86 research outputs found
Dilepton anisotropy from and collisions at BEVALAC energies
A full calculation of lepton-pair angular characteristics is carried out for
pairs created in and collisions from 1.0 to 2.1
GeV/A. It is demonstrated that the dilepton decay anisotropy depends
sensitively on the different sources and may be used for their disentangling.
Due to the dominance of the -and -Dalitz decays and only a small
anisotropy coefficient for annihilation, the expected anisotropy
coefficients show a decrease with invariant mass of the dilepton pair and
change only moderately when comparing and reactions at the
same bombarding energy.Comment: 10 pages, LaTeX, plus 4 postscript figures tarred, compressed and
uuencoded in separate fil
Baryonic contributions to the dilepton spectrum of nucleon-nucleon collisions
We study the production of dileptons in relativistic nucleon-nucleon
collisions. Additionally to the traditional dilepton production channels
(vector meson decays, meson and Delta(1232) Dalitz decays) we included in our
model as new dilepton sources the Dalitz decay of higher unflavored baryon
resonances with spin<=5/2 and mass<=2.25 GeV/c^2. The contributions of these
new channels are estimated using experimental information about the Ngamma
decays of the resonances and have large uncertainties. The obtained dilepton
spectra are compared to the experimental data by the DLS collaboration.
Predictions for the HADES detector (SIS, GSI) are also discussed. In spite of
the large uncertainties of the higher resonance Dalitz decay contributions we
are able to draw the conclusion that these contributions are negligible
compared to the other dilepton sources and do not influence the detectability
of the phi and omega vector meson peaks.Comment: 9 pages, 4 figures, version accepted for publication in Phys. Rev.
Low-mass dileptons and dropping rho meson mass
Using the transport model, we have studied dilepton production from heavy-ion
collisions at Bevalac energies. It is found that the enhanced production of
low-mass dileptons observed in the experiment by the DLS collaboration cannot
be explained by the dropping of hadron masses, in particular the -meson
mass, in dense matter.Comment: 9 pages, LaTeX, including 1 postscript figure, to appear in Phys.
Lett.
Dileptons from the strongly-interacting Quark-Gluon Plasma within the Parton-Hadron-String-Dynamics (PHSD) approach
Dilepton production in In+In collisions at 158 AGeV is studied within the
microscopic Parton-Hadron-Strings Dynamics (PHSD) transport approach, which is
based on a dynamical quasiparticle model (DQPM) matched to reproduce lattice
QCD results in thermodynamic equilibrium. A comparison to the data of the NA60
Collaboration shows that the low mass dilepton spectra are well described by
including a collisional broadening of vector mesons, while the spectra in the
intermediate mass range are dominated by off-shell quark-antiquark annihilation
in the nonperturbative QGP. In particular, the observed softening of the mT
spectra at intermediate masses is reproduced.Comment: talk given at the 21st International Conference on Ultra-Relativistic
Nucleus-Nucleus Collisions (Quark Matter 2009), to be published in
Nucl.Phys.A, 4 pages, 3 figures, elsarticle styl
Dynamics of strange, charm and high momentum hadrons in relativistic nucleus-nucleus collisions
We investigate hadron production and attenuation of hadrons with strange and
charm quarks (or antiquarks) as well as high transverse momentum hadrons in
relativistic nucleus-nucleus collisions from 2 AGeV to 21.3 ATeV within two
independent transport approaches (UrQMD and HSD). Both transport models are
based on quark, diquark, string and hadronic degrees of freedom, but do not
include any explicit phase transition to a quark-gluon plasma. From our
dynamical calculations we find that both models do not describe the maximum in
the ratio at 20 - 30 AGeV in central Au+Au collisions found
experimentally, though the excitation functions of strange mesons are
reproduced well in HSD and UrQMD. Furthermore, the transport calculations show
that the charmonium recreation by reactions is
comparable to the dissociation by 'comoving' mesons at RHIC energies contrary
to SPS energies. This leads to the final result that the total
suppression as a function of centrality at RHIC should be less than the
suppression seen at SPS energies where the 'comover' dissociation is
substantial and the backward channels play no role. Furthermore, our transport
calculations -- in comparison to experimental data on transverse momentum
spectra from , d+Au and Au+Au reactions -- show that pre-hadronic effects
are responsible for both the hardening of the hadron spectra for low transverse
momenta (Cronin effect) as well as the suppression of high hadrons. The
mutual interactions of formed hadrons are found to be negligible in central
Au+Au collisions at = 200 GeV for 6 GeV/c and the
sizeable suppression seen experimentally is attributed to a large extent to the
interactions of 'leading' pre-hadrons with the dense environment.Comment: 10 pages, incl. 7 postscript figures, invited talk presented at the
25th Course "Heavy Ion Reactions from Nuclear to Quark Matter", Erice, Sept.
