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

Status of the HADES experiment (I):
heavy-ion reactions
IPNO Participation: J. L. Boyard, T. Hennino, T. Liu, E. Morinière, B. Ramstein, M. Sudol.
Collaboration : 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
Les premiers résultats de production inclusive de dileptons en collisions Ar+KCl à 1.76 GeV par nucléon
avec le détecteur HADES sont présentés. Mesuré pour la toute première fois avec une haute résolution, le
spectre de masse invariante e+e- est comparé avec les prédictions des codes de transport UrQMD et HSD.
During the year 2009 the dielectron analysis of the
Ar+KCl run at 1.756 GeV/u has been finalized.
This experiment provided data for a moderately
large collision system, allowing to continue our
studies of dilepton production started with the light
12 C + 12 C system [1], as well as in the elementary
p + p and d + p reactions [2]. The spectroscopy of
the vector-meson region was started with this data
set as well. The reconstructed dielectron invariantmass
distribution shows for the first time at SIS
energies a clear ω peak, providing thus new and
valuable constraints for transport models.
In Figure 1, we contrast our e + e - invariant mass
spectrum with predictions by the UrQMD [3] and
the HSD models [4]. Both predictions were generated
assuming that hadron properties are not
modified by the surrounding nuclear medium, i.e.,
particles have their vacuum properties. The
UrQMD and HSD results were weighted according
to our first level trigger impact parameter selection.
Both predicted spectra were normalized to their
respective
0 multiplicities. Systematic errors are
given by the horizontal bars. They stem from the
correction on reconstruction efficiency and combinatorial
background estimates (20%), the uncertainty
in our knowledge of the multiplicity (11%).
0
Both transport codes suggest that below an invariant
mass 0.15 GeV/c 2 , the spectrum is dominated
by the dielectrons from
0 Dalitz decays. In the intermediate
mass region 0.15-0.5 GeV/c 2 , the pair
yield originates mainly from eta and Delta decays.
Dielectron decays of the light vector mesons, ω
and ρ 0 , have substantial contributions to the spectrum
only at higher invariant masses. As compared
to the UrQMD calculations, HSD includes additional
sources of dielectrons, namely the NN an N
bremsstrahlung processes. These contributions to
the pair yield, are however, for the Ar+KCl system
at given beam energy predicted to be less significant.
In [4], the HSD group published also a prediction
on how the dielectron spectrum would be changed,
if ρ 0 and ω were modified by simultaneous collisional
broadening and mass shifts a la Brown-Rho
[ 5 ]
i n
Figure 1. Invariant mass distribution of e + e - pairs
measured by HADES (points) compared with predictions
(up: UrQMD [3], down: HSD [4]). All particles
have their vacuum properties. Statistical and
systematic errors of the measurement are shown
as vertical and horizontal bars, respectively.
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