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

Status of the HADES experiments (II):
dilepton channels in elementary reactions
IPNO Participation: J.L. Boyard, T. Hennino, T. Liu, E. Morinière, B. Ramstein, M. Sudol.
Collaboration : CAS-Rez, FZR-Dresden, GSI-Darmstadt, INR-Moscow, INFN/LNSCatania,
INFN/University-Milano, ITEP-Moscow, SIP/Jag. Univ-Cracow, JW Goethe Univ.-Frankfurt, JINR-
Dubna, J Liebig Univ.-Giessen, LIP-Coimbra, TUM-München, Univ., Santiago de Compostela, Univ.
of Cyprus-Nicosia, NPI-Rez, Univ. Valencia
La production de dielectrons en réactions pp et dp à 1.25 GeV/nucléon a été étudiée avec HADES (High
Acceptance DiElectron Spectrometer) à GSI. Les résultats obtenus en canaux inclusifs et exclusifs sont
présentés et comparés à des modèles. Pour la première fois, le processus de décroissance Dalitz du (
Ne + e - ) a pu être identifié. Le taux d’événements mesurés est en accord avec le rapport de branchement
calculé en QED (4.2 10 -5 ).
Motivation
The first results from the HADES collaboration obtained
in C+C and Ar+KCl systems [1] have
pointed to the important role played by the nucleon
-nucleon bremsstrahlung and Daliz decay processes
in the dilepton production at intermediate
invariant masses ( 0.15 < M ee < 0.5 GeV/c 2 ), The
contribution of these dilepton sources has therefore
to be clarified, as a prerequisite to the study of
and in-medium properties.
In particular, the new results from the HSD transport
model [2] use the recent bremsstrahlung calculation
from [3], which is a factor 2-4 higher than
other predictions [4]. Meanwhile, other transport
models reproduce quite well the data with different
relative contributions of bremstrahlung and Dalitz
decay processes.
Inclusive dilepton spectra:
Following these motivations, inclusive e + e - spectra
(Fig.1) were measured in the p+p and quasi-free
n+p reactions studied with HADES in experiments
with the GSI proton and deuterium beams at 1.25
GeV [5], i.e. below the production threshold.
Two different approaches for the analysis of these
spectra were followed in our event generator
PLUTO [6]. The first one is based on the dominance
of the resonance. In analogy with the
transport model calculations, it provides a description
of the following channels:
N N e e - ( 0 Dalitz decay)
Ne e - ( Dalitz decay)
The details of the treatment of the production
and pionic decay could be checked quite precisely,
using the new measurements by HADES in the
hadronic channels [7]. Two options for the N-
transition form factors are used: either a constant
magnetic form factor (G M =3, G E =0, G C =0) in agreement
with the photon-point measurements, or the
two-component quark-model [8], which is mainly
based on the Vector Meson Dominance model in
our energy range. As shown on fig.1, there is a
good agreement, for both reactions, between the
dilepton yield measured at low invariant masses
and the simulation of the
0 Dalitz decay. In the
case of the pp reaction, the region of invariant
masses larger than 140 MeV/c 2 is well described
by the simulation of the Dalitz decay. The agreement
is even better when the two-component
Fig. 1: Invariant mass distribution of electron pairs
measured in p+p (top) and quasi-free n+p (bottom)
reactions at a beam energy of 1.25 GeV. The dashed
and long-dashed lines show the contributions
of
0 and Dalitz decays, respectively, in simulations
using the resonance model. The enhancement
due to the N- transition form factor [8] is
shown as the grey area. The dashed and full lines
are the results of OBE models [3] and [4], respectively.
The contribution has been added in the case
of the n+p reaction.
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