16-24, 2003, to be publ. in Prog. Part. Nucl. Phy
Perspectives of production in pp, pd and p Be reactions at SIS energies
We study dilepton production from pp, pd and p Be collisions from 1 - 5 GeV
including the , , and Dalitz decays, direct
decays of vector mesons (, ) as well as subthreshold
production via baryonic resonances (e.g. ). Our
calculations compare rather well with the pp and pd data from the DLS
Collaboration, however, overestimate slightly the 'old' p Be data from that
group. Futhermore, detailed predictions for differential dilepton spectra at
SIS energies are made with a high mass resolution that can be controlled
experimentally by the HADES Collaboration in near future.Comment: 27 pages, LaTeX, including 13 postscript figures, to be published in
Nucl. Phys.
production in reactions at SIS energies
Detailed predictions for dilepton production from reactions at SIS
energies are presented within a semi-classical BUU transport model that
includes the off-shell propagation of vector mesons nonperturbatively and
calculates the width of the vector mesons dynamically. Different scenarios of
in-medium modifications of vector mesons, such as collisional broadening and
dropping vector meson masses, are investigated and the possibilities for an
experimental observation of in-medium effects in reactions at 1--4 GeV
are discussed for a variety of nuclear targets.Comment: 38 pages, LaTeX, including 20 postscript figures, to be published in
Nucl. Phys.
Dilepton production and off-shell transport dynamics at SIS energies
Dilepton production in nucleus-nucleus collisions at 1-2 A GeV as well as in
elementary pp and pd reactions is studied within the microscopic HSD transport
approach which includes the off-shell dynamics of vector mesons explicitly. The
study addresses additionally the production of and mesons since
their Dalitz decays provide a sizeable contribution to the dilepton invariant
mass spectra up to about 0.5 GeV. Our transport results agree with the TAPS
experimental data on and multiplicities in C+C collisions from 0.8
to 2 A GeV. We find that the 'DLS-puzzle' - which addresses an underestimation
of the yield in the mass range from 0.2 to 0.5 GeV in C+C and Ca+Ca
collisions - may be solved when incorporating a stronger bremsstrahlung
contribution in line with recent OBE calculations. Moreover, the HSD results
with 'enhanced' bremsstrahlung cross sections agree very well with the HADES
experimental data for the dilepton mass spectra for C+C at 1 and 2 A GeV,
especially when including a collisional broadening in the vector-meson spectral
functions. Detailed predictions for dilepton spectra from pp and pn/pd
reactions at 1.25 GeV, 2.2 GeV and 3.5 GeV are presented which will allow to
verify/falsify the larger bremsstrahlung contributions from the experimental
side in the near future.Comment: 43 pages, 22 figures; to be published to Nucl. Phys.
Excitation functions of hadronic observables from SIS to RHIC energies
We calculate excitation functions for various dynamical quantities as well as
experimental observables from SIS to RHIC energies within the HSD transport
approach which is based on string, quark, diquark () and hadronic degrees of freedom without including any explicit
phase transition to a quark-gluon plasma (QGP). It is argued that the failure
of this more 'conventional' approach in comparison to experimental data should
indicate the presence of a different phase which might be either attributed to
space-time regions of vanishing scalar quark condensate ( = 0) or
to the presence of a QGP phase with strongly interacting partons. We study the
ratio, the low mass dilepton enhancement in the invariant mass regime
from 0.2 -- 1.2 GeV as well as charmonium suppression for central Au + Au
collisions as a function of the bombarding energy and present predictions for
these observables as well as hadron rapiditiy distributions at RHIC energies.
Whereas all observables studied within HSD show smooth increasing/decreasing
excitation functions, the experimental ratio indicates a maximum at
11 AGeV (or above) which is interpreted as a signature for a chirally
restored phase in the course of the reaction.Comment: 34 pages, LaTeX, including 14 postscript figures (high quality color
versions of figs. 3,4 are available from
http://theorie.physik.uni-giessen.de/~brat/own.html), Nucl. Phys.
e^+e^- production from pp reactions at BEVALAC energies
We have performed a detailed study of dilepton production from pp collisions
including the subthreshold production via baryonic resonances (N(1520),
N(1700)) in addition to the conventional dilepton sources as and Dalitz decays and direct decays of vector mesons (). The role of baryonic resonances in production from
nucleon-nucleon collisions is studied in comparison to the DLS data which are
well described.Comment: 19 pages, revtex, including 7 postscript figures, to be published in
Nucl. Phys.
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