrids
and high precision spectroscopy of charmonium
as well as for the exclusive electromagnetic
processes studies.
The project proposed by the IPNO group is the
determination of the electromagnetic form factors
(FF) of the proton, in the time-like region, through
the annihilation reaction pbar-p into e+e-, over a
large range of the transfer momentum q 2 up to 28
(GeV/c) 2 through the angular distribution measurement
of one of the final leptons.
The precision of this measurement is limited by
two constraints: first, the huge hadronic background,
dominated by the two charged pion channels
has an event rate about 10 6 higher than the
signal. Secondly, a sufficient sensitivity of the angular
distribution to the ratio R IGE/GMI has been
demonstrated and the statistical accuracy on this
quantity has been quantified.
Simulations studies for pbar+p→e + +e -
The differential cross section in the center of mass
reads :
d
d cos
8m
2
2
(
1)
2
2
2
G M
(1 cos ) G E
sin
2
The simulations are performed within the BaBar
framework. The detector description and the different
steps of the simulations are described in the
PANDA Physics Book (arXiv:0903.3905 [hep-ex]).
A realistic projection of the angular distribution of
the lepton, as it will be measured with PANDA is
given in Fig.1, for q 2 = 8.21 (GeV/c) 2 , and assuming
|GE|=|GM|.
Table 1. Cross section σ (σ QCD ) and number of
counts, N (N QCD ) for different values of s and of
the antiproton momentum p.
where θ is the electron production angle and τ=q 2 /
(4m 2 ) and m is the proton mass. The calculated
cross section and the number of counts are given
in Table 1, assuming IGEI=IGMI, with an integrated
luminosity of L=2 fb -1 with 100% efficiency and full
acceptance, using two parametrizations (dipolelike
form and QCD-inspired model) for the form
factors.
Fig. 1 Electron angular distribution as a function of
cos θ : generated events (black circles), reconstructed
events (red squares), acceptance and efficiency
correction (blue stars, right scale), and efficiency
corrected events (green triangles).
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